I want to set the slider Value to 10 and then make it possible to move it down or up again, but when I set a Value for the slider, I get an Error which says

System.NullReferenceException

this is the code:

        <MenuItem Header = "Edit">
            <MenuItem Header="Transparence">
                <Slider Width="100" Name="transparence" ValueChanged="transparenz_ValueChanged" 
                        IsMoveToPointEnabled="True" Value="10"/>
            </MenuItem>
        </MenuItem>

this is the method where the exception occures

    private void transparenz_ValueChanged(object sender, RoutedPropertyChangedEventArgs<double> e)
    {
        Input.Opacity = transparenz.Value * 0.1;
    }

when I remove the value from the XAML File, everything works fine

I just want to have this Slider start at the end

The Slider

CodePudding user response：

If you want something to happen before having its event handlers attached, you need to set it in your constructor in code behind instead of XAML, like this:

In your MainWindow.xaml.cs (or whatever your view is named .xaml.cs):

/// <summary>
/// Interaction logic for MainWindow.xaml
/// </summary>
public partial class MainWindow : Window {
    public MainWindow() {
        InitializeComponent();
        transparenz.Value = 10;
    }
    ... Rest of your code here
}

Then remove Value="10" from your XAML file.

CodePudding user response：

The name is wrong... change for Name="transparenz"

 <Slider Width="100" Name="transparenz" ValueChanged="transparenz_ValueChanged" 
                            IsMoveToPointEnabled="True" Value="10"/>

private void transparenz_ValueChanged(object sender, RoutedPropertyChangedEventArgs<double> e)
        {
            Input.Opacity = transparenz.Value * 0.1;
        }