C#: Enable nullability checks on Semmle.Extraction.CIL

2025-12-17 01:03:14 +01:00 · 2020-08-19 14:53:40 +02:00
parent d56a03389c
commit 9cdee63ed7
22 changed files with 286 additions and 179 deletions
--- a/csharp/extractor/Semmle.Extraction.CIL/CachedFunction.cs
+++ b/csharp/extractor/Semmle.Extraction.CIL/CachedFunction.cs
@@ -9,7 +9,7 @@ namespace Semmle.Extraction.CIL
    /// </summary>
    /// <typeparam name="SrcType">The type of the source.</typeparam>
    /// <typeparam name="TargetType">The type of the generated object.</typeparam>
-    public class CachedFunction<SrcType, TargetType>
+    public class CachedFunction<SrcType, TargetType> where SrcType : notnull
    {
        readonly Func<SrcType, TargetType> generator;
        readonly Dictionary<SrcType, TargetType> cache;
--- a/csharp/extractor/Semmle.Extraction.CIL/Context.cs
+++ b/csharp/extractor/Semmle.Extraction.CIL/Context.cs
@@ -14,13 +14,19 @@ namespace Semmle.Extraction.CIL
    /// </summary>
    partial class Context : IDisposable
    {
-        public Extraction.Context cx;
        readonly FileStream stream;
-        public readonly MetadataReader mdReader;
-        public readonly PEReader peReader;
-        public readonly string assemblyPath;
-        public Entities.Assembly assembly;
-        public PDB.IPdb pdb;
+        Entities.Assembly? assemblyNull;
+
+        public Extraction.Context cx { get; }
+        public MetadataReader mdReader { get; }
+        public PEReader peReader { get; }
+        public string assemblyPath { get; }
+        public Entities.Assembly assembly
+        {
+            get { return assemblyNull!; }
+            set { assemblyNull = value; }
+        }
+        public PDB.IPdb? pdb { get; }

        public Context(Extraction.Context cx, string assemblyPath, bool extractPdbs)
        {
@@ -105,7 +111,7 @@ namespace Semmle.Extraction.CIL
        /// </summary>
        /// <param name="handle">The handle of the method.</param>
        /// <returns>The debugging information, or null if the information could not be located.</returns>
-        public PDB.IMethod GetMethodDebugInformation(MethodDefinitionHandle handle)
+        public PDB.IMethod? GetMethodDebugInformation(MethodDefinitionHandle handle)
        {
            return pdb == null ? null : pdb.GetMethod(handle.ToDebugInformationHandle());
        }
@@ -125,7 +131,8 @@ namespace Semmle.Extraction.CIL
        }

        /// <summary>
-        /// The list of generic type parameters.
+        /// The list of generic type parameters, including type parameters of
+        /// containing types.
        /// </summary>
        public abstract IEnumerable<Entities.Type> TypeParameters { get; }

--- a/csharp/extractor/Semmle.Extraction.CIL/Entities/Assembly.cs
+++ b/csharp/extractor/Semmle.Extraction.CIL/Entities/Assembly.cs
@@ -47,9 +47,9 @@ namespace Semmle.Extraction.CIL.Entities
            trapFile.Write(cx.assemblyPath.Replace("\\", "/"));
        }

-        public override bool Equals(object obj)
+        public override bool Equals(object? obj)
        {
-            return GetType() == obj.GetType() && Equals(file, ((Assembly)obj).file);
+            return GetType() == obj?.GetType() && Equals(file, ((Assembly)obj).file);
        }

        public override int GetHashCode() => 7 * file.GetHashCode();
@@ -63,7 +63,7 @@ namespace Semmle.Extraction.CIL.Entities
            get
            {
                yield return file;
-                yield return Tuples.assemblies(this, file, FullName, assemblyName.Name, assemblyName.Version.ToString());
+                yield return Tuples.assemblies(this, file, FullName, assemblyName.Name ?? string.Empty, assemblyName.Version?.ToString() ?? string.Empty);

                if (cx.pdb != null)
                {
@@ -75,7 +75,7 @@ namespace Semmle.Extraction.CIL.Entities

                foreach (var handle in cx.mdReader.TypeDefinitions)
                {
-                    IExtractionProduct product = null;
+                    IExtractionProduct? product = null;
                    try
                    {
                        product = cx.Create(handle);
@@ -92,7 +92,7 @@ namespace Semmle.Extraction.CIL.Entities

                foreach (var handle in cx.mdReader.MethodDefinitions)
                {
-                    IExtractionProduct product = null;
+                    IExtractionProduct? product = null;
                    try
                    {
                        product = cx.Create(handle);
--- a/csharp/extractor/Semmle.Extraction.CIL/Entities/Attribute.cs
+++ b/csharp/extractor/Semmle.Extraction.CIL/Entities/Attribute.cs
@@ -27,7 +27,7 @@ namespace Semmle.Extraction.CIL.Entities
            this.@object = @object;
        }

-        public override bool Equals(object obj)
+        public override bool Equals(object? obj)
        {
            return obj is Attribute attribute && handle.Equals(attribute.handle);
        }
@@ -58,13 +58,15 @@ namespace Semmle.Extraction.CIL.Entities
                for (int index = 0; index < decoded.FixedArguments.Length; ++index)
                {
                    object value = decoded.FixedArguments[index].Value;
-                    yield return Tuples.cil_attribute_positional_argument(this, index, value == null ? "null" : value.ToString());
+                    var stringValue = value?.ToString();
+                    yield return Tuples.cil_attribute_positional_argument(this, index, stringValue ?? "null");
                }

                foreach (var p in decoded.NamedArguments)
                {
                    object value = p.Value;
-                    yield return Tuples.cil_attribute_named_argument(this, p.Name, value == null ? "null" : value.ToString());
+                    var stringValue = value?.ToString();
+                    yield return Tuples.cil_attribute_named_argument(this, p.Name, stringValue ?? "null");
                }
            }
        }
--- a/csharp/extractor/Semmle.Extraction.CIL/Entities/Event.cs
+++ b/csharp/extractor/Semmle.Extraction.CIL/Entities/Event.cs
@@ -36,7 +36,7 @@ namespace Semmle.Extraction.CIL.Entities

        public override string IdSuffix => ";cil-event";

-        public override bool Equals(object obj)
+        public override bool Equals(object? obj)
        {
            return obj is Event e && handle.Equals(e.handle);
        }
--- a/csharp/extractor/Semmle.Extraction.CIL/Entities/ExceptionRegion.cs
+++ b/csharp/extractor/Semmle.Extraction.CIL/Entities/ExceptionRegion.cs
@@ -30,7 +30,7 @@ namespace Semmle.Extraction.CIL.Entities
        {
            get
            {
-                IInstruction try_start, try_end, handler_start;
+                IInstruction? try_start, try_end, handler_start;

                if (!jump_table.TryGetValue(r.TryOffset, out try_start))
                    throw new InternalError("Failed  to retrieve handler");
@@ -44,7 +44,7 @@ namespace Semmle.Extraction.CIL.Entities

                if (r.FilterOffset != -1)
                {
-                    IInstruction filter_start;
+                    IInstruction? filter_start;
                    if (!jump_table.TryGetValue(r.FilterOffset, out filter_start))
                        throw new InternalError("ExceptionRegion filter clause");

