How do I validate user entry in a textbox in the lost focus event without the lost focus event firing repeatedly?

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I have a wpf application where the user enters a work order number in a text box. In the lost focus event of the text box, a database is queried to validate that the work order number exists. If not, a message box is displayed and the Focus() method of the textbox control is called to keep the focus on that text box.

What actually happens is that the lost focus event fires repeatedly

Here is the xaml for the textbox control:

TextBox x:Name="txtWONumber" IsReadOnly="False" Focusable="True" HorizontalAlignment="Left" Height="23" Margin="174,15,0,0" TextWrapping="Wrap" Text="" VerticalAlignment="Top" Width="120" RenderTransformOrigin="0.384,0.424" FocusManager.LostFocus="TxtWONumber_LostFocus"/>

Here is the event handler code:

private void TxtWONumber_LostFocus(object sender, RoutedEventArgs e)
		{

			string strWONumber = this.txtWONumber.Text;
			//MessageBox.Show("Work Order No: " + strWONumber);

			var qryWorkOrder =
			(from w in dataEntities.vwWOPatternCheckDatas	
			 where w.WONumber == strWONumber
			 orderby w.WONumber
			 select new { w.WONumber }).ToList();

			if (qryWorkOrder.Count() == 0)
			{
				MessageBox.Show("Work Order Number not found! Please enter a valid Work Order Number.");
				txtWONumber.Focus();
				txtWONumber.SelectAll();
			}
		}

Thank you,

Brian Miller

What I have tried:

I have tried using the LostFocus event as well as the FocusManager.LostFocus event and the behavior is the same.

I have spent a number of hours searching for relevant information on the web. Nothing seems to address this situation. The closest I have found is related to two consecutive text boxes in the tab order each with a lost focus event.

Posted 2-Mar-19 18:45pm

Member 12344083

Updated 3-Mar-19 2:46am

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Richard Deeming 5-Mar-19 12:35pm

NB: Rather than counting all the matches to determine whether there are any, it's better to use either Any or All:

if (!dataEntities.vwWOPatternCheckDatas.Any(w => w.WONumber == strWONumber)) { ... }
// Or:
if (dataEntities.vwWOPatternCheckDatas.All(w => w.WONumber != strWONumber)) { ... }

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Gerry Schmitz · Answer 1 · 2019-03-02T22:04:00

I've encountered your situation with different languages.

The "next field" will receive "focus" while the previous field is "losing focus".

"Setting focus" while "losing focus" in the "lost focus" event gets problematic because of the "next field" focus logic.

The "work around" (in my case) was to ("simply") store the field name of the current field while losing focus, and at the same time in the next field's "got focus" to do a call to the "previous fields" validation logic. If it is valid, then "accept focus" of the new field, else reject the new focus whereupon the focus stays on the last field (e.g. Windows Forms has a "Can focus" property).

Haven't reviewed WPF (because I tend to do "record-level" validation more these days than "field-level") for an equivalent "can focus", but you can probably return focus to the previous field (from the next field) using the focus statement.

Note that if your "field" is at "end of your form", you may have to resort to a "record-level" validation anyway because now you may not lose focus at all (while "saving").

(Yeah; sounds ugly but such is life when you do something most people don't: field-level validation).

As for validating the "work order #": in those cases I usually employ a "pick list", as every major software product does. There are never that many "open" work orders that you cannot query the the db for a list of work orders numbers that can be "popped up" when the user hits that field.

Dylvh · Answer 2 · 2019-03-03T02:47:00

I think this is what's happening. You're showing a message, but immediately giving focus to the textbox again. Now, you have the message box open, and you want to click ok to close it, but then the textbox is losing focus again.

Maybe try the following:

private void TxtWONumber_LostFocus(object sender, RoutedEventArgs e)
		{
                        if (checkNumber) // Should number be checked
                        {
                            checkNumber = false;
        			string strWONumber = this.txtWONumber.Text;
        			//MessageBox.Show("Work Order No: " + strWONumber);
    
        			var qryWorkOrder =
        			(from w in dataEntities.vwWOPatternCheckDatas	
        			 where w.WONumber == strWONumber
        			 orderby w.WONumber
        			 select new { w.WONumber }).ToList();
    
        			if (qryWorkOrder.Count() == 0)
        			{
        				if (MessageBox.Show("Work Order Number not found! Please enter a valid Work Order Number.") == DialogResult.OK)
                                        {
                                		txtWONumber.Focus();
                        			txtWONumber.SelectAll();
                                                checkNumber = true;
                                        }
        			}
                        }
                        
		}

So, you have a global bool variable checkNumber which is set to true, that indicates that you want to check the works order number. Once it enters the if statement, you set it to false, and set it back to true after you've clicked the ok button on the message box.

AnkushK1 · Answer 3 · 2019-03-02T21:47:00

Rather using LostFocus, you might also think to create and use DependencyProperty of string type in code behind and bind it 2 way to Text property of TextBox. Dependency property provides a callback event that you can use to validate the text. Refer to below link for details on its usage

WPF Tutorial | Dependency Properties[^]

How do I validate user entry in a textbox in the lost focus event without the lost focus event firing repeatedly?

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