This is how it can be done
private void textBox2_TextChanged(object sender, EventArgs e)
bool validOne = Double.TryParse(textBox1.Text, out val1);
bool validTwo = Double.TryParse(textBox2.Text, out val2);
if (validOne && validTwo)
double result = val1 + val2;
textBox3.Text = result.ToString();
I strongly suggest you get acquainted your self with technology and programming methodology. This is a very basic task and you should have done it yourself.