--- a/csharp/extractor/Semmle.Extraction.CIL/Entities/Field.cs
+++ b/csharp/extractor/Semmle.Extraction.CIL/Entities/Field.cs
@@ -91,7 +91,7 @@ namespace Semmle.Extraction.CIL.Entities
            fd = cx.mdReader.GetFieldDefinition(handle);
        }

-        public override bool Equals(object obj)
+        public override bool Equals(object? obj)
        {
            return obj is DefinitionField field && handle.Equals(field.handle);
        }
@@ -153,7 +153,7 @@ namespace Semmle.Extraction.CIL.Entities
            declType = (Type)cx.CreateGeneric(gc, mr.Parent);
        }

-        public override bool Equals(object obj)
+        public override bool Equals(object? obj)
        {
            return obj is MemberReferenceField field && Handle.Equals(field.Handle);
        }
--- a/csharp/extractor/Semmle.Extraction.CIL/Entities/File.cs
+++ b/csharp/extractor/Semmle.Extraction.CIL/Entities/File.cs
@@ -25,9 +25,9 @@ namespace Semmle.Extraction.CIL.Entities
            trapFile.Write(Semmle.Extraction.Entities.File.PathAsDatabaseId(path));
        }

-        public override bool Equals(object obj)
+        public override bool Equals(object? obj)
        {
-            return GetType() == obj.GetType() && path == ((File)obj).path;
+            return GetType() == obj?.GetType() && path == ((File)obj).path;
        }

        public override int GetHashCode() => 11 * path.GetHashCode();
@@ -36,7 +36,11 @@ namespace Semmle.Extraction.CIL.Entities
        {
            get
            {
-                var parent = cx.CreateFolder(System.IO.Path.GetDirectoryName(path));
+                var directoryName = System.IO.Path.GetDirectoryName(path);
+                if (directoryName == null)
+                    throw new InternalError($"Directory name for path '{path}' is null.");
+
+                var parent = cx.CreateFolder(directoryName);
                yield return parent;
                yield return Tuples.containerparent(parent, this);
                yield return Tuples.files(this, path, System.IO.Path.GetFileNameWithoutExtension(path), System.IO.Path.GetExtension(path).Substring(1));
--- a/csharp/extractor/Semmle.Extraction.CIL/Entities/Folder.cs
+++ b/csharp/extractor/Semmle.Extraction.CIL/Entities/Folder.cs
@@ -41,7 +41,7 @@ namespace Semmle.Extraction.CIL.Entities
            }
        }

-        public override bool Equals(object obj)
+        public override bool Equals(object? obj)
        {
            return obj is Folder folder && path == folder.path;
        }
--- a/csharp/extractor/Semmle.Extraction.CIL/Entities/Instruction.cs
+++ b/csharp/extractor/Semmle.Extraction.CIL/Entities/Instruction.cs
@@ -365,6 +365,11 @@ namespace Semmle.Extraction.CIL.Entities
            {
                int offset = Offset;

+                if (Method.Implementation is null)
+                {
+                    yield break;
+                }
+
                yield return Tuples.cil_instruction(this, (int)OpCode, Index, Method.Implementation);

                switch (PayloadType)
@@ -414,11 +419,17 @@ namespace Semmle.Extraction.CIL.Entities
                        break;
                    case Payload.Arg8:
                    case Payload.Arg16:
-                        yield return Tuples.cil_access(this, Method.Parameters[(int)UnsignedPayloadValue]);
+                        if (!(Method.Parameters is null))
+                        {
+                            yield return Tuples.cil_access(this, Method.Parameters[(int)UnsignedPayloadValue]);
+                        }
                        break;
                    case Payload.Local8:
                    case Payload.Local16:
-                        yield return Tuples.cil_access(this, Method.LocalVariables[(int)UnsignedPayloadValue]);
+                        if (!(Method.LocalVariables is null))
+                        {
+                            yield return Tuples.cil_access(this, Method.LocalVariables[(int)UnsignedPayloadValue]);
+                        }
                        break;
                    case Payload.None:
                    case Payload.Target8:
@@ -439,7 +450,7 @@ namespace Semmle.Extraction.CIL.Entities
        public IEnumerable<IExtractionProduct> JumpContents(Dictionary<int, IInstruction> jump_table)
        {
            int target;
-            IInstruction inst;
+            IInstruction? inst;

            switch (PayloadType)
            {
--- a/csharp/extractor/Semmle.Extraction.CIL/Entities/Method.cs
+++ b/csharp/extractor/Semmle.Extraction.CIL/Entities/Method.cs
@@ -23,25 +23,29 @@ namespace Semmle.Extraction.CIL.Entities
    /// </summary>
    abstract class Method : TypeContainer, IMethod
    {
+        protected MethodTypeParameter[]? genericParams;
+        protected GenericContext gc;
+        protected MethodSignature<ITypeSignature> signature;
+
        protected Method(GenericContext gc) : base(gc.cx)
        {
            this.gc = gc;
        }

-        public override IEnumerable<Type> TypeParameters => gc.TypeParameters.Concat(declaringType.TypeParameters);
+        public override IEnumerable<Type> TypeParameters => gc.TypeParameters.Concat(DeclaringType.TypeParameters);

        public override IEnumerable<Type> MethodParameters =>
            genericParams == null ? gc.MethodParameters : gc.MethodParameters.Concat(genericParams);

        public int GenericParameterCount => signature.GenericParameterCount;

-        public virtual Method SourceDeclaration => this;
+        public virtual Method? SourceDeclaration => this;

        public abstract Type DeclaringType { get; }
        public abstract string Name { get; }

-        public virtual IList<LocalVariable> LocalVariables => throw new NotImplementedException();
-        public IList<Parameter> Parameters { get; private set; }
+        public virtual IList<LocalVariable>? LocalVariables => throw new NotImplementedException();
+        public IList<Parameter>? Parameters { get; private set; }

        public override void WriteId(TextWriter trapFile) => WriteMethodId(trapFile, DeclaringType, NameLabel);

@@ -70,12 +74,6 @@ namespace Semmle.Extraction.CIL.Entities
            trapFile.Write(')');
        }

-        protected MethodTypeParameter[] genericParams;
-        protected Type declaringType;
-        protected GenericContext gc;
-        protected MethodSignature<ITypeSignature> signature;
-        protected string name;
-
        public override string IdSuffix => ";cil-method";

        protected void PopulateParameters(IEnumerable<Type> parameterTypes)
@@ -145,13 +143,15 @@ namespace Semmle.Extraction.CIL.Entities
    {
        readonly Handle handle;
        readonly MethodDefinition md;
-        readonly PDB.IMethod methodDebugInformation;
+        readonly PDB.IMethod? methodDebugInformation;
+        readonly Type declaringType;

