WinForms Data Validation

This article was originally published in the "Chris Sells on .NET" newsletter

You should never trust your users. I don't mean don't trust them to pay (which is a completely separate issue that I won't go into here…). I mean don't trust the data that they enter. They may not give you all the data that you need. They may not give you data in the correct format. List boxes, radio buttons and all of the other controls that give users choices all do so to make sure that the provide data in the correct format. However, sometimes you need to validate more free form data entry, like what a user types into a text box. For that, you need to handle a control's Validating event:

void applicantNameTextBox_Validating(object sender, CancelEventArgs e) {
  if( applicantNameTextBox.Text.Length == 0 ) {
    MessageBox.Show("Please enter a name", "Error");
    e.Cancel = true;
  }
}

The Validating will be called when moving focus from a control on the dialog that has the CausesValidation property set to true to another control that has the CausesValidation property set to true, e.g. from a TextBox control to the OK button. The Validating event gives the handler the chance to cancel the move of focus by setting the CancelEventArgs.Cancel property to true. In this example, if the user doesn't enter a name into the text box, than the Validating event handler notifies the user of their transgression and cancels the event, which will keep the focus on the text box with invalid data.
If the Validating event is not canceled, the dialog will get notified with the Validated event:

void applicantNameTextBox_Validated(object sender, EventArgs e) {
  MessageBox.Show("Nice name, " + applicantNameTextBox.Text);
}

Since each control has CausesValidation set to true by default, to allow the user to cancel the dialog without entering valid data, make sure to set the CausesValidation property to false for your Cancel or Close button:

void InitializeComponent() {
  ...
  this.cancelButton.CausesValidation = false;
  ...
}

Data Format Notification

As much as I lean on the message box in my test development, I prefer not to use it for actual applications. A better way is to use the ErrorProvider component, which shows the user that there's a problem and provides a tooltip with extra information, as shown in Figure 1.

Figure 1: Example use of the ErrorProvider component

When the user attempts to chance focus out of the empty text box, we use an instance of the ErrorProvider component to set an error associated with that text box, causing the icon to be displayed to the right of the text box and making the tool tip available if they need more info. This behavior was achieved by dragging an ErrorProvider component onto the dialog and handling the Validating event like so:

void applicantNameTextBox_Validating(object sender, CancelEventArgs e) {
  string error = null;
  if( applicantNameTextBox.Text.Length == 0 ) {
    error = "Please enter a name";
    e.Cancel = true;
  }
  errorProvider1.SetError((Control)sender, error);
}

Notice the call to ErrorProvider.SetError, passing in the control the error is associated with (which we get from the sender argument to the Validating event), along with the error string, which will be used as the tool tip. If there is no problem, the error will be null, which will cause the error indicator on the dialog to go away.