From 51b3e82701aa0c098df03c641a49c5740e2dea84 Mon Sep 17 00:00:00 2001
From: Ryan Brandenburg <rybrande@microsoft.com>
Date: Wed, 25 Jan 2017 17:01:14 -0800
Subject: [PATCH] OnStarting throws when Response.HasStarted

---
 .../ResponseFeature.cs                         |  5 +++++
 .../ResponseFeatureTests.cs                    | 18 ++++++++++++++++++
 2 files changed, 23 insertions(+)

diff --git a/src/Microsoft.AspNetCore.TestHost/ResponseFeature.cs b/src/Microsoft.AspNetCore.TestHost/ResponseFeature.cs
index 73a17f6d33..f736481810 100644
--- a/src/Microsoft.AspNetCore.TestHost/ResponseFeature.cs
+++ b/src/Microsoft.AspNetCore.TestHost/ResponseFeature.cs
@@ -36,6 +36,11 @@ namespace Microsoft.AspNetCore.TestHost
 
         public void OnStarting(Func<object, Task> callback, object state)
         {
+            if (HasStarted)
+            {
+                throw new InvalidOperationException();
+            }
+
             var prior = _responseStartingAsync;
             _responseStartingAsync = async () =>
             {
diff --git a/test/Microsoft.AspNetCore.TestHost.Tests/ResponseFeatureTests.cs b/test/Microsoft.AspNetCore.TestHost.Tests/ResponseFeatureTests.cs
index 9ab936bb81..f50d68a815 100644
--- a/test/Microsoft.AspNetCore.TestHost.Tests/ResponseFeatureTests.cs
+++ b/test/Microsoft.AspNetCore.TestHost.Tests/ResponseFeatureTests.cs
@@ -1,6 +1,7 @@
 // Copyright (c) .NET Foundation. All rights reserved.
 // Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
 
+using System;
 using System.Threading.Tasks;
 using Xunit;
 
@@ -22,5 +23,22 @@ namespace Microsoft.AspNetCore.TestHost
 
             Assert.True(responseInformation.HasStarted);
         }
+
+        [Fact]
+        public void OnStarting_ThrowsWhenHasStarted()
+        {
+            // Arrange
+            var responseInformation = new ResponseFeature();
+            responseInformation.HasStarted = true;
+
+            // Act & Assert
+            Assert.Throws<InvalidOperationException>(() =>
+            {
+                responseInformation.OnStarting((status) =>
+                {
+                    return Task.FromResult(string.Empty);
+                }, state: "string");
+            });
+        }
     }
 }
\ No newline at end of file