Minimal APIs - return string aren't setting content type header #33860

src/Http/Http.Extensions/src/RequestDelegateFactory.cs

@@ -32,7 +32,7 @@ public static partial class RequestDelegateFactory
         private static readonly MethodInfo ExecuteObjectReturnMethod = typeof(RequestDelegateFactory).GetMethod(nameof(ExecuteObjectReturn), BindingFlags.NonPublic | BindingFlags.Static)!;
         private static readonly MethodInfo GetRequiredServiceMethod = typeof(ServiceProviderServiceExtensions).GetMethod(nameof(ServiceProviderServiceExtensions.GetRequiredService), BindingFlags.Public | BindingFlags.Static, new Type[] { typeof(IServiceProvider) })!;
         private static readonly MethodInfo ResultWriteResponseAsyncMethod = typeof(RequestDelegateFactory).GetMethod(nameof(ExecuteResultWriteResponse), BindingFlags.NonPublic | BindingFlags.Static)!;
-        private static readonly MethodInfo StringResultWriteResponseAsyncMethod = GetMethodInfo<Func<HttpResponse, string, Task>>((response, text) => HttpResponseWritingExtensions.WriteAsync(response, text, default));
+        private static readonly MethodInfo StringResultWriteResponseAsyncMethod = typeof(RequestDelegateFactory).GetMethod(nameof(ExecuteWriteStringResponseAsync), BindingFlags.NonPublic | BindingFlags.Static)!;
         private static readonly MethodInfo JsonResultWriteResponseAsyncMethod = GetMethodInfo<Func<HttpResponse, object, Task>>((response, value) => HttpResponseJsonExtensions.WriteAsJsonAsync(response, value, default));
         private static readonly MethodInfo LogParameterBindingFailureMethod = GetMethodInfo<Action<HttpContext, string, string, string>>((httpContext, parameterType, parameterName, sourceValue) =>
             Log.ParameterBindingFailed(httpContext, parameterType, parameterName, sourceValue));
@@ -444,7 +444,7 @@ private static Expression AddResponseWritingToMethodCall(Expression methodCall,
             }
             else if (returnType == typeof(string))
             {
-                return Expression.Call(StringResultWriteResponseAsyncMethod, HttpResponseExpr, methodCall, Expression.Constant(CancellationToken.None));
+                return Expression.Call(StringResultWriteResponseAsyncMethod, HttpContextExpr, methodCall);
             }
             else if (returnType.IsValueType)
             {
@@ -704,6 +704,7 @@ private static async Task ExecuteObjectReturn(object? obj, HttpContext httpConte
             }
             else if (obj is string stringValue)
             {
+                SetPlaintextContentType(httpContext);
                 await httpContext.Response.WriteAsync(stringValue);
             }
             else
@@ -732,8 +733,10 @@ static async Task ExecuteAwaited(Task<T> task, HttpContext httpContext)
 
         private static Task ExecuteTaskOfString(Task<string?> task, HttpContext httpContext)
         {
-            EnsureRequestTaskNotNull(task);
 
+            SetPlaintextContentType(httpContext);
+            EnsureRequestTaskNotNull(task);
+
             static async Task ExecuteAwaited(Task<string> task, HttpContext httpContext)
             {
                 await httpContext.Response.WriteAsync(await task);
@@ -747,6 +750,12 @@ static async Task ExecuteAwaited(Task<string> task, HttpContext httpContext)
             return ExecuteAwaited(task!, httpContext);
         }
 
+        private static Task ExecuteWriteStringResponseAsync(HttpContext httpContext, string text)
+        {
+             SetPlaintextContentType(httpContext);
+             return httpContext.Response.WriteAsync(text);
+        }
+
         private static Task ExecuteValueTask(ValueTask task)
         {
             static async Task ExecuteAwaited(ValueTask task)
@@ -779,6 +788,8 @@ static async Task ExecuteAwaited(ValueTask<T> task, HttpContext httpContext)
 
         private static Task ExecuteValueTaskOfString(ValueTask<string?> task, HttpContext httpContext)
         {
+            SetPlaintextContentType(httpContext);
+
             static async Task ExecuteAwaited(ValueTask<string> task, HttpContext httpContext)
             {
                 await httpContext.Response.WriteAsync(await task);
@@ -884,5 +895,10 @@ private static IResult EnsureRequestResultNotNull(IResult? result)
 
             return result;
         }
+
+        private static void SetPlaintextContentType(HttpContext httpContext)
+        {
+            httpContext.Response.ContentType ??= "text/plain; charset=utf-8";
+        }
     }
 }

src/Http/Http.Extensions/test/RequestDelegateFactoryTests.cs

@@ -1174,7 +1174,7 @@ public static IEnumerable<object[]> StringResult
 
         [Theory]
         [MemberData(nameof(StringResult))]
-        public async Task RequestDelegateWritesStringReturnValueAsJsonResponseBody(Delegate @delegate)
+        public async Task RequestDelegateWritesStringReturnValueAndSetContentTypeWhenNull(Delegate @delegate)
         {
             var httpContext = new DefaultHttpContext();
             var responseBodyStream = new MemoryStream();
@@ -1187,6 +1187,21 @@ public async Task RequestDelegateWritesStringReturnValueAsJsonResponseBody(Deleg
             var responseBody = Encoding.UTF8.GetString(responseBodyStream.ToArray());
 
             Assert.Equal("String Test", responseBody);
+            Assert.Equal("text/plain; charset=utf-8", httpContext.Response.ContentType);
+        }
+
+        [Theory]
+        [MemberData(nameof(StringResult))]
+        public async Task RequestDelegateWritesStringReturnDoNotChangeContentType(Delegate @delegate)
+        {
+            var httpContext = new DefaultHttpContext();
+            httpContext.Response.ContentType = "application/json; charset=utf-8";
+
+            var requestDelegate = RequestDelegateFactory.Create(@delegate);
+
+            await requestDelegate(httpContext);
+
+            Assert.Equal("application/json; charset=utf-8", httpContext.Response.ContentType);
         }
 
         public static IEnumerable<object[]> IntResult