-        LocalVariable[] locals;
+        string name;
+        LocalVariable[]? locals;

-        public MethodImplementation Implementation { get; private set; }
+        public MethodImplementation? Implementation { get; private set; }

-        public override IList<LocalVariable> LocalVariables => locals;
+        public override IList<LocalVariable>? LocalVariables => locals;

        public DefinitionMethod(GenericContext gc, MethodDefinitionHandle handle) : base(gc)
        {
@@ -167,7 +167,7 @@ namespace Semmle.Extraction.CIL.Entities
            methodDebugInformation = cx.GetMethodDebugInformation(handle);
        }

-        public override bool Equals(object obj)
+        public override bool Equals(object? obj)
        {
            return obj is DefinitionMethod method && handle.Equals(method.handle);
        }
@@ -208,7 +208,7 @@ namespace Semmle.Extraction.CIL.Entities

                PopulateParameters(typeSignature.ParameterTypes);

-                foreach (var c in Parameters)
+                foreach (var c in Parameters!)
                    yield return c;

                foreach (var c in PopulateFlags)
@@ -315,8 +315,8 @@ namespace Semmle.Extraction.CIL.Entities
            // The sequence point gives the location of each instruction.
            // The location of an instruction is given by the sequence point *after* the
            // instruction.
-            IEnumerator<PDB.SequencePoint> nextSequencePoint = null;
-            PdbSourceLocation instructionLocation = null;
+            IEnumerator<PDB.SequencePoint>? nextSequencePoint = null;
+            PdbSourceLocation? instructionLocation = null;

            if (methodDebugInformation != null)
            {
@@ -401,9 +401,9 @@ namespace Semmle.Extraction.CIL.Entities
    {
        readonly MemberReferenceHandle handle;
        readonly MemberReference mr;
-        readonly Type declType;
+        readonly Type declaringType;
        readonly GenericContext parent;
-        readonly Method sourceDeclaration;
+        readonly Method? sourceDeclaration;

        public MemberReferenceMethod(GenericContext gc, MemberReferenceHandle handle) : base(gc)
        {
@@ -416,22 +416,24 @@ namespace Semmle.Extraction.CIL.Entities
            parent = (GenericContext)cx.CreateGeneric(gc, mr.Parent);

            var parentMethod = parent as Method;
-            nameLabel = cx.GetString(mr.Name);

-            declType = parentMethod == null ? parent as Type : parentMethod.DeclaringType;
+            var declType = parentMethod == null ? parent as Type : parentMethod.DeclaringType;

            if (declType is null)
                throw new InternalError("Parent context of method is not a type");

-            var typeSourceDeclaration = declType.SourceDeclaration;
-            sourceDeclaration = typeSourceDeclaration == declType ? (Method)this : typeSourceDeclaration.LookupMethod(mr.Name, mr.Signature);
+            declaringType = declType;
+            nameLabel = cx.GetString(mr.Name);
+
+            var typeSourceDeclaration = declaringType.SourceDeclaration;
+            sourceDeclaration = typeSourceDeclaration == declaringType ? (Method)this : typeSourceDeclaration.LookupMethod(mr.Name, mr.Signature);
        }

        private readonly string nameLabel;

        public override string NameLabel => nameLabel;

-        public override bool Equals(object obj)
+        public override bool Equals(object? obj)
        {
            return obj is MemberReferenceMethod method && handle.Equals(method.handle);
        }
@@ -441,11 +443,11 @@ namespace Semmle.Extraction.CIL.Entities
            return handle.GetHashCode();
        }

-        public override Method SourceDeclaration => sourceDeclaration;
+        public override Method? SourceDeclaration => sourceDeclaration;

        public override bool IsStatic => !signature.Header.IsInstance;

-        public override Type DeclaringType => declType;
+        public override Type DeclaringType => declaringType;

        public override string Name => cx.ShortName(mr.Name);

@@ -465,7 +467,7 @@ namespace Semmle.Extraction.CIL.Entities
                var typeSignature = mr.DecodeMethodSignature(cx.TypeSignatureDecoder, this);

                PopulateParameters(typeSignature.ParameterTypes);
-                foreach (var p in Parameters) yield return p;
+                foreach (var p in Parameters!) yield return p;

                foreach (var f in PopulateFlags) yield return f;

@@ -486,6 +488,7 @@ namespace Semmle.Extraction.CIL.Entities
        readonly MethodSpecification ms;
        readonly Method unboundMethod;
        readonly ImmutableArray<Type> typeParams;
+        readonly Type declaringType;

        public MethodSpecificationMethod(GenericContext gc, MethodSpecificationHandle handle) : base(gc)
        {
@@ -501,7 +504,7 @@ namespace Semmle.Extraction.CIL.Entities
            unboundMethod.WriteId(trapFile);
            trapFile.Write('<');
            int index = 0;
-            foreach(var param in typeParams)
+            foreach (var param in typeParams)
            {
                trapFile.WriteSeparator(",", ref index);
                trapFile.WriteSubId(param);
@@ -511,7 +514,7 @@ namespace Semmle.Extraction.CIL.Entities

        public override string NameLabel => throw new NotImplementedException();

-        public override bool Equals(object obj)
+        public override bool Equals(object? obj)
        {
            return obj is MethodSpecificationMethod method && handle.Equals(method.handle) && typeParams.SequenceEqual(method.typeParams);
        }
@@ -548,7 +551,7 @@ namespace Semmle.Extraction.CIL.Entities
                }

                PopulateParameters(constructedTypeSignature.ParameterTypes);
-                foreach (var p in Parameters)
+                foreach (var p in Parameters!)
                    yield return p;

                foreach (var f in PopulateFlags)
--- a/csharp/extractor/Semmle.Extraction.CIL/Entities/Namespace.cs
+++ b/csharp/extractor/Semmle.Extraction.CIL/Entities/Namespace.cs
@@ -17,7 +17,7 @@ namespace Semmle.Extraction.CIL.Entities
    /// </summary>
    public sealed class Namespace : TypeContainer, INamespace
    {
-        public Namespace ParentNamespace;
+        public Namespace? ParentNamespace;
        public readonly string Name;

        public bool IsGlobalNamespace => ParentNamespace == null;
@@ -35,7 +35,7 @@ namespace Semmle.Extraction.CIL.Entities
            trapFile.Write(Name);
        }

-        public override bool Equals(object obj)
+        public override bool Equals(object? obj)
        {
            if (obj is Namespace ns && Name == ns.Name)
            {
@@ -63,7 +63,7 @@ namespace Semmle.Extraction.CIL.Entities
            return i == -1 ? fqn : fqn.Substring(i + 1);
        }

-        static Namespace createParentNamespace(Context cx, string fqn)
+        static Namespace? createParentNamespace(Context cx, string fqn)
        {
            if (fqn == "") return null;
            var i = fqn.LastIndexOf('.');
@@ -74,7 +74,7 @@ namespace Semmle.Extraction.CIL.Entities
        {
        }

