I am working on Windows XP with Visual Studio 2010 with SP1. I was using TypeMock Isolator 7.0.0 successfully within Visual Studio with MS test. Then I uninstalled TypeMock Isolator 7.0.0 and checked if any file related with typemock remains in documents and files folder. There was none. I installed TypeMock Isolator 126.96.36.199 and have seen that Isolator Essential successfully installed on my computer. I can see TypeMock in Visual Studio toolbar and enabled test with 3rd party application option. I tried to run your example(about mocking showdialog function) so I created a VS test project with .NET version 4, gave reference to Typemock and Typemock C# dlls. And within test class I pasted your example. I successfully build VS test project and tried to run test method by selecting it from VS test view window. For a while test is shown as running but then in the VS test results window it is shown as result that test "not executed". when I clicked on the test result, a dialog with Microsoft Visual Studio caption says that "The test has not been executed." Then I erased code within test method and just wrote "Assert.AreEqual(true, true)" and ran the test again but I again got the result as test not executed. When I disabled test with 3rd party application option, I was able to ran the test and the result was passed. How can I successfully run tests within Visual Studio 2010 with Typemock version 188.8.131.52?