Fix Content-Type header set when request content is optional

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

@@ -259,7 +259,8 @@ private MethodBodyStatements BuildMessage(
             }
             else
             {
-                statements.AddRange(AppendHeaderParameters(request, operation, paramMap));
+                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));
             }
 
@@ -377,7 +378,7 @@ private PropertyProvider GetClassifier(InputOperation operation)
             throw new InvalidOperationException($"Unexpected status codes for operation {operation.Name}");
         }
 
-        private IEnumerable<MethodBodyStatement> AppendHeaderParameters(HttpRequestApi request, InputOperation operation, Dictionary<string, ParameterProvider> paramMap, bool isNextLink = false)
+        private IEnumerable<MethodBodyStatement> AppendHeaderParameters(HttpRequestApi request, InputOperation operation, Dictionary<string, ParameterProvider> paramMap, bool isNextLink = false, ParameterProvider? contentParam = null)
         {
             List<MethodBodyStatement> statements = new(operation.Parameters.Count);
 
@@ -425,6 +426,18 @@ private IEnumerable<MethodBodyStatement> AppendHeaderParameters(HttpRequestApi r
                 {
                     statement = BuildQueryOrHeaderOrPathParameterNullCheck(type, valueExpression, statement);
                 }
+                // If this is a Content-Type header and there's an optional content parameter, wrap in content null check
+                else if (inputHeaderParameter.IsContentType && contentParam != null)
+                {
+                    // Check if any body parameter in the operation is optional
+                    var hasOptionalBody = operation.Parameters.Any(p =>
+                        p is InputBodyParameter bodyParam && !bodyParam.IsRequired);
+
+                    if (hasOptionalBody)
+                    {
+                        statement = new IfStatement(contentParam.NotEqual(Null)) { statement };
+                    }
+                }
 
                 statements.Add(statement);
             }

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

@@ -1492,5 +1492,97 @@ public void TestApiVersionParameterReinjectedInCreateNextRequestMethod()
             Assert.That(file.Content, Contains.Substring("api-version"));
             Assert.That(file.Content, Contains.Substring("maxpagesize"));
         }
+
+        [Test]
+        public void ContentTypeHeaderWrappedInNullCheckWhenContentIsOptional()
+        {
+            // Test that when there's an optional body parameter with a Content-Type header,
+            // the Content-Type header setting is wrapped in a null check for the content parameter
+            var contentTypeParam = InputFactory.HeaderParameter(
+                "Content-Type",
+                InputFactory.Literal.String("application/json"),
+                isRequired: true,
+                isContentType: true,
+                scope: InputParameterScope.Constant);
+            var bodyParam = InputFactory.BodyParameter(
+                "body",
+                InputPrimitiveType.String,
+                isRequired: false);
+            var operation = InputFactory.Operation(
+                "TestOperation",
+                requestMediaTypes: ["application/json"],
+                parameters: [contentTypeParam, 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.Headers.Set(""Content-Type"", ""application/json"");
+}
+";
+            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 void ContentTypeHeaderNotWrappedInNullCheckWhenContentIsRequired()
+        {
+            // Test that when there's a required body parameter with a Content-Type header,
+            // the Content-Type header setting is NOT wrapped in a null check
+            var contentTypeParam = InputFactory.HeaderParameter(
+                "Content-Type",
+                InputFactory.Literal.String("application/json"),
+                isRequired: true,
+                isContentType: true,
+                scope: InputParameterScope.Constant);
+            var bodyParam = InputFactory.BodyParameter(
+                "body",
+                InputPrimitiveType.String,
+                isRequired: true);
+            var operation = InputFactory.Operation(
+                "TestOperation",
+                requestMediaTypes: ["application/json"],
+                parameters: [contentTypeParam, 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);
+
+            // Verify there's no if statement wrapping the Content-Type header
+            var wrappedStatement = @"if ((content != null))
+{
+    request.Headers.Set(""Content-Type"", ""application/json"");
+}
+";
+            var statementsString = string.Join("\n", statements!.Select(s => s.ToDisplayString()));
+            var hasIfWrappedContentType = statements!.Any(s => s.ToDisplayString().Contains(wrappedStatement));
+            Assert.IsFalse(hasIfWrappedContentType,
+                $"Content-Type should NOT be wrapped in an if statement for required content, but found:\n{statementsString}");
+        }
     }
 }