TargetNullValue causes null value in Slider Controls

[GrafJ-AUMA]

Description

I am using a slider, bound to a nullable decimal in a view model. It's a percentage value from 0 to 100 in with 0.1 accuarcy. I defined a TargetNullValue at 50. If I click on the slider to move it a slinge step (IsMoveToPointEnabled="False") and I reach the TargetNullValue, the value get set to null.

Reproduction Steps

1. Create a new empty WPF Project
2. Add a slider bound to a nullable decimal in a view model
3. Define a TargetNullValue for the bound value
4. Set Min to 0 and max to 100
5. Click as long as needed to move the slider to the target null value

Expected behavior

The value gets set and is not null. If I moved the slider before (Sliding aroung), it works as expected.

Actual behavior

The bound value is set to null.

Regression?

No response

Known Workarounds

No response

Impact

No response

Configuration

.NET Framework 4.8 on Windows 11 x64

Other information

No response

[lindexi]

@GrafJ-AUMA Could you attach the code project? I'm worried that this is the expected behavior.

[GrafJ-AUMA]

This is my XAML:

<Window x:Class="Playground.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        xmlns:local="clr-namespace:Playground"
        mc:Ignorable="d"
        Title="MainWindow" Height="350" Width="525"
        d:DataContext="{d:DesignInstance local:MainViewModel}">
	<Window.DataContext>
		<local:MainViewModel />
	</Window.DataContext>
	<StackPanel Orientation="Vertical">
	    <TextBlock Text="My simple Slider"/>
	    
	    <TextBox Text="{Binding DecimalValue, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}"/>
	    <Slider
		    AutomationProperties.AutomationId="Slider_1"
		    Minimum="0"
		    Maximum="100"
		    Margin="4"
		    VerticalAlignment="Center"
		    Value="{Binding DecimalValue, Mode=TwoWay, TargetNullValue=50, FallbackValue=50}"/>
    </StackPanel>
</Window>

This is my ViewModel:

using GalaSoft.MvvmLight;


namespace Playground;

public class MainViewModel : ViewModelBase
{
	private decimal? _decimalValue;
	public decimal? DecimalValue
	{
		get => _decimalValue;
		set
		{
			if (_decimalValue.Equals(value))
			{
				return;
			}

			_decimalValue = value;
			RaisePropertyChanged();
		}
	}
}

This is a minimal showcase for my issue. The decimal value needs to be nullable. The slider sets the value only to null, if I click on it to move it one step for- or backward if I reach the TargetNullValue. This won't happen again, after I slided once.

[lindexi]

@GrafJ-AUMA After I test your demo code, I found that the behavior met my expectations. May you need only the FallbackValue, not the TargetNullValue property, such as:

        <Slider
            AutomationProperties.AutomationId="Slider_1"
            Minimum="0"
            Maximum="100"
            Margin="4"
            VerticalAlignment="Center"
            Value="{Binding DecimalValue, Mode=TwoWay,  FallbackValue=50}"/>

[GrafJ-AUMA]

Basically yes, but then I get some errors in my debug output: System.Windows.Data Warning: 6 : 'SystemConvertConverter' converter failed to convert value '<null>' (type '<null>'); fallback value will be used, if available. BindingExpression:Path=DecimalValue;

[lindexi]

@GrafJ-AUMA I think that this debug error output as my expected. converter failed xx; fallback value will be used... and then use the FallbackValue property.

[GrafJ-AUMA]

Okay, but how can I handle NULL values without any warning? Shouldn't be the TargetNulLValue be used to handle this? Or am I wrong at this point?

[lindexi]

Okay, but how can I handle NULL values without any warning? Shouldn't be the TargetNulLValue be used to handle this? Or am I wrong at this point?

@GrafJ-AUMA Good question. I'm also looking for the best practice methods.