Why we use interface in c# ?

Interfaces are contract between architect and developer. Interfaces are used when the functionality is going to be common.

 Only declaration is there in interface because the definitions are open to 

developer, as per their need they can define the method. Lets suppose one common method is there in interface and definition should be different for different class as per requirement then all classes which implement interfaces can provide their definitions. 

                                If you will define methods in interfaces then method definition will be same every where in derived classes. but i don't want to keep same definitions. Definitions should be different.

 Please see below example : The value of title, id , height and width will not be same for all derived classes like Textbox, Label. If we define the members then all derived controls will have same value of id, title, height and width. So here we need interface where all members are common and derived class can provide the implementations.