-        public Namespace(Context cx, string name, Namespace parent) : base(cx)
+        public Namespace(Context cx, string name, Namespace? parent) : base(cx)
        {
            Name = name;
            ParentNamespace = parent;
@@ -86,7 +86,7 @@ namespace Semmle.Extraction.CIL.Entities
            {
                yield return Tuples.namespaces(this, Name);
                if (!IsGlobalNamespace)
-                    yield return Tuples.parent_namespace(this, ParentNamespace);
+                    yield return Tuples.parent_namespace(this, ParentNamespace!);
            }
        }
    }
--- a/csharp/extractor/Semmle.Extraction.CIL/Entities/Parameter.cs
+++ b/csharp/extractor/Semmle.Extraction.CIL/Entities/Parameter.cs
@@ -33,7 +33,7 @@ namespace Semmle.Extraction.CIL.Entities
            trapFile.Write(index);
        }

-        public override bool Equals(object obj)
+        public override bool Equals(object? obj)
        {
            return obj is Parameter param && method.Equals(param.method) && index == param.index;
        }
--- a/csharp/extractor/Semmle.Extraction.CIL/Entities/Property.cs
+++ b/csharp/extractor/Semmle.Extraction.CIL/Entities/Property.cs
@@ -48,7 +48,7 @@ namespace Semmle.Extraction.CIL.Entities
            trapFile.Write(")");
        }

-        public override bool Equals(object obj)
+        public override bool Equals(object? obj)
        {
            return obj is Property property && Equals(handle, property.handle);
        }
--- a/csharp/extractor/Semmle.Extraction.CIL/Entities/SourceLocation.cs
+++ b/csharp/extractor/Semmle.Extraction.CIL/Entities/SourceLocation.cs
@@ -32,7 +32,7 @@ namespace Semmle.Extraction.CIL.Entities
            trapFile.Write(location.EndColumn);
        }

-        public override bool Equals(object obj)
+        public override bool Equals(object? obj)
        {
            return obj is PdbSourceLocation l && location.Equals(l.location);
        }
--- a/csharp/extractor/Semmle.Extraction.CIL/Entities/Type.cs
+++ b/csharp/extractor/Semmle.Extraction.CIL/Entities/Type.cs
@@ -8,6 +8,7 @@ using System.Reflection;
 using Semmle.Util;
 using System.IO;
 using System.Text;
+using System.Diagnostics.CodeAnalysis;

 namespace Semmle.Extraction.CIL.Entities
 {
@@ -103,7 +104,7 @@ namespace Semmle.Extraction.CIL.Entities
        /// shortcut to comparing the signature bytes since handles are unique.
        /// </param>
        /// <returns>The method, or 'null' if not found or not supported.</returns>
-        internal virtual Method LookupMethod(StringHandle methodName, BlobHandle signature)
+        internal virtual Method? LookupMethod(StringHandle methodName, BlobHandle signature)
        {
            return null;
        }
@@ -160,7 +161,7 @@ namespace Semmle.Extraction.CIL.Entities
            get;
        }

-        public virtual TypeContainer Parent => (TypeContainer)ContainingType ?? Namespace;
+        public virtual TypeContainer Parent => (TypeContainer?)ContainingType ?? Namespace!;

        public override IEnumerable<IExtractionProduct> Contents
        {
@@ -177,9 +178,9 @@ namespace Semmle.Extraction.CIL.Entities

        public abstract string Name { get; }

-        public abstract Namespace Namespace { get; }
+        public abstract Namespace? Namespace { get; }

-        public abstract Type ContainingType { get; }
+        public abstract Type? ContainingType { get; }

        public abstract Type Construct(IEnumerable<Type> typeArguments);

@@ -233,7 +234,7 @@ namespace Semmle.Extraction.CIL.Entities
        /// </summary>
        /// <param name="t">The resulting primitive type, or null.</param>
        /// <returns>True if this type is a primitive type.</returns>
-        public bool TryGetPrimitiveType(out PrimitiveType t)
+        public bool TryGetPrimitiveType([NotNullWhen(true)] out PrimitiveType? t)
        {
            if (TryGetPrimitiveTypeCode(out var code))
            {
@@ -249,7 +250,7 @@ namespace Semmle.Extraction.CIL.Entities

        private bool TryGetPrimitiveTypeCode(out PrimitiveTypeCode code)
        {
-            if (ContainingType == null && Namespace.Name == cx.SystemNamespace.Name)
+            if (ContainingType == null && Namespace?.Name == cx.SystemNamespace.Name)
            {
                switch (Name)
                {
@@ -341,7 +342,7 @@ namespace Semmle.Extraction.CIL.Entities
            typeParams = new Lazy<IEnumerable<TypeTypeParameter>>(MakeTypeParameters);
        }

-        public override bool Equals(object obj)
+        public override bool Equals(object? obj)
        {
            return obj is TypeDefinitionType t && handle.Equals(t.handle);
        }
@@ -390,9 +391,9 @@ namespace Semmle.Extraction.CIL.Entities

        public override Namespace Namespace => cx.Create(td.NamespaceDefinition);

-        readonly Type declType;
+        readonly Type? declType;

-        public override Type ContainingType => declType;
+        public override Type? ContainingType => declType;

        public override int ThisTypeParameters
        {
@@ -562,18 +563,14 @@ namespace Semmle.Extraction.CIL.Entities
        readonly TypeReference tr;
        readonly Lazy<TypeTypeParameter[]> typeParams;

-        public TypeReferenceType(Context cx, TypeReferenceHandle handle) : this(cx, handle, cx.mdReader.GetTypeReference(handle))
-        {
-            typeParams = new Lazy<TypeTypeParameter[]>(MakeTypeParameters);
-        }
-
-        public TypeReferenceType(Context cx, TypeReferenceHandle handle, TypeReference tr) : base(cx)
+        public TypeReferenceType(Context cx, TypeReferenceHandle handle) : base(cx)
        {
+            this.typeParams = new Lazy<TypeTypeParameter[]>(MakeTypeParameters);
            this.handle = handle;
-            this.tr = tr;
+            this.tr = cx.mdReader.GetTypeReference(handle);
        }

-        public override bool Equals(object obj)
+        public override bool Equals(object? obj)
        {
            return obj is TypeReferenceType t && handle.Equals(t.handle);
        }
@@ -637,7 +634,7 @@ namespace Semmle.Extraction.CIL.Entities
            }
        }

-        public override Type ContainingType
+        public override Type? ContainingType
        {
            get
            {
@@ -654,7 +651,7 @@ namespace Semmle.Extraction.CIL.Entities
            switch (tr.ResolutionScope.Kind)
            {
                case HandleKind.TypeReference:
-                    ContainingType.WriteAssemblyPrefix(trapFile);
+                    ContainingType!.WriteAssemblyPrefix(trapFile);
                    break;
                case HandleKind.AssemblyReference:
                    var assemblyDef = cx.mdReader.GetAssemblyReference((AssemblyReferenceHandle)tr.ResolutionScope);
@@ -684,7 +681,7 @@ namespace Semmle.Extraction.CIL.Entities
            var ct = ContainingType;
            if (ct != null)
            {
-                ContainingType.GetId(trapFile, inContext);
+                ct.GetId(trapFile, inContext);
            }
            else
            {
@@ -719,9 +716,11 @@ namespace Semmle.Extraction.CIL.Entities
    public sealed class ConstructedType : Type
    {
        readonly Type unboundGenericType;
-        readonly Type[] thisTypeArguments;

