Visual Studio 2013测试运行程序（VSTest）大大降低了测试执行速度

Question

I have a test suite that is using Visual Studio Unit Test framework. It was initially developed on .NET 4.0 in VS2010.
After upgrade to .NET 4.5/VS2013 (no logic changes, just bumping .NET version in csproj files) same tests run x2 times slower. It happens both when executed within IDE or from console (vsconsole/vstest).
After I port few upgraded tests to use NUnit, execution times are back to normal @4.5/2013. I have absolutely no idea what is the root cause and how to fix this. The primary suspect was InIsolation parameter for VSTest, but it doesn't seem to have any effect on execution time. Any ideas about all of the above? I can't port all tests because there are a lot of them and this approach will require much additional efforts on changing of build scripts.

Answer 1

It turned out Shims infrastructure has major performance overhead when a lot of classes are generated. We solved the problem by creating explicit "no shims" context for method that doesn't use them and marking only required classes to have shims generated:

/// <summary>
/// Executing method without ShimsContext being created.
/// </summary>
/// <param name="action">Action to execute.</param>
/// <remarks>Usage: ExecuteWithoutShimsContext(()=> TestMethod()); </remarks>
public static void ExecuteWithoutShimsContext(FakesDelegates.Action action)
{
    if (action == null)
    {
        throw new ArgumentNullException("action");
    }
    using (UnitTestIsolationRuntime.AcquireProtectingThreadContext())
    {
        action();
    }
}

System.fakes configuration file example:

<Fakes xmlns="http://schemas.microsoft.com/fakes/2011/">
  <Assembly Name="System" Version="4.0.0.0"/>
    <ShimGeneration>
        <Clear/>
        <Add FullName="System.Diagnostics.Process"/>
        <Add FullName="System.Net.Sockets.TcpClient"/>
        <Add FullName="System.Net.Sockets.NetworkStream"/>
        <Add FullName="System.ComponentModel.BackgroundWorker"/>
        <Add FullName="System.ComponentModel.Component"/>
        <Add FullName="System.Timers.Timer"/>
    </ShimGeneration>
</Fakes>