Dealing with HttpResponse in unit tests

Question

I have a simple website that has one page. The function I am unit testing is

public void WriteFileToClient(string fileContent, string fileName)
        {
            StringWriter stringWriter = new StringWriter();
            stringWriter.Write(fileContent);
            Response.ContentType = "text/plain";
            Response.AddHeader("content-disposition", string.Format("attachment;filename={0}.txt", fileName));
            Response.Clear();

            using (StreamWriter writer = new StreamWriter(Response.OutputStream, Encoding.UTF8))
            {
                writer.Write(stringWriter.ToString());
            }
            Response.End();
        }

When I run my test it says that Response is not available

Here is my Unit test

[TestMethod]
        [TestCategory("Unit")]
        public void WriteFileToClient_ShouldSucceedForCorrectInput()
        {
            //Arrange
            Migrate migrate = new Migrate();
            Guid g = Guid.Parse("fffe49c1-a838-46bc-b5c6-4e0bbd3e4c32");

            string fileContent = migrate.GenerateFlatFileText(g);
            string fileName = string.Format("flat file - {0}.txt", g);

            //Ask
            migrate.WriteFileToClient(fileContent, fileName);

            //Assert
            migrate.Response.OutputStream.Should().NotBeNull();
            migrate.Response.ContentType.Should().Be("text/plain");
            migrate.Response.Headers.Should().HaveCount(1);
        }

Any suggestions on how to mock the response? honestly I don't understand why the Response object is not available, my Migrate class inherits Page and to my understanding should has the Response included

Answer 1

The Response object is part of System.Web, which is a strong indicator it runs inside a web server. You are running a "unit test" on web code without invoking a web server. Can you guess why it might be failing?

UI is not normally something you can easily unit test, if at all. An exception is a controller in MVC, but the controller must be isolated from dependencies to be unit tested beyond "did it call the correct view". When you start dealing with tests on things like response stream, you need to bring up the code in the context of a web server. This means the actual code with Response has to be tested using an HTTP Request and then examining the response. This is more of an integration test than a unit test, unless you mock up the dependency.

Make sense?