fix(http-client-csharp): gate required nullable null fallback on patch check

Co-authored-by: jorgerangel-msft <102122018+jorgerangel-msft@users.noreply.github.com>

Author: Copilot

.chronus/changes/fix-required-nullable-patch-duplicate-2026-6-29-14-9-14.md

@@ -0,0 +1,7 @@
+---
+changeKind: fix
+packages:
+  - "@typespec/http-client-csharp"
+---
+
+Avoid emitting duplicate output for required nullable properties during patch serialization. The null fallback for a required nullable property is now gated on `!Patch.Contains(...)` so patched values remain the single source of output for that property.

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

@@ -2558,10 +2558,17 @@ private MethodBodyStatement CreateConditionalSerializationStatement(
                         _utf8JsonWriterSnippet.WriteNull(serializedName));
                 }
 
+                // When the property path is already present in the patch, the patched value is
+                // emitted by Patch.WriteTo(writer), so the null fallback must be gated on the patch
+                // check to avoid writing the property twice.
+                MethodBodyStatement writeNullStatement = patchCheck != null
+                    ? new IfStatement(patchCheck) { _utf8JsonWriterSnippet.WriteNull(serializedName) }
+                    : _utf8JsonWriterSnippet.WriteNull(serializedName);
+
                 return new IfElseStatement(
                     condition,
                     writePropertySerializationStatement,
-                    _utf8JsonWriterSnippet.WriteNull(serializedName));
+                    writeNullStatement);
             }
 
             if (shouldCheckJsonPath)

packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator.ClientModel/test/Providers/MrwSerializationTypeDefinitions/DynamicModelSerializationTests.cs

@@ -531,6 +531,33 @@ public void WriteMultiplePrimitiveProperties()
             Assert.AreEqual(Helpers.GetExpectedFromFile(), file.Content);
         }
 
+        [Test]
+        public void WriteRequiredNullableProperty()
+        {
+            var inputModel = InputFactory.Model(
+               "dynamicModel",
+               isDynamicModel: true,
+               properties:
+               [
+                    InputFactory.Property("foo", new InputNullableType(InputPrimitiveType.String), isRequired: true)
+               ]);
+
+            MockHelpers.LoadMockGenerator(inputModels: () => [inputModel]);
+            var model = ScmCodeModelGenerator.Instance.TypeFactory.CreateModel(inputModel) as ClientModel.Providers.ScmModelProvider;
+
+            Assert.IsNotNull(model);
+            Assert.IsTrue(model!.IsDynamicModel);
+            var serialization = model.SerializationProviders.SingleOrDefault();
+            Assert.IsNotNull(serialization);
+
+            var writer = new TypeProviderWriter(new FilteredMethodsTypeProvider(
+                serialization!,
+                name => name is "JsonModelWriteCore" or "Write"));
+
+            var file = writer.Write();
+            Assert.AreEqual(Helpers.GetExpectedFromFile(), file.Content);
+        }
+
         [Test]
         public void WriteModelPropertyType()
         {

packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator.ClientModel/test/Providers/MrwSerializationTypeDefinitions/TestData/DynamicModelSerializationTests/WriteRequiredNullableProperty.cs

@@ -0,0 +1,56 @@
+// <auto-generated/>
+
+#nullable disable
+
+using System;
+using System.ClientModel.Primitives;
+using System.Text.Json;
+using Sample.Models;
+
+namespace Sample
+{
+    public partial class DynamicModel
+    {
+        global::System.BinaryData global::System.ClientModel.Primitives.IPersistableModel<global::Sample.Models.DynamicModel>.Write(global::System.ClientModel.Primitives.ModelReaderWriterOptions options) => this.PersistableModelWriteCore(options);
+
+        void global::System.ClientModel.Primitives.IJsonModel<global::Sample.Models.DynamicModel>.Write(global::System.Text.Json.Utf8JsonWriter writer, global::System.ClientModel.Primitives.ModelReaderWriterOptions options)
+        {
+#pragma warning disable SCME0001 // Type is for evaluation purposes only and is subject to change or removal in future updates.
+            if (Patch.Contains("$"u8))
+            {
+                writer.WriteRawValue(Patch.GetJson("$"u8));
+                return;
+            }
+#pragma warning restore SCME0001 // Type is for evaluation purposes only and is subject to change or removal in future updates.
+
+            writer.WriteStartObject();
+            this.JsonModelWriteCore(writer, options);
+            writer.WriteEndObject();
+        }
+
+        protected virtual void JsonModelWriteCore(global::System.Text.Json.Utf8JsonWriter writer, global::System.ClientModel.Primitives.ModelReaderWriterOptions options)
+        {
+            string format = (options.Format == "W") ? ((global::System.ClientModel.Primitives.IPersistableModel<global::Sample.Models.DynamicModel>)this).GetFormatFromOptions(options) : options.Format;
+            if ((format != "J"))
+            {
+                throw new global::System.FormatException($"The model {nameof(global::Sample.Models.DynamicModel)} does not support writing '{format}' format.");
+            }
+#pragma warning disable SCME0001 // Type is for evaluation purposes only and is subject to change or removal in future updates.
+            if ((global::Sample.Optional.IsDefined(Foo) && !Patch.Contains("$.foo"u8)))
+            {
+                writer.WritePropertyName("foo"u8);
+                writer.WriteStringValue(Foo);
+            }
+            else
+            {
+                if (!Patch.Contains("$.foo"u8))
+                {
+                    writer.WriteNull("foo"u8);
+                }
+            }
+
+            Patch.WriteTo(writer);
+#pragma warning restore SCME0001 // Type is for evaluation purposes only and is subject to change or removal in future updates.
+        }
+    }
+}

packages/http-client-csharp/generator/TestProjects/Local/Sample-TypeSpec/src/Generated/Models/DynamicModel.Serialization.cs

@@ -223,7 +223,10 @@ protected virtual void JsonModelWriteCore(Utf8JsonWriter writer, ModelReaderWrit
             }
             else
             {
-                writer.WriteNull("requiredNullableDictionary"u8);
+                if (!Patch.Contains("$.requiredNullableDictionary"u8))
+                {
+                    writer.WriteNull("requiredNullableDictionary"u8);
+                }
             }
             if (!Patch.Contains("$.primitiveDictionary"u8))
             {