WPF - NullReferenceException on isEnabled

Question

I'm new to WPF, in the past I've used Windows Forms. I'm having an issue here that I like someone to explain to me. The below is a really simple example.

I've got a XAML page on which I've got a single checkbox, a button, and a text field. The checkbox is checked by default.

When the checkbox is unchecked, I want to enable the button and the text field, e.g.

private void UseDefaultFoldersCB_Checked(object sender, RoutedEventArgs e)
{
      //MessageBox.Show("");
      if (StartDirLocationTB.IsEnabled == false)
      {
           StartDirLocationTB.IsEnabled = true;
      }

      if (SelectStartLocationBtn.IsEnabled == false)
      {
            SelectStartLocationBtn.IsEnabled = true;
      }
}

XAML:

<CheckBox Content="Use Default Folders" IsChecked="True" Height="16" HorizontalAlignment="Left" Margin="10,14,0,0" Name="UseDefaultFoldersCB" VerticalAlignment="Top" Checked="UseDefaultFoldersCB_Checked" />
<TextBox Height="23" IsEnabled="False" HorizontalAlignment="Left" Margin="9,38,0,0" Name="StartDirLocationTB" VerticalAlignment="Top" Width="403" Background="WhiteSmoke" />
<Button Content="Select Start Folder" IsEnabled="False" Height="23" HorizontalAlignment="Right" Margin="0,38,6,0" Name="SelectStartLocationBtn" VerticalAlignment="Top" Width="139" />

Stack Trace:

System.NullReferenceException was unhandled by user code
Message=Object reference not set to an instance of an object.
Source=TestProject StackTrace: at TestProject.MainWindow.UseDefaultFoldersCB_Checked(Object sender, RoutedEventArgs e) in C:\Users\jc\Desktop\Test\TestProject\MainWindow.xaml.cs:line 611 at System.Windows.EventRoute.InvokeHandlersImpl(Object source, RoutedEventArgs args, Boolean reRaised) at System.Windows.UIElement.RaiseEventImpl(DependencyObject sender, RoutedEventArgs args) at System.Windows.Controls.Primitives.ToggleButton.OnIsCheckedChanged(DependencyObject d, DependencyPropertyChangedEventArgs e) at System.Windows.DependencyObject.OnPropertyChanged(DependencyPropertyChangedEventArgs e) at System.Windows.FrameworkElement.OnPropertyChanged(DependencyPropertyChangedEventArgs e) at System.Windows.DependencyObject.NotifyPropertyChange(DependencyPropertyChangedEventArgs args) at System.Windows.DependencyObject.UpdateEffectiveValue(EntryIndex entryIndex, DependencyProperty dp, PropertyMetadata metadata, EffectiveValueEntry oldEntry, EffectiveValueEntry& newEntry, Boolean coerceWithDeferredReference, Boolean coerceWithCurrentValue, OperationType operationType) at System.Windows.DependencyObject.SetValueCommon(DependencyProperty dp, Object value, PropertyMetadata metadata, Boolean coerceWithDeferredReference, Boolean coerceWithCurrentValue, OperationType operationType, Boolean isInternal) at System.Windows.DependencyObject.SetValue(DependencyProperty dp, Object value) at MS.Internal.Xaml.Runtime.ClrObjectRuntime.SetValue(Object inst, XamlMember property, Object value)

When I launch the application from Visual Studio, I get a NullReferenceException on the above code. Why does this code execute when the application launches? I'd have thought it would only execute when the checkbox is checked/unchecked? Why the NullReferenceException?

Thanks.

Answer 1

The answer is to stop coding as if you are in WinForms. Use Databinding instead. I found a nice sample for you here. If you still want to do it in event handlers, keep it in the 'Checked' event and just add null checks. If you use the Click event, keyboard shortcuts won't work. Null-checking sample:

private void UseDefaultFoldersCB_Checked(object sender, RoutedEventArgs e)
{
    if (StartDirLocationTB != null && StartDirLocationTB.IsEnabled == false)
    {
         StartDirLocationTB.IsEnabled = true;
    }

    if (SelectStartLocationBtn != null && SelectStartLocationBtn.IsEnabled == false)
    {
         SelectStartLocationBtn.IsEnabled = true;
    }
}

Answer 2

The Reason why you are receiving call on this Event Handler is because while initializing the Page , XAML pareser sees that an Event is attached to Checked attribute of the Checkbox(i.e. this event will be called when ever checkbox IsChecked property is true) thus it is calling the event at the time of loading itself.

I will suggest that you use Click event of the checkbox, so that you can have an event whenever checkbox state is changed.

So you XAML will be something Like this.

<CheckBox Content="Use Default Folders" IsChecked="True" Height="16" HorizontalAlignment="Left" Margin="10,14,0,0" Name="UseDefaultFoldersCB" VerticalAlignment="Top" Click="UseDefaultFoldersCB_Click" />
<TextBox Height="23" IsEnabled="False" HorizontalAlignment="Left" Margin="9,38,0,0" Name="StartDirLocationTB" VerticalAlignment="Top" Width="403" Background="WhiteSmoke" />
<Button Content="Select Start Folder" IsEnabled="False" Height="23" HorizontalAlignment="Right" Margin="0,38,6,0" Name="SelectStartLocationBtn" VerticalAlignment="Top" Width="139" />

and the Eventhandler code remains same as..

private void UseDefaultFoldersCB_Click(object sender, RoutedEventArgs e)
{
      //MessageBox.Show("");
      if (StartDirLocationTB.IsEnabled == false)
      {
           StartDirLocationTB.IsEnabled = true;
      }

      if (SelectStartLocationBtn.IsEnabled == false)
      {
            SelectStartLocationBtn.IsEnabled = true;
      }
}