Question

I have created a custom user control (JCUserControl), and I am using it in the Main Window. And my Main Window has no codebehind.

I have this in the JCUserControl codebehind:

Private Sub ImmediateRadioButton_Checked(sender As System.Object, e As System.Windows.RoutedEventArgs) Handles ImmediateRadioButton.Checked
    SomeTextBox.IsEnabled = False
End Sub

When I run it, it fails with the NullReferenceException. If I comment out the SomeTextBox.IsEnabled = false, it runs without any problems.

Can somebody help?

Edit:

Found out that I could just check if the radio buttons are loaded before doing whatever I want to do.

Private Sub ImmediateRadioButton_Checked(sender As System.Object, e As System.Windows.RoutedEventArgs) Handles ImmediateRadioButton.Checked

If ImmediateRadioButton.IsLoaded Then
SomeTextBox.IsEnabled = False
End If
End Sub

Answer 1

Isn’t it obvious? SomeTextBox is null, this is possible because not all controls can be initialized at the same time, if the RadioButton appears earlier in the parsing order its checked event can fire before the TextBox exits.

You should approach this more declarativly anyway, e.g. bind the TextBox.IsEnabled to the RadioButton.IsChecked or add a new boolean property and bind both to that.

e.g. for the former:

<RadioButton Name="rb"/>
<!-- Apply Binding.Converter as needed, or use a DataTrigger instead -->
<TextBox IsEnabled="{Binding IsChecked, ElementName=rb}"/>