Can’t add method to class in C#?

Question

Can’t add method to class in C#? When I Open ClassView from my View menu. I wanted to add one method and one property. Acc. To Figure 3. http://www.c-sharpcorner.com/UploadFile/mahesh/dll12222005064058AM/dll.aspx But when I Right click on Class1-> I don’t find as in Figure 4. http://www.c-sharpcorner.com/UploadFile/mahesh/dll12222005064058AM/dll.aspx It gives these options: View class diagram Go to definition Browse definition Find all references Filter to type Create instance Invoke static method Copy Rename Sort by best match sort by alphabatically sort by object type sort by object access Group by object type But not add method, what should I do?