Question

Just a weird exception that I can’t get my head around

My code:

<ListBox Grid.Row="1" Grid.Column="0" Background="{StaticResource ContentBackgroundColour}" BorderThickness="0" Margin="0,3,0,3" ItemsSource="{Binding Path=Scenarios}" HorizontalContentAlignment="Stretch" Padding="0">
    <ListBox.ItemTemplate>
        <DataTemplate>
            <Button Content="{Binding}" HorizontalContentAlignment="Left" Command="{Binding Path=OpenScenario}" CommandParameter="{Binding}"></Button>
        </DataTemplate>
    </ListBox.ItemTemplate>
</ListBox>

And:

public ICommand OpenScenario
{
    get { return new RelayCommand(param => _OpenScenario((ManageScenarioModel)param), 
        param => ((ManageScenarioModel)param).IsOpen); }
}

private void _OpenScenario(ManageScenarioModel toOpen)
{
    toOpen.IsOpen = true;
    new ManageScenario(this).Show();
}

However the part param => ((ManageScenarioModel)param).IsOpen) throws a null reference exception upon start up, can someone please explain why this is?

Answer 1

Because WPF is calling your “CanExecute” expression to determine whether or not the button should be enabled. Apparently your binding is resulting in a null command parameter. You should probably check for a null parameter and return false.

param => (param != null && ((ManageScenarioModel)param).IsOpen)