How to get typeof generic interface

i am having

interface ITestInterface<TSource,TDestination>

i want the

void TestFunction(Type t1, Type t2)
{
    var x = typeof(ITestInterface<t1, t2>)
}

what is difference between genericMethod.Invoke(this, null) and calling method direct i.e TestFunction(typeof(Emp), typeof(Dept)). so that i can change function to

void TestFunction<TSource, TDestination>()
{
var x = typeof(ITestInterface<TSource, TDestination>)
}