Mandate

gistfile1.txt

public static class Mandate
{
    public static void That<TException>(bool condition, string message) where TException : Exception
    {
        if (!condition)
        {
            var exceptionInstance = (TException)Activator.CreateInstance(typeof(TException), message);

            throw exceptionInstance;
        }
    }
}

Cons: it will build and allocate the exception message string on each test, even when not throwing.