Using event in WPF - wpf

Question

I have two textboxes in WPF. named txt1 and txt2.
In the lostFocus of txt1 I write
If txt1.Text is nothing then
txt1.Focus
End If
In the lostFocus event of txt2 I write
If txt2.Text is nothing then
txt2.Focus
End If
Now, If txt1 and txt2 are both empty and user presses TAB key in txt1 the problem occurs. Program goes in infinite loop. I mean cursor comes to txt1 and goes to txt2 infinite times.I know This is normal behavior according to my code.
So I want to have validating event to avoid the problems like above. But I cannot find one in WPF.
So which event should I use?

Answer 1

I am not a VB coder so can't write exact code for you but here is what you should do. Add event handler for event PreviewLostKeyboardFocus. inside the event handler set e.Handled to true if the text is empty.
Sample C# code. I have writter a generic handler.
private void TextBox_PreviewLostKeyboardFocus(object sender, KeyboardFocusChangedEventArgs e)
{
if (string.IsNullOrEmpty((sender as TextBox).Text))
{
e.Handled = true;
}
}

Answer 2

A better solution might be to allow the user to navigate away from the empty/null text box but either revert the text to the initial value (if there was one) or provide a validation error. Providing validation errors is relatively easy with IDataErrorInfo.
As a software user myself I get annoyed when an application prevents me from navigating away from a field.
Reset Value Approach
See this stackoverflow appraoch for how to maintain and get the previous value. In the LostFocus event you can set your member variable back to _oldValue if the current value is invalid.
determine a textbox's previous value in its lost focused event? WPF
Validation Approach
Those two dates are stored in a model or a view model class. In that class implement IDataErrorInfo (http://msdn.microsoft.com/en-us/library/system.componentmodel.idataerrorinfo(v=vs.95).aspx). Then in your xaml you can show the validation errors.
//This is your model/viewmodel validation logic
public string this[string columnName]
{
get
{
string result = null;
if (columnName == "FirstName")
{
if (string.IsNullOrEmpty(FirstName))
result = "Please enter a First Name";
}
if (columnName == "LastName")
{
if (string.IsNullOrEmpty(LastName))
result = "Please enter a Last Name";
}
if (columnName == "Age")
{
if (Age < = 0 || Age >= 99)
result = "Please enter a valid age";
}
return result;
}
}
//Here is a sample of a xaml text block
<textbox x:Name="tbFirstName" Grid.Row="0" Grid.Column="1" Validation.Error="Validation_Error" Text="{Binding UpdateSourceTrigger=LostFocus, Path=FirstName, ValidatesOnDataErrors=true, NotifyOnValidationError=true}" />
You can also look at these other StackOverflow posts-
What is IDataErrorInfo and how does it work with WPF?
IDataErrorInfo notification