Receiving windows message in Unity

now, I can send message from unity to C# but i have one question I can send message from C# to C# not to send message from C# to unity How do I receive messages from other programs to unity.

     protected override void WndProc(ref Message m)
     {
         switch (m.Msg)
         {
             case Win32.WM_COPYDATA:
                 Win32.CopyDataStruct st = (Win32.CopyDataStruct)Marshal.PtrToStructure(m.LParam, typeof(Win32.CopyDataStruct));
                 string strData = Marshal.PtrToStringUni(st.lpData);
         }
     }