Fix CodeWriter field dedup to update references via MemberExpression

cspell.yaml

@@ -57,6 +57,7 @@ words:
   - debugpy
   - Declipse
   - dedented
+  - Dedup
   - Dedupes
   - deps
   - destructures

packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator/src/Expressions/MemberExpression.cs

@@ -1,6 +1,7 @@
 // Copyright (c) Microsoft Corporation. All rights reserved.
 // Licensed under the MIT License.
 
+using Microsoft.TypeSpec.Generator.Primitives;
 using Microsoft.TypeSpec.Generator.Providers;
 using Microsoft.TypeSpec.Generator.Statements;
 
@@ -10,16 +11,25 @@ public sealed record MemberExpression(ValueExpression? Inner, string MemberName)
     {
         public ValueExpression? Inner { get; internal set; } = Inner;
         public string MemberName { get; private set; } = MemberName;
+        internal CodeWriterDeclaration? Declaration { get; set; }
         internal override void Write(CodeWriter writer)
         {
             if (Inner is not null)
             {
                 Inner.Write(writer);
                 writer.AppendRaw(".");
             }
-            // workaround to avoid Roslyn reducing properties named Object to object
-            // Should come up with a better approach - https://github.com/microsoft/typespec/issues/4724
-            writer.AppendRaw(MemberName == "Object" && Inner == null ? $"this.{MemberName}" : MemberName);
+
+            if (Declaration is not null)
+            {
+                writer.Append(Declaration, referenceOnly: true);
+            }
+            else
+            {
+                // workaround to avoid Roslyn reducing properties named Object to object
+                // Should come up with a better approach - https://github.com/microsoft/typespec/issues/4724
+                writer.AppendRaw(MemberName == "Object" && Inner == null ? $"this.{MemberName}" : MemberName);
+            }
         }
 
         internal override ValueExpression? Accept(LibraryVisitor visitor, MethodProvider method)

packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator/src/Providers/FieldProvider.cs

@@ -101,6 +101,10 @@ public void Update(
                 _variable?.Update(name: name);
                 _asMember?.Update(memberName: name);
                 _declaration = null;
+                if (_asMember != null)
+                {
+                    _asMember.Declaration = Declaration;
+                }
                 InitializeParameter();
             }
 
@@ -148,6 +152,6 @@ private void InitializeParameter()
         }
 
         private MemberExpression? _asMember;
-        public static implicit operator MemberExpression(FieldProvider field) => field._asMember ??= new MemberExpression(null, field.Name);
+        public static implicit operator MemberExpression(FieldProvider field) => field._asMember ??= new MemberExpression(null, field.Name) { Declaration = field.Declaration };
     }
 }

packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator/src/Writers/CodeWriter.cs

@@ -1004,12 +1004,16 @@ public CodeScope AmbientScope()
             return codeWriterScope;
         }
 
-        internal void Append(CodeWriterDeclaration declaration)
+        internal void Append(CodeWriterDeclaration declaration, bool referenceOnly = false)
         {
             if (declaration.HasBeenDeclared(_scopes))
             {
                 WriteIdentifier(declaration.GetActualName(_scopes.Peek()));
             }
+            else if (referenceOnly)
+            {
+                WriteIdentifier(declaration.RequestedName);
+            }
             else
             {
                 WriteDeclaration(declaration);

packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator/test/Writers/CodeWriterDeclarationTests.cs

@@ -168,6 +168,35 @@ public void ScopeDeclaredTwiceForMethodSignatureParam()
             Assert.AreEqual(Helpers.GetExpectedFromFile(), codeWriter.ToString(false));
         }
 
+        [Test]
+        public void FieldDedupUpdatesReferencesViaMemberExpression()
+        {
+            var type = new TestTypeProvider();
+            var field1 = new FieldProvider(FieldModifiers.Private | FieldModifiers.ReadOnly, typeof(string), "_scope", type);
+            var field2 = new FieldProvider(FieldModifiers.Private | FieldModifiers.ReadOnly, typeof(string), "_scope", type);
+
+            using var codeWriter = new CodeWriter();
+            // Write both field declarations - the second should be deduped to _scope0
+            codeWriter.WriteField(field1);
+            codeWriter.WriteField(field2);
+            // Write a reference to the second field via AsValueExpression (implicit MemberExpression)
+            field2.AsValueExpression.Write(codeWriter);
+            Assert.AreEqual(Helpers.GetExpectedFromFile(), codeWriter.ToString(false));
+        }
+
+        [Test]
+        public void FieldReferenceBeforeDeclarationUsesRequestedName()
+        {
+            var type = new TestTypeProvider();
+            var field = new FieldProvider(FieldModifiers.Private | FieldModifiers.ReadOnly, typeof(string), "_token", type);
+
+            using var codeWriter = new CodeWriter();
+            // Write a reference BEFORE the field declaration — should use the original name, not create a declaration
+            field.AsValueExpression.Write(codeWriter);
+            codeWriter.WriteField(field);
+            Assert.AreEqual(Helpers.GetExpectedFromFile(), codeWriter.ToString(false));
+        }
+
         private Dictionary<CodeWriter.CodeScope, string> GetDeclarationScopes(CodeWriterDeclaration declaration)
         {
             var namesDictionaryField = typeof(CodeWriterDeclaration).GetField("_actualNames", BindingFlags.NonPublic | BindingFlags.Instance);

packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator/test/Writers/TestData/CodeWriterDeclarationTests/FieldDedupUpdatesReferencesViaMemberExpression.cs

@@ -0,0 +1,3 @@
+private readonly string _scope;
+private readonly string _scope0;
+_scope0

packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator/test/Writers/TestData/CodeWriterDeclarationTests/FieldReferenceBeforeDeclarationUsesRequestedName.cs

@@ -0,0 +1 @@
+_tokenprivate readonly string _token;