So you’re sitting down planning some tests for your shiny new code, only to find that your code uses WebClient to download a file. No problem – you’ve been reading about Microsoft’s new Fakes framework, so you just right-click the System reference in your test project and select “Create Fakes” and you get a bunch of cool fakes to work with.

But going a bit deeper into the Rabbit Hole, you realize that there are no fakes for System.Net classes. What gives?

The White-List

It turns out that when you right-click a reference and select “Add Fakes” a fakes file is created for that assembly in the Fakes folder. When you add a Fakes lib for System, you in fact get 2 fakes files: one for mscorlib and one for System.


Since System is a large library, the Fakes framework doesn’t automatically generate you a fake for every System class. One of the classes that doesn’t have a fake created by default is System.Net.WebClient. In order to force a fake Shim for this class, you’ll need to override the default “white-list” of classes. Fortunately this is relatively straight-forward to do.

The Fakes Config File

Double-click System.fakes to open it in the editor – it’s just an XML file. It does come with an XSD, so as you type in this file IntelliSense will guide you (just like the Force, Luke). Within the  tag, add a  tag and within that an  tag and you’re good to go. Here’s my Fakes file for adding a WebClient fake:

<fakes xmlns=""><br>  <assembly name="System" version=""><br>  <shimgeneration><br>    <add fullname="System.Net.WebClient"><br>  </add></shimgeneration><br></assembly></fakes><br>

Remember to recompile!

Here’s some example code using the WebClient fake:

public class HttpFunctions<br>{<br>    public void Download(string url, string fileName)<br>    {<br>        var w = new WebClient();<br>        w.DownloadFile(url, fileName);<br>        <br>    }<br>}<br><br>[TestClass]<br>public class UnitTest1<br>{<br>    [TestMethod]<br>    public void TestFakeWebClient()<br>    {<br>        using (ShimsContext.Create())<br>        {<br>            ShimWebClient.AllInstances.DownloadFileStringString = (w, url, fileName) =&gt;<br>            {<br>                File.WriteAllText(fileName, url);<br>            };<br><br>            var c = new HttpFunctions();<br>            string guidFileName = Guid.NewGuid().ToString() + ".html";<br>            c.Download("", guidFileName);<br><br>            Assert.IsTrue(File.Exists(guidFileName));<br>            string contents = File.ReadAllText(guidFileName);<br>            Assert.AreEqual("", contents);<br>            File.Delete(guidFileName);<br>        }<br>    }<br>}<br>

Happy faking!