Fix optional request body to wrap Content-Type header in null check

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

@@ -264,7 +264,7 @@ private MethodBodyStatements BuildMessage(
             {
                 var contentParam = signature.Parameters.FirstOrDefault(p => p.Name == "content" && p.Location == ParameterLocation.Body);
                 statements.AddRange(AppendHeaderParameters(request, operation, paramMap, contentParam: contentParam));
-                statements.AddRange(GetSetContent(request, signature.Parameters));
+                statements.AddRange(GetSetContent(request, signature.Parameters, operation));
             }
 
             // Apply request options and return message
@@ -327,11 +327,33 @@ private Dictionary<string, ParameterProvider> GetReinjectedParametersMap(
             return reinjectedParamsMap;
         }
 
-        private IReadOnlyList<MethodBodyStatement> GetSetContent(HttpRequestApi request, IReadOnlyList<ParameterProvider> parameters)
+        private IReadOnlyList<MethodBodyStatement> GetSetContent(HttpRequestApi request, IReadOnlyList<ParameterProvider> parameters, InputOperation operation)
         {
             var contentParam = parameters.FirstOrDefault(
                 p => p.Location == ParameterLocation.Body);
-            return contentParam is null ? [] : [request.Content().Assign(contentParam).Terminate()];
+            if (contentParam is null)
+            {
+                return [];
+            }
+
+            // If body is optional, check if content assignment is already handled
+            var hasOptionalBody = operation.Parameters.Any(p =>
+                p is InputBodyParameter bodyParam && !bodyParam.IsRequired);
+            if (hasOptionalBody)
+            {
+                // If there's a Content-Type header, the content assignment is already inside
+                // the if (content != null) block in AppendHeaderParameters
+                var hasContentTypeHeader = operation.Parameters.Any(p =>
+                    p is InputHeaderParameter h && h.IsContentType);
+                if (hasContentTypeHeader)
+                {
+                    return [];
+                }
+                // No Content-Type header but optional body - wrap content assignment in null check
+                return [new IfStatement(contentParam.NotEqual(Null)) { request.Content().Assign(contentParam).Terminate() }];
+            }
+
+            return [request.Content().Assign(contentParam).Terminate()];
         }
 
         private Dictionary<List<int>, PropertyProvider> BuildPipelineMessage20xClassifiers()
@@ -445,7 +467,11 @@ private IEnumerable<MethodBodyStatement> AppendHeaderParameters(HttpRequestApi r
 
                     if (hasOptionalBody)
                     {
-                        statement = new IfStatement(contentParam.NotEqual(Null)) { statement };
+                        statement = new IfStatement(contentParam.NotEqual(Null))
+                        {
+                            statement,
+                            request.Content().Assign(contentParam).Terminate()
+                        };
                     }
                 }
 

packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator.ClientModel/test/Providers/RestClientProviders/RestClientProviderTests.cs

@@ -1690,6 +1690,7 @@ public void ContentTypeHeaderWrappedInNullCheckWhenContentIsOptional()
             var expectedStatement = @"if ((content != null))
 {
     request.Headers.Set(""Content-Type"", ""application/json"");
+    request.Content = content;
 }
 ";
             var statementsString = string.Join("\n", statements!.Select(s => s.ToDisplayString()));
@@ -1744,6 +1745,45 @@ public void ContentTypeHeaderNotWrappedInNullCheckWhenContentIsRequired()
                 $"Content-Type should NOT be wrapped in an if statement for required content, but found:\n{statementsString}");
         }
 
+        [Test]
+        public void ContentAssignmentWrappedInNullCheckWhenBodyIsOptionalWithoutContentType()
+        {
+            // Test that when there's an optional body parameter without a Content-Type header,
+            // the content assignment is still wrapped in a null check
+            var bodyParam = InputFactory.BodyParameter(
+                "body",
+                InputPrimitiveType.String,
+                isRequired: false);
+            var operation = InputFactory.Operation(
+                "TestOperation",
+                requestMediaTypes: ["application/json"],
+                parameters: [bodyParam]);
+            var inputServiceMethod = InputFactory.BasicServiceMethod("Test", operation);
+            var inputClient = InputFactory.Client("TestClient", methods: [inputServiceMethod]);
+            MockHelpers.LoadMockGenerator(clients: () => [inputClient]);
+
+            var client = ScmCodeModelGenerator.Instance.TypeFactory.CreateClient(inputClient);
+            Assert.IsNotNull(client);
+
+            var restClient = client!.RestClient;
+            Assert.IsNotNull(restClient);
+
+            var createMethod = restClient.Methods.FirstOrDefault(m => m.Signature.Name == "CreateTestOperationRequest");
+            Assert.IsNotNull(createMethod, "CreateTestOperationRequest method not found");
+
+            var statements = createMethod!.BodyStatements as MethodBodyStatements;
+            Assert.IsNotNull(statements);
+
+            var expectedStatement = @"if ((content != null))
+{
+    request.Content = content;
+}
+";
+            var statementsString = string.Join("\n", statements!.Select(s => s.ToDisplayString()));
+            Assert.IsTrue(statements!.Any(s => s.ToDisplayString() == expectedStatement),
+                $"Expected to find statement:\n{expectedStatement}\nBut got statements:\n{statementsString}");
+        }
+
         [Test]
         public async Task PageSizeParameterCasingPreservedFromLastContractView()
         {