-        public override IEnumerable<Type> ThisTypeArguments => thisTypeArguments;
+        // Either null or notEmpty
+        readonly Type[]? thisTypeArguments;
+
+        public override IEnumerable<Type> ThisTypeArguments => thisTypeArguments.EnumerateNull();

        public override IEnumerable<Type> ThisGenericArguments => thisTypeArguments.EnumerateNull();

@@ -751,7 +750,6 @@ namespace Semmle.Extraction.CIL.Entities

            unboundGenericType = unboundType;
            var thisParams = unboundType.ThisTypeParameters;
-            var parentParams = suppliedArgs - thisParams;

            if (typeArguments.Count() == thisParams)
            {
@@ -760,18 +758,21 @@ namespace Semmle.Extraction.CIL.Entities
            }
            else if (thisParams == 0)
            {
-                containingType = unboundType.ContainingType.Construct(typeArguments);
+                // all type arguments belong to containing type
+                containingType = unboundType.ContainingType!.Construct(typeArguments);
            }
            else
            {
-                containingType = unboundType.ContainingType.Construct(typeArguments.Take(parentParams));
+                // some type arguments belong to containing type
+                var parentParams = suppliedArgs - thisParams;
+                containingType = unboundType.ContainingType!.Construct(typeArguments.Take(parentParams));
                thisTypeArguments = typeArguments.Skip(parentParams).ToArray();
            }
        }

-        public override bool Equals(object obj)
+        public override bool Equals(object? obj)
        {
-            if(obj is ConstructedType t && Equals(unboundGenericType, t.unboundGenericType) && Equals(containingType, t.containingType))
+            if (obj is ConstructedType t && Equals(unboundGenericType, t.unboundGenericType) && Equals(containingType, t.containingType))
            {
                if (thisTypeArguments is null) return t.thisTypeArguments is null;
                if (!(t.thisTypeArguments is null)) return thisTypeArguments.SequenceEqual(t.thisTypeArguments);
@@ -788,12 +789,12 @@ namespace Semmle.Extraction.CIL.Entities
            return h;
        }

-        readonly Type containingType;
-        public override Type ContainingType => containingType;
+        readonly Type? containingType;
+        public override Type? ContainingType => containingType;

        public override string Name => unboundGenericType.Name;

-        public override Namespace Namespace => unboundGenericType.Namespace;
+        public override Namespace Namespace => unboundGenericType.Namespace!;

        public override int ThisTypeParameters => thisTypeArguments == null ? 0 : thisTypeArguments.Length;

@@ -851,7 +852,7 @@ namespace Semmle.Extraction.CIL.Entities
            typeCode = tc;
        }

-        public override bool Equals(object obj)
+        public override bool Equals(object? obj)
        {
            return obj is PrimitiveType pt && typeCode == pt.typeCode;
        }
@@ -871,7 +872,7 @@ namespace Semmle.Extraction.CIL.Entities

        public override Namespace Namespace => cx.SystemNamespace;

-        public override Type ContainingType => null;
+        public override Type? ContainingType => null;

        public override int ThisTypeParameters => 0;

@@ -894,19 +895,17 @@ namespace Semmle.Extraction.CIL.Entities
        readonly Type elementType;
        readonly int rank;

-        public ArrayType(Context cx, Type element, ArrayShape shape) : base(cx)
+        public ArrayType(Context cx, Type elementType, int rank) : base(cx)
        {
-            rank = shape.Rank;
-            elementType = element;
+            this.rank = rank;
+            this.elementType = elementType;
        }

-        public ArrayType(Context cx, Type element) : base(cx)
+        public ArrayType(Context cx, Type elementType) : this(cx, elementType, 1)
        {
-            rank = 1;
-            elementType = element;
        }

-        public override bool Equals(object obj)
+        public override bool Equals(object? obj)
        {
            return obj is ArrayType array && elementType.Equals(array.elementType) && rank == array.rank;
        }
@@ -929,7 +928,7 @@ namespace Semmle.Extraction.CIL.Entities

        public override Namespace Namespace => cx.SystemNamespace;

-        public override Type ContainingType => null;
+        public override Type? ContainingType => null;

        public override int ThisTypeParameters => elementType.ThisTypeParameters;

@@ -972,9 +971,9 @@ namespace Semmle.Extraction.CIL.Entities
            this.gc = gc;
        }

-        public override Namespace Namespace => null;
+        public override Namespace? Namespace => null;

-        public override Type ContainingType => null;
+        public override Type? ContainingType => null;

        public override int ThisTypeParameters => 0;

@@ -1034,7 +1033,7 @@ namespace Semmle.Extraction.CIL.Entities
            this.index = index;
        }

-        public override bool Equals(object obj)
+        public override bool Equals(object? obj)
        {
            return obj is MethodTypeParameter tp && method.Equals(tp.method) && index == tp.index;
        }
@@ -1072,7 +1071,7 @@ namespace Semmle.Extraction.CIL.Entities
            type = t;
        }

-        public override bool Equals(object obj)
+        public override bool Equals(object? obj)
        {
            return obj is TypeTypeParameter tp && type.Equals(tp.type) && index == tp.index;
        }
@@ -1089,7 +1088,7 @@ namespace Semmle.Extraction.CIL.Entities
            trapFile.Write(index);
        }

-        public override TypeContainer Parent => type ?? gc as TypeContainer;
+        public override TypeContainer Parent => type;
        public override string Name => "!" + index;

        public override IEnumerable<Type> TypeParameters => Enumerable.Empty<Type>();
@@ -1119,7 +1118,7 @@ namespace Semmle.Extraction.CIL.Entities
            this.pointee = pointee;
        }

-        public override bool Equals(object obj)
+        public override bool Equals(object? obj)
        {
            return obj is PointerType pt && pointee.Equals(pt.pointee);
        }
@@ -1138,9 +1137,9 @@ namespace Semmle.Extraction.CIL.Entities

        public override string Name => pointee.Name + "*";

-        public override Namespace Namespace => pointee.Namespace;
+        public override Namespace? Namespace => pointee.Namespace;

-        public override Type ContainingType => pointee.ContainingType;
+        public override Type? ContainingType => pointee.ContainingType;

        public override TypeContainer Parent => pointee.Parent;

@@ -1180,7 +1179,7 @@ namespace Semmle.Extraction.CIL.Entities

        public override Namespace Namespace => cx.GlobalNamespace;

-        public override Type ContainingType => null;
+        public override Type? ContainingType => null;

        public override int ThisTypeParameters => 0;

