How to call this method when types are known only in runtime?

I'm having troubles invoking this method when the types are unknown at compile time.

public static IObservable<TRet> WhenAnyValue<TSender, TRet>(this TSender This, Expression<Func<TSender, TRet>> property1)

The Expression is typed and method call is also typed.

I thought of using reflection, but the code I get is really tricky and brittle. Is there a good way to "parse" this kind of calls to make them more dynamic?