What is the difference between string datatype and String class in c#?

String : is an class (system.String) and it's an reference type (class) 

and 

string: is an alis name of String class that is created by microsoft, it's a data type, is a c# keyword and is a complier shortcut for System.String class 

so as a per above points when you use a string keyword, it reachs the System.String class , so we can say that both the (String, string ) are the same. 

Regardes 

both are same

"string" is an alias to System.String Class