There are several patterns to call a synchronous method asynchronously as shown below.
1- BeginInvoke and EndInvoke
BeginInvoke and EndInvoke Pattern
As you see that first message written in console is worked in main thread (ThreadID is 1), But, DoWork() method is called asynchronously, so that, second message is worked on different thread (ThreadID is 3).
DoWork() method returns int value and without parameters, so that, BeginInvoke method shows two parameters(callback, @object) as shown in Figure-2
Notice that EndInvoke() method takes asyncResult variable returned by BeginInvoke() method and it also returns int value because of Func delegate in Figure-3.