C# Form Control Validation Guide

1. C# - FCL/Form & Control Validation John Kelleher

2. Validating Controls • Ensure correct data is input • Validating event • Raised when control is asked to validate contents • Programmer writes event handler • If contents found to be invalid programmer can notify user • If contents found to be valid, Validated event is raised and focus is passed to next control

3. Validating Event • Only raised… • if control has CausesValidation property set to true …and • when focus is passed to next control whose CausesValidation property set to true • Programmer write response: e.g. inform user of error

4. Validating Event (contd.) • Event handler is of type CancelEventHandler • this passes CancelEventArgs object • Event handler sets Cancel flag to true for the CancelEventArgs object if data is invalid • Focus does not pass to next control! • Event handler sets Cancel flag to false for the CancelEventArgs object if data is valid • Validated event is raised

5. Example private ErrorProvider errorProvider = new ErrorProvider(); … Private void textbox_Validating(object sender, CancelEventArgs e) { string errText = “”; if (textbox.Text.Length == 0) { e.Cancel = true; errText = “This field must not be empty”; } errorProvider.SetError(textbox, errText); }

6. Code explanation • Basic error messaging system • If control validation fails, focus remains with control • Send empty string to errorProvider to clear error message/status • Ensure CausesValidation property for Cancel button is set to false • This ensures that focus can pass to the button and the user may cancel the form