What is asynchronous and synchronous callback

search this in google:
:D

https://www.google.com/#q=What+is+asynchronous+and+synchronous+callback[^]

Callbacks, synchronous and asynchronous[^] is a great blog for information on this topic.

A synchronous call means control waits till the operation is complete before moving to the next statement.
Example:

Class Main()
{
var ResultOfA = ProcessA(); //Only upon completion of this method, the control can move to next line to process method ProcessB() and then to subsequent methos.
ProcessB();
ProcessC();
ProcessD(ResultOfA);
}

In Asynchronous call

If you decide to call ProcessA, ProcessB and ProcessC in parallel (as only ProcessD requires ProcessA to complete to receive the parameters).

It goes like this:

        Task<string> t1 = Task<string>.Factory.StartNew(() => a.ProcessA());
        Task t2 = Task.Factory.StartNew(() => a.ProcessB());
        Task t3 = Task.Factory.StartNew(() => a.ProcessC());
        Task.WaitAll(t1, t2, t3);

        a.ProcessD(t1.Result);
</string></string>

This means as soon as the method A is called, the control, without waiting for its completion, executes the next line which is to processB and same happens untill it reaches the statement Task.WaitAll() which forces the control to wait till all the three methods complete before executing the fourth method ProcessD.