Microsoft Visual Studio 2013 includes Fakes (Premium and above) to isolate existing code. This involves intercepting calls to the framework and rerouting them to Shims. These Shims are created by right clicking a reference and selecting “Add Fakes Assembly”. But sometimes a little manual override is needed to get things working.

[TestMethod]
public void Stub_environment_and_dns() {
   // arrange
   using (ShimsContext.Create()) {
      System.Fakes.ShimEnvironment.MachineNameGet = () => "hello";
      System.Net.Fakes.ShimDns.GetHostName = () => "world";

   // act
      var result = string.Format("{0} {1}",
         System.Environment.MachineName,
         System.Net.Dns.GetHostName());

   // assert
      Assert.AreEqual("hello world", result);
   }
}

To get the above code to work edit the mscorlib.fakes and System.fakes files. We need to instruct the generator to create fakes for System.Net.Dns and System.Environment.

<Fakes xmlns="http://schemas.microsoft.com/fakes/2011/">
   <Assembly Name="System" Version="4.0.0.0" />
   <ShimGeneration>
      <Add FullName="System.Net.Dns"/>
   </ShimGeneration>
</Fakes>

<Fakes xmlns="http://schemas.microsoft.com/fakes/2011/">
   <Assembly Name="mscorlib" Version="4.0.0.0"/>
   <ShimGeneration>
      <Add FullName="System.Environment"/>
   </ShimGeneration>
</Fakes>

Still the build output contains warnings:

1>mscorlib.fakes : warning : Some fakes could not be generated. For complete details, set Diagnostic attribute of the Fakes element in this file to ‘true’ and rebuild the project.
1>System.fakes : warning : Some fakes could not be generated. For complete details, set Diagnostic attribute of the Fakes element in this file to ‘true’ and rebuild the project.

This makes me reserved in using this for my projects.