File 1:

    public delegate void CallBack(int a, int b);

File 2:

 ......{
            CalledBack += OnCalledBack;
        }

        private void OnCalledBack(int a, int b)
        {
            
        }

        public event CallBack CalledBack;

Go to file 1 and change it to be:

    public delegate void CallBack(string s);

The error in file 2 is detected only when you open the file.

Issue was closed