Please help me, What is use of KnownType Attribute in WCF.

The main purpose is, it tells the "deserialization" engine what type of object it was while deserialize. The below tutorial gives a nice overview about the "KnownType" Attribute:

 

 

https://msdn.microsoft.com/en-us/library/ms730167(v=vs.110).aspx 

 

Hope this helps!