//defining first interface
    interface Ifirst
    {
        void sum(int a, int b);
    }
 
//defining second interface which inherits first interface
    interface Isecond:Ifirst
    {
        void sub(int a, int b);
    }
 
//defining class which inherits second interface
 
    class newClass : Isecond
    {
//implementing interface defined methods
        public void sum(int a, int b)
        {
            MessageBox.Show("Sum is: " + (a + b).ToString());
        }
        public void sub(int a, int b)
        {
            MessageBox.Show("Difference is: " + (a - b).ToString());
        }
    }

Demonstrating u

//creating instance of class
newClass c = new newClass();
private void btnSum_Click(object sender, EventArgs e)
        {       
//calling sum() of Ifirst interface     
c.sum(Convert.ToInt32(txtNum1.Text), Convert.ToInt32(txtNum2.Text));
        }