diff --git a/csharp/extractor/Semmle.Extraction.CSharp/Entities/IParameter.cs b/csharp/extractor/Semmle.Extraction.CSharp/Entities/IParameter.cs
new file mode 100644
index 00000000000..d3f3a04c08f
--- /dev/null
+++ b/csharp/extractor/Semmle.Extraction.CSharp/Entities/IParameter.cs
@@ -0,0 +1,9 @@
+namespace Semmle.Extraction.CSharp.Entities
+{
+    /// <summary>
+    /// Marker interface for parameter entities.
+    /// </summary>
+    internal interface IParameter : IEntity
+    {
+    }
+}
diff --git a/csharp/extractor/Semmle.Extraction.CSharp/Entities/Parameter.cs b/csharp/extractor/Semmle.Extraction.CSharp/Entities/Parameter.cs
index 49ef9a4a6e9..9cca0683f00 100644
--- a/csharp/extractor/Semmle.Extraction.CSharp/Entities/Parameter.cs
+++ b/csharp/extractor/Semmle.Extraction.CSharp/Entities/Parameter.cs
@@ -7,16 +7,23 @@ using Semmle.Extraction.CSharp.Populators;
 
 namespace Semmle.Extraction.CSharp.Entities
 {
-    internal class Parameter : CachedSymbol<IParameterSymbol>, IExpressionParentEntity
+    internal class Parameter : CachedSymbol<IParameterSymbol>, IExpressionParentEntity, IParameter
     {
         protected IEntity? Parent { get; set; }
         protected Parameter Original { get; }
+        private int PositionOffset { get; set; }
 
-        protected Parameter(Context cx, IParameterSymbol init, IEntity? parent, Parameter? original)
+        private Parameter(Context cx, IParameterSymbol init, IEntity? parent, Parameter? original, int positionOffset)
             : base(cx, init)
         {
             Parent = parent;
             Original = original ?? this;
+            PositionOffset = positionOffset;
+        }
+
+        protected Parameter(Context cx, IParameterSymbol init, IEntity? parent, Parameter? original)
+            : this(cx, init, parent, original, 0)
+        {
         }
 
         public override Microsoft.CodeAnalysis.Location ReportingLocation => Symbol.GetSymbolLocation();
@@ -32,7 +39,7 @@ namespace Semmle.Extraction.CSharp.Entities
             RefReadOnly = 6
         }
 
-        protected virtual int Ordinal => Symbol.Ordinal;
+        protected virtual int Ordinal => Symbol.Ordinal + PositionOffset;
 
         private Kind ParamKind
         {
@@ -55,23 +62,25 @@ namespace Semmle.Extraction.CSharp.Entities
                         if (Ordinal == 0)
                         {
                             if (Symbol.ContainingSymbol is IMethodSymbol method && method.IsExtensionMethod)
+                            {
                                 return Kind.This;
+                            }
                         }
                         return Kind.None;
                 }
             }
         }
 
-        public static Parameter Create(Context cx, IParameterSymbol param, IEntity parent, Parameter? original = null)
+        public static Parameter Create(Context cx, IParameterSymbol param, IEntity parent, Parameter? original = null, int positionOffset = 0)
         {
             var cachedSymbol = cx.GetPossiblyCachedParameterSymbol(param);
-            return ParameterFactory.Instance.CreateEntity(cx, cachedSymbol, (cachedSymbol, parent, original));
+            return ParameterFactory.Instance.CreateEntity(cx, cachedSymbol, (cachedSymbol, parent, original, positionOffset));
         }
 
         public static Parameter Create(Context cx, IParameterSymbol param)
         {
             var cachedSymbol = cx.GetPossiblyCachedParameterSymbol(param);
-            return ParameterFactory.Instance.CreateEntity(cx, cachedSymbol, (cachedSymbol, null, null));
+            return ParameterFactory.Instance.CreateEntity(cx, cachedSymbol, (cachedSymbol, null, null, 0));
         }
 
         public override void WriteId(EscapingTextWriter trapFile)
@@ -79,6 +88,9 @@ namespace Semmle.Extraction.CSharp.Entities
             if (Parent is null)
                 Parent = Method.Create(Context, Symbol.ContainingSymbol as IMethodSymbol);
 
+            if (Parent is null && Symbol.ContainingSymbol is INamedTypeSymbol type && type.IsExtension)
+                Parent = Type.Create(Context, type);
+
             if (Parent is null)
                 throw new InternalError(Symbol, "Couldn't get parent of symbol.");
 
@@ -194,11 +206,11 @@ namespace Semmle.Extraction.CSharp.Entities
             return syntax?.Default;
         }
 
-        private class ParameterFactory : CachedEntityFactory<(IParameterSymbol, IEntity?, Parameter?), Parameter>
+        private class ParameterFactory : CachedEntityFactory<(IParameterSymbol, IEntity?, Parameter?, int), Parameter>
         {
             public static ParameterFactory Instance { get; } = new ParameterFactory();
 
-            public override Parameter Create(Context cx, (IParameterSymbol, IEntity?, Parameter?) init) => new Parameter(cx, init.Item1, init.Item2, init.Item3);
+            public override Parameter Create(Context cx, (IParameterSymbol, IEntity?, Parameter?, int) init) => new Parameter(cx, init.Item1, init.Item2, init.Item3, init.Item4);
         }
 
         public override TrapStackBehaviour TrapStackBehaviour => TrapStackBehaviour.OptionalLabel;
diff --git a/csharp/extractor/Semmle.Extraction.CSharp/Trap/Tuples.cs b/csharp/extractor/Semmle.Extraction.CSharp/Trap/Tuples.cs
index b789eaa2e9c..c1d082bbee2 100644
--- a/csharp/extractor/Semmle.Extraction.CSharp/Trap/Tuples.cs
+++ b/csharp/extractor/Semmle.Extraction.CSharp/Trap/Tuples.cs
@@ -292,10 +292,10 @@ namespace Semmle.Extraction.CSharp
         internal static void overrides(this TextWriter trapFile, Method overriding, Method overridden) =>
             trapFile.WriteTuple("overrides", overriding, overridden);
 
-        internal static void param_location(this TextWriter trapFile, Parameter param, Location location) =>
+        internal static void param_location(this TextWriter trapFile, IParameter param, Location location) =>
             trapFile.WriteTuple("param_location", param, location);
 
-        internal static void @params(this TextWriter trapFile, Parameter param, string name, Type type, int child, Parameter.Kind mode, IEntity method, Parameter originalDefinition) =>
+        internal static void @params(this TextWriter trapFile, IParameter param, string name, Type type, int child, Parameter.Kind mode, IEntity method, IParameter originalDefinition) =>
             trapFile.WriteTuple("params", param, name, type, child, (int)mode, method, originalDefinition);
 
         internal static void parent_namespace(this TextWriter trapFile, IEntity type, Namespace parent) =>