@@ -1202,13 +1201,20 @@ namespace Semmle.Extraction.CIL.Entities
    {
        struct Array : ITypeSignature
        {
-            public ITypeSignature elementType;
-            public ArrayShape shape;
+            private readonly ITypeSignature elementType;
+            private readonly ArrayShape shape;
+
+            public Array(ITypeSignature elementType, ArrayShape shape) : this()
+            {
+                this.elementType = elementType;
+                this.shape = shape;
+            }
+
            public void WriteId(TextWriter trapFile, GenericContext gc)
            {
                elementType.WriteId(trapFile, gc);
                trapFile.Write('[');
-                for (int i=1; i<shape.Rank; ++i)
+                for (int i = 1; i < shape.Rank; ++i)
                    trapFile.Write(',');
                trapFile.Write(']');
            }
@@ -1216,7 +1222,12 @@ namespace Semmle.Extraction.CIL.Entities

        struct ByRef : ITypeSignature
        {
-            public ITypeSignature elementType;
+            private readonly ITypeSignature elementType;
+
+            public ByRef(ITypeSignature elementType)
+            {
+                this.elementType = elementType;
+            }

            public void WriteId(TextWriter trapFile, GenericContext gc)
            {
@@ -1227,7 +1238,12 @@ namespace Semmle.Extraction.CIL.Entities

        struct FnPtr : ITypeSignature
        {
-            public MethodSignature<ITypeSignature> signature;
+            private readonly MethodSignature<ITypeSignature> signature;
+
+            public FnPtr(MethodSignature<ITypeSignature> signature)
+            {
+                this.signature = signature;
+            }

            public void WriteId(TextWriter trapFile, GenericContext gc)
            {
@@ -1236,25 +1252,32 @@ namespace Semmle.Extraction.CIL.Entities
        }

        ITypeSignature IConstructedTypeProvider<ITypeSignature>.GetArrayType(ITypeSignature elementType, ArrayShape shape) =>
-            new Array { elementType = elementType, shape = shape };
+            new Array(elementType, shape);

        ITypeSignature IConstructedTypeProvider<ITypeSignature>.GetByReferenceType(ITypeSignature elementType) =>
-            new ByRef { elementType = elementType };
+            new ByRef(elementType);

        ITypeSignature ISignatureTypeProvider<ITypeSignature, object>.GetFunctionPointerType(MethodSignature<ITypeSignature> signature) =>
-            new FnPtr { signature = signature };
+            new FnPtr(signature);

        class Instantiation : ITypeSignature
        {
-            public ITypeSignature genericType;
-            public ImmutableArray<ITypeSignature> typeArguments;
+            private readonly ITypeSignature genericType;
+            private readonly ImmutableArray<ITypeSignature> typeArguments;
+
+            public Instantiation(ITypeSignature genericType, ImmutableArray<ITypeSignature> typeArguments)
+            {
+                this.genericType = genericType;
+                this.typeArguments = typeArguments;
+            }
+

            public void WriteId(TextWriter trapFile, GenericContext gc)
            {
                genericType.WriteId(trapFile, gc);
                trapFile.Write('<');
                int index = 0;
-                foreach(var arg in typeArguments)
+                foreach (var arg in typeArguments)
                {
                    trapFile.WriteSeparator(",", ref index);
                    arg.WriteId(trapFile, gc);
@@ -1264,12 +1287,18 @@ namespace Semmle.Extraction.CIL.Entities
        }

        ITypeSignature IConstructedTypeProvider<ITypeSignature>.GetGenericInstantiation(ITypeSignature genericType, ImmutableArray<ITypeSignature> typeArguments) =>
-            new Instantiation { genericType = genericType, typeArguments = typeArguments };
+            new Instantiation(genericType, typeArguments);

        class GenericMethodParameter : ITypeSignature
        {
-            public object innerGc;
-            public int index;
+            private readonly object innerGc;
+            private readonly int index;
+
+            public GenericMethodParameter(object innerGc, int index)
+            {
+                this.innerGc = innerGc;
+                this.index = index;
+            }

            public void WriteId(TextWriter trapFile, GenericContext outerGc)
            {
@@ -1284,7 +1313,12 @@ namespace Semmle.Extraction.CIL.Entities

        class GenericTypeParameter : ITypeSignature
        {
-            public int index;
+            private readonly int index;
+
+            public GenericTypeParameter(int index)
+            {
+                this.index = index;
+            }

            public void WriteId(TextWriter trapFile, GenericContext gc)
            {
@@ -1294,16 +1328,23 @@ namespace Semmle.Extraction.CIL.Entities
        }

        ITypeSignature ISignatureTypeProvider<ITypeSignature, object>.GetGenericMethodParameter(object genericContext, int index) =>
-            new GenericMethodParameter { innerGc = genericContext, index = index };
+            new GenericMethodParameter(genericContext, index);

        ITypeSignature ISignatureTypeProvider<ITypeSignature, object>.GetGenericTypeParameter(object genericContext, int index) =>
-            new GenericTypeParameter { index = index };
+            new GenericTypeParameter(index);

        class Modified : ITypeSignature
        {
-            public ITypeSignature modifier;
-            public ITypeSignature unmodifiedType;
-            public bool isRequired;
+            private readonly ITypeSignature modifier;
+            private readonly ITypeSignature unmodifiedType;
+            private readonly bool isRequired;
+
+            public Modified(ITypeSignature modifier, ITypeSignature unmodifiedType, bool isRequired)
+            {
+                this.modifier = modifier;
+                this.unmodifiedType = unmodifiedType;
+                this.isRequired = isRequired;
+            }

            public void WriteId(TextWriter trapFile, GenericContext gc)
            {
@@ -1313,12 +1354,17 @@ namespace Semmle.Extraction.CIL.Entities

        ITypeSignature ISignatureTypeProvider<ITypeSignature, object>.GetModifiedType(ITypeSignature modifier, ITypeSignature unmodifiedType, bool isRequired)
        {
-            return new Modified { modifier = modifier, unmodifiedType = unmodifiedType, isRequired = isRequired };
+            return new Modified(modifier, unmodifiedType, isRequired);
        }

        class Pinned : ITypeSignature
        {
-            public ITypeSignature elementType;
+            private readonly ITypeSignature elementType;
+
+            public Pinned(ITypeSignature elementType)
+            {
+                this.elementType = elementType;
+            }

            public void WriteId(TextWriter trapFile, GenericContext gc)
            {
@@ -1329,12 +1375,17 @@ namespace Semmle.Extraction.CIL.Entities

        ITypeSignature ISignatureTypeProvider<ITypeSignature, object>.GetPinnedType(ITypeSignature elementType)
        {
-            return new Pinned { elementType = elementType };
+            return new Pinned(elementType);
        }

        class PointerType : ITypeSignature
        {
-            public ITypeSignature elementType;
+            private readonly ITypeSignature elementType;
+
+            public PointerType(ITypeSignature elementType)
+            {
+                this.elementType = elementType;
+            }

            public void WriteId(TextWriter trapFile, GenericContext gc)
            {
@@ -1345,12 +1396,17 @@ namespace Semmle.Extraction.CIL.Entities

