Adding unit test for ViewResult

test/Microsoft.AspNet.Mvc.Core.Test/ViewResultTest.cs

@@ -15,6 +15,9 @@ namespace Microsoft.AspNet.Mvc.Core.Test
 {
     public class ViewResultTest
     {
+        // The buffer size of the StreamWriter used in ViewResult.
+        private const int ViewResultStreamWriterBufferSize = 1024;
+
         [Fact]
         public async Task ExecuteResultAsync_WritesOutputWithoutBOM()
         {
@@ -51,5 +54,46 @@ namespace Microsoft.AspNet.Mvc.Core.Test
             // Assert
             Assert.Equal(expected, memoryStream.ToArray());
         }
+
+        // The StreamWriter used by ViewResult an internal buffer and consequently anything written to this buffer
+        // prior to it filling up will not be written to the underlying stream once an exception is thrown.
+        [Theory]
+        [InlineData(30, 0)]
+        [InlineData(ViewResultStreamWriterBufferSize + 30, ViewResultStreamWriterBufferSize)]
+        public async Task ExecuteResultAsync_DoesNotWriteToResponse_OnceExceptionIsThrown(int writtenLength, int expectedLength)
+        {
+            // Arrange
+            var longString = new string('a', writtenLength);
+            var memoryStream = new MemoryStream();
+            var response = new Mock<HttpResponse>();
+            response.SetupGet(r => r.Body)
+                   .Returns(memoryStream);
+            var context = new Mock<HttpContext>();
+            context.SetupGet(c => c.Response)
+                   .Returns(response.Object);
+            var routeDictionary = new Dictionary<string, object>();
+            var actionContext = new ActionContext(context.Object,
+                                                  Mock.Of<IRouter>(),
+                                                  routeDictionary,
+                                                  new ActionDescriptor());
+            var view = new Mock<IView>();
+            view.Setup(v => v.RenderAsync(It.IsAny<ViewContext>()))
+                 .Callback((ViewContext v) =>
+                 {
+                     v.Writer.Write(longString);
+                     throw new Exception();
+                 });
+            var serviceProvider = Mock.Of<IServiceProvider>();
+            var viewEngine = new Mock<IViewEngine>();
+            viewEngine.Setup(v => v.FindView(routeDictionary, It.IsAny<string>()))
+                      .Returns(ViewEngineResult.Found("MyView", view.Object));
+            var viewResult = new ViewResult(serviceProvider, viewEngine.Object);
+
+            // Act
+            await Record.ExceptionAsync(() => viewResult.ExecuteResultAsync(actionContext));
+
+            // Assert
+            Assert.Equal(expectedLength, memoryStream.Length);
+        }
     }
 }