how to calculation in textboxes

I'd use code similar to the following:

        private void button1_Click(object sender, EventArgs e)

        {

            int i;

            bool isValid = Int32.TryParse(textBox1.Text, out i);

            if (isValid)

            {

               // do something with 'i' which will contain the integer input 

             

            }                            

            else

            {

               MessageBox.Show("You haven't entered a valid integer. Please amend");

               textBox1.Focus();

            } 

        }

If it's a number with a decimal point, then use a double or decimal variable rather than an int variable together with the Double.TryParse() or Decimal.TryParse() methods.