        ITypeSignature IConstructedTypeProvider<ITypeSignature>.GetPointerType(ITypeSignature elementType)
        {
-            return new PointerType { elementType = elementType };
+            return new PointerType(elementType);
        }

        class Primitive : ITypeSignature
        {
-            public PrimitiveTypeCode typeCode;
+            private readonly PrimitiveTypeCode typeCode;
+
+            public Primitive(PrimitiveTypeCode typeCode)
+            {
+                this.typeCode = typeCode;
+            }

            public void WriteId(TextWriter trapFile, GenericContext gc)
            {
@@ -1360,12 +1416,18 @@ namespace Semmle.Extraction.CIL.Entities

        ITypeSignature ISimpleTypeProvider<ITypeSignature>.GetPrimitiveType(PrimitiveTypeCode typeCode)
        {
-            return new Primitive { typeCode = typeCode };
+            return new Primitive(typeCode);
        }

        class SzArrayType : ITypeSignature
        {
-            public ITypeSignature elementType;
+            private readonly ITypeSignature elementType;
+
+            public SzArrayType(ITypeSignature elementType)
+            {
+                this.elementType = elementType;
+            }
+
            public void WriteId(TextWriter trapFile, GenericContext gc)
            {
                elementType.WriteId(trapFile, gc);
@@ -1375,41 +1437,53 @@ namespace Semmle.Extraction.CIL.Entities

        ITypeSignature ISZArrayTypeProvider<ITypeSignature>.GetSZArrayType(ITypeSignature elementType)
        {
-            return new SzArrayType { elementType = elementType };
+            return new SzArrayType(elementType);
        }

        class TypeDefinition : ITypeSignature
        {
-            public TypeDefinitionHandle handle;
-            public byte rawTypeKind;
-            Type type;
+            private readonly TypeDefinitionHandle handle;
+            private readonly byte rawTypeKind;
+
+            public TypeDefinition(TypeDefinitionHandle handle, byte rawTypeKind)
+            {
+                this.handle = handle;
+                this.rawTypeKind = rawTypeKind;
+            }
+
            public void WriteId(TextWriter trapFile, GenericContext gc)
            {
-                type = (Type)gc.cx.Create(handle);
+                var type = (Type)gc.cx.Create(handle);
                type.WriteId(trapFile);
            }
        }

        ITypeSignature ISimpleTypeProvider<ITypeSignature>.GetTypeFromDefinition(MetadataReader reader, TypeDefinitionHandle handle, byte rawTypeKind)
        {
-            return new TypeDefinition { handle = handle, rawTypeKind = rawTypeKind };
+            return new TypeDefinition(handle, rawTypeKind);
        }

        class TypeReference : ITypeSignature
        {
-            public TypeReferenceHandle handle;
-            public byte rawTypeKind; // struct/class (not used)
-            Type type;
+            private readonly TypeReferenceHandle handle;
+            private readonly byte rawTypeKind; // struct/class (not used)
+
+            public TypeReference(TypeReferenceHandle handle, byte rawTypeKind)
+            {
+                this.handle = handle;
+                this.rawTypeKind = rawTypeKind;
+            }
+
            public void WriteId(TextWriter trapFile, GenericContext gc)
            {
-                type = (Type)gc.cx.Create(handle);
+                var type = (Type)gc.cx.Create(handle);
                type.WriteId(trapFile);
            }
        }

        ITypeSignature ISimpleTypeProvider<ITypeSignature>.GetTypeFromReference(MetadataReader reader, TypeReferenceHandle handle, byte rawTypeKind)
        {
-            return new TypeReference { handle = handle, rawTypeKind = rawTypeKind };
+            return new TypeReference(handle, rawTypeKind);
        }

        ITypeSignature ISignatureTypeProvider<ITypeSignature, object>.GetTypeFromSpecification(MetadataReader reader, object genericContext, TypeSpecificationHandle handle, byte rawTypeKind)
@@ -1433,7 +1507,7 @@ namespace Semmle.Extraction.CIL.Entities
        }

        Type IConstructedTypeProvider<Type>.GetArrayType(Type elementType, ArrayShape shape) =>
-            cx.Populate(new ArrayType(cx, elementType, shape));
+            cx.Populate(new ArrayType(cx, elementType, shape.Rank));

        Type IConstructedTypeProvider<Type>.GetByReferenceType(Type elementType) =>
            elementType;  // ??
--- a/csharp/extractor/Semmle.Extraction.CIL/Factories.cs
+++ b/csharp/extractor/Semmle.Extraction.CIL/Factories.cs
@@ -41,7 +41,7 @@ namespace Semmle.Extraction.CIL
                    e.WriteId(writer);
                    var id = writer.ToString();

-                    if (debugLabels.TryGetValue(id, out IExtractedEntity previousEntity))
+                    if (debugLabels.TryGetValue(id, out IExtractedEntity? previousEntity))
                    {
                        cx.Extractor.Message(new Message("Duplicate trap ID", id, null, severity: Util.Logging.Severity.Warning));
                    }
--- a/csharp/extractor/Semmle.Extraction.CIL/PDB/MetadataPdbReader.cs
+++ b/csharp/extractor/Semmle.Extraction.CIL/PDB/MetadataPdbReader.cs
@@ -25,7 +25,7 @@ namespace Semmle.Extraction.PDB

            public string Path { get; private set; }

-            public string Contents => File.Exists(Path) ? File.ReadAllText(Path, System.Text.Encoding.Default) : null;
+            public string? Contents => File.Exists(Path) ? File.ReadAllText(Path, System.Text.Encoding.Default) : null;
        }

        // Turns out to be very important to keep the MetadataReaderProvider live
@@ -41,7 +41,7 @@ namespace Semmle.Extraction.PDB

        public IEnumerable<ISourceFile> SourceFiles => reader.Documents.Select(handle => new SourceFile(reader, handle));

-        public IMethod GetMethod(MethodDebugInformationHandle handle)
+        public IMethod? GetMethod(MethodDebugInformationHandle handle)
        {
            var debugInfo = reader.GetMethodDebugInformation(handle);

@@ -51,10 +51,10 @@ namespace Semmle.Extraction.PDB
                Where(p => p.Location.File.Path != null).
                ToArray();

-            return sequencePoints.Any() ? new Method() { SequencePoints = sequencePoints } : null;
+            return sequencePoints.Any() ? new Method(sequencePoints) : null;
        }

