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Allow Form to close when invalid data is present

, 24 May 2014 CPOL
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Allowing a User to cancel a Form when Validation is in place and invalid data has been entered.

Introduction

This Tip shows ways of allowing Users to cancel out of a form when Validation events would have them enter valid data before proceeding .

Background

After testing an solution to a Quick Answers question on DataGridView cell validation, I couldn't close the form I had been working with...

The Scenario

I had put some code in a CellValidating event, deliberately entered incorrect data and proven that the validation was working.

However, the Cell still had incorrect data in it when I attempted to close the Form. So the CellValidating code was fired before the click event on the Close[X] and set the focus back to the offending control.

In other words, I was stuck on that control until I entered valid data. Forcing users to do something (like enter valid data) when all they want to do is get out of there, is going to annoy them pretty quickly so I tried to find a solution.

Ways Around the problem

Disable AutoValidate method

Most of my research pointed me in the direction of turning off AutoValidate all together. This does work, but you must remember to validate all the controls when the User was finished data entry and had not tried to just cancel (Note you can do this by calling the ValidateChildren method of the Form).

WndProc method

The principle with this method is to disable AutoValidate only when trying to close the form via Close[X]...

C# Example:
private const int WM_CLOSE = 0x0010;  
...
protected override void WndProc(ref Message m)
{
        if (m.Msg == WM_CLOSE) // Attempting to close Form
                AutoValidate = AutoValidate.Disable; //this stops (all) validations

        base.WndProc(ref m);    //call the base method to handle other messages
}
     
VB.NET Example:
Private Const WM_CLOSE As Integer = &H10
...
Protected Overrides Sub WndProc(ByRef m As Message)
        If m.Msg = WM_CLOSE Then
                AutoValidate = AutoValidate.Disable
        End If
        MyBase.WndProc(m)
End Sub 

Cancel button method

The solution above worked well until I added a "Cancel" button to the form. Again, the CellValidating event is fired before the Button_Click event so I was back to square-one.

After fiddling around trying to find an appropriate Message to capture, I realised that it was as easy as setting the CausesValidation property on the Cancel button to false.

Form_Closing event method

After playing around with all the above I finally came across this article User Input Validation in Windows Forms. Tucked away at the bottom of the article were these wise words...

Quote:
However, in some cases, you might want to let the user close the form regardless of whether the values in the controls are valid. You can override validation and close a form that still contains invalid data by creating a handler for the form's Closing event. In the event, set the Cancel property to false. This forces the form to close
In other words it was a simple as
private void Form1_FormClosing(object sender, FormClosingEventArgs e)
{
        e.Cancel = false;
}
or in VB.NET
Private Sub Form1_FormClosing(ByVal sender As System.Object, ByVal e As System.Windows.Forms.FormClosingEventArgs) Handles MyBase.FormClosing
    e.Cancel = False
End Sub

Note with this approach you will still need the CausesValidation property set to false if you use a Cancel button.

History

24 May 2014 - First Version

License

This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)

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About the Author

CHill60
Software Developer (Senior)
United Kingdom United Kingdom
Works as a Contract Developer mainly in the Financial Services sector.
 
"Abandon all hope of ever being understood, all ye who have a good idea but don't say it perfectly." Doug Bernard

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