Question

According to Jon Skeet, “You can only call BeginInvoke on a delegate which has a single target invocation.”

Why is that? What’s the real reason?

Note: For clarification (and because I made this mistake), I am talking about the BeginInvoke on delegates, not on controls.

Answer 1

I think Jon Skeet does a good job explaining in the post you linked:

How do you want the threading to work? Do you have to run each
invocation synchronously, but run the whole thing asynchronously with
respect to the calling thread, or could you run each invocation
asynchronously?

If it’s the former, just run a single threadpool work item which calls
the delegate synchronously. If it’s the latter, get the invocation list
with Delegate.GetInvocationList and call BeginInvoke on element of the
list in turn.

Basically calling BeginInvoke on a MulticastDelegate is ambiguous, do you want the delegates to wait for each other or not? While in theory it could decide for you, the choice has been made to force you to explicitly select the method you want by calling the delegates in a different fashion.

In other words it is a design choice to avoid confusion. Also it is important to note that BeginInvoke has fallen out of favor and newer methods of asynchronous programming are available making updating this old standard unlikely, so even if they wanted to change now, there is no reason to.