Dim d1 As Double = val(e.form.controls("TextBox1").Text)
Dim d2 As Double = val(e.form.controls("TextBox2").Text)
Dim d3 As Double = val(e.form.controls("TextBox3").Text)
e.form.controls("TextBox4").Text = iif(d1=0 or d2=0, 0, d1*d2+d3)