Monotouch: how to verify UITextFields

I have several UITextFields that I want to do a numeric check on when the user taps the Next key on the keyboard. I kno开发者_Python百科w I can do it when the k/b becomes first responder... I want to check the field before the user goes on to the next field.

How do I do this?

void HandleTxtHIhandleEditingDidEnd (object sender, EventArgs e) {
if(!decimal.TryParse(txtHI.Text, out x)) {