Visual Studio 2013 is unable to open the Test Window

Question

When I open Visual Studio 2013 and load my solution I'm greeted by an error message telling me the Test Window is unable to load.

The composition produced a single composition error. The root cause is provided below. Review the CompositionException.Errors property for more detailed information.

1) Cannot compose part 'Microsoft.VisualStudio.TestWindow.Controller.TestsServiceInternal' because a cycle exists in the dependencies between the exports being composed. To break this cycle, consider changing some imports from constructor to property injection.
Element: Microsoft.VisualStudio.TestWindow.Controller.TestsServiceInternal -->  Microsoft.VisualStudio.TestWindow.Controller.TestsServiceInternal

Resulting in: Cannot get export 'Microsoft.VisualStudio.TestWindow.Controller.TestsServiceInternal (ContractName="Microsoft.VisualStudio.TestWindow.Data.TestsService+ITestsServiceInternal")' from part 'Microsoft.VisualStudio.TestWindow.Controller.TestsServiceInternal'.
Element: Microsoft.VisualStudio.TestWindow.Controller.TestsServiceInternal (ContractName="Microsoft.VisualStudio.TestWindow.Data.TestsService+ITestsServiceInternal") -->  Microsoft.VisualStudio.TestWindow.Controller.TestsServiceInternal

The composition produced a single composition error. The root cause is provided below. Review the CompositionException.Errors property for more detailed information.

1) An operation is not legal in the current state. (Exception from HRESULT: 0x80131509)

Resulting in: An exception occurred while trying to create an instance of type 'Microsoft.VisualStudio.TestWindow.VsHost.TestWindowHost'.

Resulting in: Cannot activate part 'Microsoft.VisualStudio.TestWindow.VsHost.TestWindowHost'.
Element: Microsoft.VisualStudio.TestWindow.VsHost.TestWindowHost -->  Microsoft.VisualStudio.TestWindow.VsHost.TestWindowHost

Resulting in: Cannot get export 'Microsoft.VisualStudio.TestWindow.VsHost.TestWindowHost (ContractName="Microsoft.VisualStudio.TestWindow.Controller.IOpenResult")' from part 'Microsoft.VisualStudio.TestWindow.VsHost.TestWindowHost'.
Element: Microsoft.VisualStudio.TestWindow.VsHost.TestWindowHost (ContractName="Microsoft.VisualStudio.TestWindow.Controller.IOpenResult") -->  Microsoft.VisualStudio.TestWindow.VsHost.TestWindowHost

Resulting in: Cannot set import 'Microsoft.VisualStudio.TestWindow.Controller.TestsServiceInternal..ctor (Parameter="openResult", ContractName="Microsoft.VisualStudio.TestWindow.Controller.IOpenResult")' on part 'Microsoft.VisualStudio.TestWindow.Controller.TestsServiceInternal'.
Element: Microsoft.VisualStudio.TestWindow.Controller.TestsServiceInternal..ctor (Parameter="openResult", ContractName="Microsoft.VisualStudio.TestWindow.Controller.IOpenResult") -->  Microsoft.VisualStudio.TestWindow.Controller.TestsServiceInternal

Resulting in: Cannot get export 'Microsoft.VisualStudio.TestWindow.Controller.TestsServiceInternal (ContractName="Microsoft.VisualStudio.TestWindow.Data.TestsService+ITestsServiceInternal")' from part 'Microsoft.VisualStudio.TestWindow.Controller.TestsServiceInternal'.
Element: Microsoft.VisualStudio.TestWindow.Controller.TestsServiceInternal (ContractName="Microsoft.VisualStudio.TestWindow.Data.TestsService+ITestsServiceInternal") -->  Microsoft.VisualStudio.TestWindow.Controller.TestsServiceInternal

Visual Studio is unable to run tests at all. Closing an re-opening the test window shows the same error.

Answer 1

This seems to be caused by a corruption in the Component Model Cache.

Close all instances of Visual Studio, then navigate to folder and delete it.

%LOCALAPPDATA%\Microsoft\VisualStudio\12.0\ComponentModelCache

Now restart Visual Studio and it should work again.