c# - P/Invoke Erorr - The modifier 'extern' is ...

When I try to declare some PInvoke classes for one of my test application, I have one problem as mentioned in the email's subject. 

The problem is that I directly add the DllImport under the namespace root, in order to use the P/Invoke you will need to enclose them in a class. 

References:

PInvoke - SendMessage Problem.