C# WPF Label Visibility Exception

Question

I'm trying to show a label based on the selected value of a combo box. I'm using VS2010.

Here's the code:

private void pointsSettings_SelectionChanged(object sender, SelectionChangedEventArgs e)
    {
        if (this.pointsSettings.SelectedValue.ToString() == "between")
        {
           pointsAboveLabel.Visibility = Visibility.Collapsed;
        }
        else
        {
           pointsAboveLabel.Visibility = Visibility.Visible;
        }
    }

And the XAML:

 <ComboBox Name="pointsSettings" SelectionChanged="pointsSettings_SelectionChanged">
     <ComboBoxItem Content="between" />
     <ComboBoxItem Content="above" IsSelected="True" />
     <ComboBoxItem Content="below" />
 </ComboBox>
 <Label Content="points" Name="pointsAboveLabel" />
 <Label Content="and" Name="pointsBetweenLabel" Visibility="Collapsed" />

And here is the error I got: Text reads: "Object reference not set to an instance of an object".

I'm aware that there is another question very similar this - with the same error in fact- however the solution provided (moving a variable initialization to above the InitializeComponent() method call) is not appropriate for me as I have no object initialization.

Any and all help is appreciated most warmly.

Answer 1

This happens because the pointsSettings_SelectionChanged happens before the UI is fully initialized (you can see in the callstack that it comes from the InitializeComponents() call). So the label has not been initialized in the UI, and therefore your variable in the codebehind pointing to that label is null. So it is effectively the same problem as the other question, however it is the WPF framework that is initializing the variable, not you.

Your XAML is evaluated sequentially, so it hits the ComboBox and initializes it (including the initially selected value) and triggers the SelectionChanged event before the label has been initialized. You'll notice if you change the order of your XAML so that the Label is above the ComboBox, you won't have the problem because the Label will be initialized before the event is fired.

To solve this, I would suggest adding a condition to the pointsSettings_SelectionChanged method to return if the pointsAboveLabel is null:

private void pointsSettings_SelectionChanged(object sender, SelectionChangedEventArgs e)
{
    if(pointsAboveLabel == null) return;
    //rest of code
}

And since you are hard-coding the initially selected value in the ComboBox, you can hard-code the initial Visibility of the Label.

But you've got another problem in your code - the SelectedValue of the ComboBox will never be a string as you've got it setup now. The SelectedValue will be a ComboBoxItem. In order to see the string value you're putting in the ComboBoxItem, you'll need to look at the Content property. So you'll need something like this:

ComboBoxItem item = pointsSettings.SelectedValue as ComboBoxItem;
if(item.Content.ToString() == "between")
{
  ...
}