Mixed-Mode Assembly MSTest Failing in VS2015

mstest, unit-testing, visual-studio-2015

Solution

Alternative 1: Configuration

Add the startup configuration to `C:\Program Files (x86)\Microsoft Visual Studio 14.0\Common7\IDE\CommonExtensions\Microsoft\TestWindow\TE.ProcessHost.Managed.exe.config`:

<startup useLegacyV2RuntimeActivationPolicy="true">
</startup>

Alternative 2: At Runtime

This may stop working.

Simply add this class to the unit test project (source):

using System;
using System.Runtime.CompilerServices;
using System.Runtime.InteropServices;
using Microsoft.VisualStudio.TestTools.UnitTesting;

[TestClass]
public static class RuntimePolicyHelper
{
    [AssemblyInitialize]
    public static void SetPolicy(TestContext ctx)
    {
        var clrRuntimeInfo =
            (ICLRRuntimeInfo)RuntimeEnvironment.GetRuntimeInterfaceAsObject(
                Guid.Empty,
                typeof(ICLRRuntimeInfo).GUID);

        // Allow errors to propagate so as to fail the tests.
        clrRuntimeInfo.BindAsLegacyV2Runtime();
    }

    [ComImport]
    [InterfaceType(ComInterfaceType.InterfaceIsIUnknown)]
    [Guid("BD39D1D2-BA2F-486A-89B0-B4B0CB466891")]
    private interface ICLRRuntimeInfo
    {
        void xGetVersionString();
        void xGetRuntimeDirectory();
        void xIsLoaded();
        void xIsLoadable();
        void xLoadErrorString();
        void xLoadLibrary();
        void xGetProcAddress();
        void xGetInterface();
        void xSetDefaultStartupFlags();
        void xGetDefaultStartupFlags();

        [MethodImpl(MethodImplOptions.InternalCall, MethodCodeType = MethodCodeType.Runtime)]
        void BindAsLegacyV2Runtime();
    }
}

Problem

When attempting to run unit tests that use mixed mode assemblies in VS2015 the tests fail to execute with the usual message: System.IO.FileLoadException: Mixed mode assembly is built against version 'v2.0.50727' of the runtime and cannot be loaded in the 4.0 runtime without additional configuration information. Creating an app.config and adding `useLegacyV2RuntimeActivationPolicy` to it has no effect - it seems as though this configuration is impossible to change. This previously worked with no manual steps in VS2013.

Original source