Why does MSTest and TestDriven.NET behave differently using this code?

Question

Check out this code:

internal static readonly Dictionary<Type, Func<IModel>> typeToCreator = new Dictionary<Type, Func<IModel>>();
protected static object _lock;        


public virtual void Register<T>(Func<IModel> creator)
{
    lock (_lock)
    {
    if (typeToCreator.ContainsKey(typeof(T)))
        typeToCreator[typeof(T)] = creator;
    else
        typeToCreator.Add(typeof(T), creator);
    }
}

When I use run the code in this test (testframework is MSTest):

[TestMethod]
public void Must_Be_BasePresenterType()
{
     var sut = new ListTilbudPresenter(_tilbudView);
     Assert.IsInstanceOfType(sut, typeof(BasePresenter));
}

...MSTest passes it and TestDriven.NET fails it because _lock is null.

Why does MSTest NOT fail the test???

Answer 1

When you debug it, what happens on the lock statement? Surely it should crash?