fix(http-client-csharp): emit else-if for required nullable null fallback

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

@@ -4,4 +4,4 @@ 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.
+Avoid emitting duplicate output for required nullable properties during patch serialization.

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

@@ -2560,15 +2560,20 @@ private MethodBodyStatement CreateConditionalSerializationStatement(
 
                 // 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);
+                // check to avoid writing the property twice. Emit it as an "else if" so the fallback
+                // is skipped entirely when the path is patched.
+                if (patchCheck != null)
+                {
+                    return new IfElseStatement(
+                        new IfStatement(condition) { writePropertySerializationStatement },
+                        [new IfStatement(patchCheck) { _utf8JsonWriterSnippet.WriteNull(serializedName) }],
+                        null);
+                }
 
                 return new IfElseStatement(
                     condition,
                     writePropertySerializationStatement,
-                    writeNullStatement);
+                    _utf8JsonWriterSnippet.WriteNull(serializedName));
             }
 
             if (shouldCheckJsonPath)

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

@@ -41,12 +41,9 @@ protected virtual void JsonModelWriteCore(global::System.Text.Json.Utf8JsonWrite
                 writer.WritePropertyName("foo"u8);
                 writer.WriteStringValue(Foo);
             }
-            else
+            else if (!Patch.Contains("$.foo"u8))
             {
-                if (!Patch.Contains("$.foo"u8))
-                {
-                    writer.WriteNull("foo"u8);
-                }
+                writer.WriteNull("foo"u8);
             }
 
             Patch.WriteTo(writer);

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

@@ -221,12 +221,9 @@ protected virtual void JsonModelWriteCore(Utf8JsonWriter writer, ModelReaderWrit
                 Patch.WriteTo(writer, "$.requiredNullableDictionary"u8);
                 writer.WriteEndObject();
             }
-            else
+            else if (!Patch.Contains("$.requiredNullableDictionary"u8))
             {
-                if (!Patch.Contains("$.requiredNullableDictionary"u8))
-                {
-                    writer.WriteNull("requiredNullableDictionary"u8);
-                }
+                writer.WriteNull("requiredNullableDictionary"u8);
             }
             if (!Patch.Contains("$.primitiveDictionary"u8))
             {