From edb2ed8df278890c96267cf49a1498f29a73a1a8 Mon Sep 17 00:00:00 2001
From: Michael Nebel <michaelnebel@github.com>
Date: Wed, 11 Feb 2026 14:10:14 +0100
Subject: [PATCH] C#: Extract the implicit property backing field when
 referenced via the field keyword.

---
 .../Entities/Expressions/Factory.cs           |  3 ++
 .../Expressions/PropertyFieldAccess.cs        | 28 +++++++++++
 .../Entities/Field.cs                         |  2 +-
 .../Entities/PropertyField.cs                 | 50 +++++++++++++++++++
 4 files changed, 82 insertions(+), 1 deletion(-)
 create mode 100644 csharp/extractor/Semmle.Extraction.CSharp/Entities/Expressions/PropertyFieldAccess.cs
 create mode 100644 csharp/extractor/Semmle.Extraction.CSharp/Entities/PropertyField.cs

diff --git a/csharp/extractor/Semmle.Extraction.CSharp/Entities/Expressions/Factory.cs b/csharp/extractor/Semmle.Extraction.CSharp/Entities/Expressions/Factory.cs
index 0da8de1e5d5..ed8dae3738f 100644
--- a/csharp/extractor/Semmle.Extraction.CSharp/Entities/Expressions/Factory.cs
+++ b/csharp/extractor/Semmle.Extraction.CSharp/Entities/Expressions/Factory.cs
@@ -160,6 +160,9 @@ namespace Semmle.Extraction.CSharp.Entities.Expressions
                     case SyntaxKind.ThisExpression:
                         return This.CreateExplicit(info);
 
+                    case SyntaxKind.FieldExpression:
+                        return PropertyFieldAccess.Create(info);
+
                     case SyntaxKind.AddressOfExpression:
                         return Unary.Create(info.SetKind(ExprKind.ADDRESS_OF));
 
diff --git a/csharp/extractor/Semmle.Extraction.CSharp/Entities/Expressions/PropertyFieldAccess.cs b/csharp/extractor/Semmle.Extraction.CSharp/Entities/Expressions/PropertyFieldAccess.cs
new file mode 100644
index 00000000000..a9d2afa84c9
--- /dev/null
+++ b/csharp/extractor/Semmle.Extraction.CSharp/Entities/Expressions/PropertyFieldAccess.cs
@@ -0,0 +1,28 @@
+using System.IO;
+using Microsoft.CodeAnalysis;
+using Microsoft.CodeAnalysis.CSharp.Syntax;
+using Semmle.Extraction.Kinds;
+
+namespace Semmle.Extraction.CSharp.Entities.Expressions
+{
+    internal class PropertyFieldAccess : Expression<FieldExpressionSyntax>
+    {
+        private PropertyFieldAccess(ExpressionNodeInfo info) : base(info.SetKind(ExprKind.FIELD_ACCESS)) { }
+
+        public static Expression Create(ExpressionNodeInfo info) => new PropertyFieldAccess(info).TryPopulate();
+
+        protected override void PopulateExpression(TextWriter trapFile)
+        {
+            var symbolInfo = Context.GetSymbolInfo(Syntax);
+            if (symbolInfo.Symbol is IFieldSymbol field)
+            {
+                var target = PropertyField.Create(Context, field);
+                trapFile.expr_access(this, target);
+                if (!field.IsStatic)
+                {
+                    This.CreateImplicit(Context, field.ContainingType, Location, this, -1);
+                }
+            }
+        }
+    }
+}
diff --git a/csharp/extractor/Semmle.Extraction.CSharp/Entities/Field.cs b/csharp/extractor/Semmle.Extraction.CSharp/Entities/Field.cs
index 9a010aad376..329115f11c7 100644
--- a/csharp/extractor/Semmle.Extraction.CSharp/Entities/Field.cs
+++ b/csharp/extractor/Semmle.Extraction.CSharp/Entities/Field.cs
@@ -10,7 +10,7 @@ namespace Semmle.Extraction.CSharp.Entities
 {
     internal class Field : CachedSymbol<IFieldSymbol>, IExpressionParentEntity
     {
-        private Field(Context cx, IFieldSymbol init)
+        protected Field(Context cx, IFieldSymbol init)
             : base(cx, init)
         {
             type = new Lazy<Type>(() => Entities.Type.Create(cx, Symbol.Type));
diff --git a/csharp/extractor/Semmle.Extraction.CSharp/Entities/PropertyField.cs b/csharp/extractor/Semmle.Extraction.CSharp/Entities/PropertyField.cs
new file mode 100644
index 00000000000..7567a9e8960
--- /dev/null
+++ b/csharp/extractor/Semmle.Extraction.CSharp/Entities/PropertyField.cs
@@ -0,0 +1,50 @@
+using System.IO;
+using Microsoft.CodeAnalysis;
+using Semmle.Extraction.CSharp.Util;
+using Semmle.Extraction.Kinds;
+
+namespace Semmle.Extraction.CSharp.Entities
+{
+    /// <summary>
+    /// Represents the autogenerated backing field `field` for a property.
+    /// It is only created for properties that use the `field` keyword in their getter or setter, and
+    /// is not created for auto-properties.
+    /// </summary>
+    internal class PropertyField : Field
+    {
+        protected PropertyField(Context cx, IFieldSymbol init)
+            : base(cx, init)
+        {
+        }
+
+        public static new PropertyField Create(Context cx, IFieldSymbol field) => PropertyFieldFactory.Instance.CreateEntity(cx, (field, field.AssociatedSymbol), field);
+
+        public override bool NeedsPopulation => true;
+
+        public override void Populate(TextWriter trapFile)
+        {
+            PopulateNullability(trapFile, Symbol.GetAnnotatedType());
+
+            var unboundFieldKey = PropertyField.Create(Context, Symbol.OriginalDefinition);
+            var name = Symbol.AssociatedSymbol is not null ? $"{Symbol.AssociatedSymbol.GetName()}.field" : Symbol.Name;
+            trapFile.fields(this, VariableKind.None, name, ContainingType!, Type.TypeRef, unboundFieldKey);
+
+            if (Context.OnlyScaffold)
+            {
+                return;
+            }
+
+            if (Context.ExtractLocation(Symbol))
+            {
+                WriteLocationsToTrap(trapFile.field_location, this, Locations);
+            }
+        }
+
+        private class PropertyFieldFactory : CachedEntityFactory<IFieldSymbol, PropertyField>
+        {
+            public static PropertyFieldFactory Instance { get; } = new PropertyFieldFactory();
+
+            public override PropertyField Create(Context cx, IFieldSymbol init) => new PropertyField(cx, init);
+        }
+    }
+}