-        public static MetadataPdbReader CreateFromAssembly(string assemblyPath, PEReader peReader)
+        public static MetadataPdbReader? CreateFromAssembly(string assemblyPath, PEReader peReader)
        {
            foreach (var provider in peReader.
                ReadDebugDirectory().
--- a/csharp/extractor/Semmle.Extraction.CIL/PDB/NativePdbReader.cs
+++ b/csharp/extractor/Semmle.Extraction.CIL/PDB/NativePdbReader.cs
@@ -23,7 +23,7 @@ namespace Semmle.Extraction.PDB
            public Document(ISymUnmanagedDocument doc)
            {
                document = doc;
-                contents = new Lazy<string>(() =>
+                contents = new Lazy<string?>(() =>
                {
                    bool isEmbedded;
                    if (document.HasEmbeddedSource(out isEmbedded) == 0 && isEmbedded)
@@ -38,7 +38,7 @@ namespace Semmle.Extraction.PDB
                });
            }

-            public override bool Equals(object obj)
+            public override bool Equals(object? obj)
            {
                var otherDoc = obj as Document;
                return otherDoc != null && Path.Equals(otherDoc.Path);
@@ -50,14 +50,14 @@ namespace Semmle.Extraction.PDB

            public override string ToString() => Path;

-            readonly Lazy<string> contents;
+            readonly Lazy<string?> contents;

-            public string Contents => contents.Value;
+            public string? Contents => contents.Value;
        }

        public IEnumerable<ISourceFile> SourceFiles => reader.GetDocuments().Select(d => new Document(d));

-        public IMethod GetMethod(MethodDebugInformationHandle h)
+        public IMethod? GetMethod(MethodDebugInformationHandle h)
        {
            int methodToken = MetadataTokens.GetToken(h.ToDefinitionHandle());
            var method = reader.GetMethod(methodToken);
@@ -72,7 +72,7 @@ namespace Semmle.Extraction.PDB
                    Select(sp => new SequencePoint(sp.Offset, new Location(new Document(sp.Document), sp.StartLine, sp.StartColumn, sp.EndLine, sp.EndColumn))).
                    ToArray();

-                return s.Any() ? new Method { SequencePoints = s } : null;
+                return s.Any() ? new Method(s) : null;
            }
            return null;
        }
@@ -87,7 +87,7 @@ namespace Semmle.Extraction.PDB
        readonly ISymUnmanagedReader5 reader;
        readonly FileStream pdbStream;

-        public static NativePdbReader CreateFromAssembly(string assemblyPath, PEReader peReader)
+        public static NativePdbReader? CreateFromAssembly(string assemblyPath, PEReader peReader)
        {
            // The Native PDB reader uses an unmanaged Windows DLL
            // so only works on Windows.
@@ -123,7 +123,7 @@ namespace Semmle.Extraction.PDB
        {
        }

-        public object GetMetadataImport() => null;
+        public object? GetMetadataImport() => null;

        public unsafe bool TryGetStandaloneSignature(int standaloneSignatureToken, out byte* signature, out int length) =>
            throw new NotImplementedException();
--- a/csharp/extractor/Semmle.Extraction.CIL/PDB/PdbReader.cs
+++ b/csharp/extractor/Semmle.Extraction.CIL/PDB/PdbReader.cs
@@ -55,7 +55,7 @@ namespace Semmle.Extraction.PDB
            return string.Format("({0},{1})-({2},{3})", StartLine, StartColumn, EndLine, EndColumn);
        }

-        public override bool Equals(object obj)
+        public override bool Equals(object? obj)
        {
            var otherLocation = obj as Location;

@@ -91,7 +91,12 @@ namespace Semmle.Extraction.PDB

    class Method : IMethod
    {
-        public IEnumerable<SequencePoint> SequencePoints { get; set; }
+        public IEnumerable<SequencePoint> SequencePoints { get; }
+
+        public Method(IEnumerable<SequencePoint> sequencePoints)
+        {
+            SequencePoints = sequencePoints;
+        }

        public Location Location => SequencePoints.First().Location;
    }
@@ -111,7 +116,7 @@ namespace Semmle.Extraction.PDB
        /// null if the contents are unavailable.
        /// E.g. if the PDB file exists but the corresponding source files are missing.
        /// </summary>
-        string Contents { get; }
+        string? Contents { get; }
    }

    /// <summary>
@@ -131,7 +136,7 @@ namespace Semmle.Extraction.PDB
        /// </summary>
        /// <param name="methodHandle">The handle to query.</param>
        /// <returns>The method information, or null if the method does not have debug information.</returns>
-        IMethod GetMethod(MethodDebugInformationHandle methodHandle);
+        IMethod? GetMethod(MethodDebugInformationHandle methodHandle);
    }

    class PdbReader
@@ -140,11 +145,11 @@ namespace Semmle.Extraction.PDB
        /// Returns the PDB information associated with an assembly.
        /// </summary>
        /// <param name="assemblyPath">The path to the assembly.</param>
-        /// <param name="peReader">The PE reader for the assembky.</param>
+        /// <param name="peReader">The PE reader for the assembly.</param>
        /// <returns>A PdbReader, or null if no PDB information is available.</returns>
-        public static IPdb Create(string assemblyPath, PEReader peReader)
+        public static IPdb? Create(string assemblyPath, PEReader peReader)
        {
-            return (IPdb)MetadataPdbReader.CreateFromAssembly(assemblyPath, peReader) ??
+            return (IPdb?)MetadataPdbReader.CreateFromAssembly(assemblyPath, peReader) ??
                NativePdbReader.CreateFromAssembly(assemblyPath, peReader);
        }
    }
--- a/csharp/extractor/Semmle.Extraction.CIL/Semmle.Extraction.CIL.csproj
+++ b/csharp/extractor/Semmle.Extraction.CIL/Semmle.Extraction.CIL.csproj
@@ -7,6 +7,7 @@
    <GenerateAssemblyInfo>false</GenerateAssemblyInfo>
    <AllowUnsafeBlocks>true</AllowUnsafeBlocks>
    <RuntimeIdentifiers>win-x64;linux-x64;osx-x64</RuntimeIdentifiers>
+    <Nullable>enable</Nullable>
  </PropertyGroup>

  <PropertyGroup Condition="'$(Configuration)|$(Platform)'=='Debug|AnyCPU'">
--- a/csharp/extractor/Semmle.Util/IEnumerableExtensions.cs
+++ b/csharp/extractor/Semmle.Util/IEnumerableExtensions.cs
@@ -64,7 +64,7 @@ namespace Semmle.Util
        /// Enumerates a possibly null enumerable.
        /// If the enumerable is null, the list is empty.
        /// </summary>
-        public static IEnumerable<T> EnumerateNull<T>(this IEnumerable<T> items)
+        public static IEnumerable<T> EnumerateNull<T>(this IEnumerable<T>? items)
        {
            if (items == null) yield break;
            foreach (var item in items) yield return item;
@@ -93,7 +93,7 @@ namespace Semmle.Util
        /// <typeparam name="T">The type of the item.</typeparam>
        /// <param name="items">The list of items to hash.</param>
        /// <returns>The hash code.</returns>
-        public static int SequenceHash<T>(this IEnumerable<T> items) where T: notnull
+        public static int SequenceHash<T>(this IEnumerable<T> items) where T : notnull
        {
            int h = 0;
            foreach (var i in items)