'member' : interface members cannot have a definition
Methods that are declared in an interface must be implemented in a class that inherits from it and not in the interface itself.
The following sample generates CS0531:
namespace x { public interface clx { int xclx() { // CS0531, cannot define xclx return 0; } // try the following declaration instead // int xclx(); } public class cly { public static void Main() { } } }