WPF XAML C# - A TextBox defined in XAML is null

Question

I am definig several TextBoxes with XAML, but the Textboxes do not behave similar although the code for me is exactly the same.`

... /* this is the definition of 4 TextBoxes which work as expected  */
    <TextBox  x:Name="TBXRed"    Text ="0" TextAlignment="Center" Grid.Column="2" Grid.Row="1" Margin="5,320,130,10"  TextChanged="redChanged"   />
    <TextBox  x:Name="TBXGreen"  Text ="0" TextAlignment="Center" Grid.Column="2" Grid.Row="1" Margin="45,320,90,10"  TextChanged="greenChanged" />
    <TextBox  x:Name="TBXBlue"   Text ="0" TextAlignment="Center" Grid.Column="2" Grid.Row="1" Margin="85,320,50,10"  TextChanged="blueChanged" />
    <TextBox  x:Name="TBXBright" Text ="0" TextAlignment="Center" Grid.Column="2" Grid.Row="1" Margin="125,320,10,10"  TextChanged="brightChanged" />

...

           <Label   x:Name="LBLRadius" Content="radius "  Grid.Column="2" Grid.Row="1" FontSize="13" FontWeight="DemiBold" Foreground="MediumBlue" Height="30" />
        <Slider   x:Name="sliderRadius"  Orientation="Horizontal"  Width="100" Height="20"  TickFrequency="1" Value="20"
                  ValueChanged="sliderRadiusChanged" Interval="1" Minimum="0"  Maximum="80"  />
        <TextBox x:Name="TBXRadius" Text="20" TextAlignment="Center" Width="30"  Height="20" TextChanged="radiusChanged" HorizontalAlignment="Center" VerticalAlignment="Center"  />

in the code behind file there is the handling for the TextChanged Event (one example).

    private void sliderRedChanged(object sender, fRoutedPropertyChangedEventArgs<double> e)
    {
        double newVal = e.NewValue;
        adjustValueDisplay(0, newVal);
        sliderRed.Value = newVal;
    }`
       private void redChanged(object sender, TextChangedEventArgs e)
    {
        redCor = TBXRed.Text;
        redCorI = Convert.ToInt32(redCor);
        sliderRed.Value =Convert.ToDouble(redCorI);
    }

    private void adjustValueDisplay(int aPos, double aValue)
    {
        switch (aPos)
        {
            case 0:
                redCor  = Convert.ToString(Math.Truncate(aValue));
                redCorI = Convert.ToInt32(redCor);
                TBXRed.TextChanged -= redChanged;
                TBXRed.Text = redCor;
                TBXRed.TextChanged += redChanged;
                ProjectParms.SetAsString("redCor", redCor);
                break;
                .......
            default:
                break;
        }
    }

This works very fine, but a little bit later I wont to define the parameters for an unsharp masking function

<StackPanel x:Name="SharpenVals" Grid.Row="3" Orientation="Horizontal" Grid.ColumnSpan="3" Grid.RowSpan="1" >
        <Label  Name="LBLSharpen"  Content="Sharpen:    on/off " FontSize="14" FontWeight="DemiBold" Foreground="MediumBlue" Height="30" Width="125" />
        <CheckBox x:Name="CBXDoSharpen"  Content="   " FontSize="13" FontWeight="DemiBold" Foreground="MediumBlue" Height="16" RenderTransformOrigin="-0.197,0.815" Width="30" />
        <Label   x:Name="LBLRadius" Content="radius "  Grid.Column="2" Grid.Row="1" FontSize="13" FontWeight="DemiBold" Foreground="MediumBlue" Height="30" />
        <Slider   x:Name="sliderRadius"  Orientation="Horizontal"  Width="100" Height="20"  TickFrequency="1" Value="20"
                  ValueChanged="sliderRadiusChanged" Interval="1" Minimum="0"  Maximum="80"  />
        <TextBox x:Name="TBXRadius" Text="20" TextAlignment="Center" Width="30"  Height="20" TextChanged="radiusChanged" HorizontalAlignment="Center" VerticalAlignment="Center"  />
        
        <Label   x:Name="LBLStrength" Content="  strength "  Grid.Column="2" Grid.Row="1" FontSize="13" FontWeight="DemiBold" Foreground="MediumBlue" Height="30" />
        <Slider   x:Name="sliderStrength"  Orientation="Horizontal"  Width="80" Height="20"  TickFrequency="1" Value="20"
                  ValueChanged="sliderStrengthChanged" Interval="1" Minimum="0"  Maximum="80"  />
        <TextBox Name="TBXStrength" Text="55" TextAlignment="Center" Width="30"  Height="20" TextChanged="strengthChanged"  />

        <Label  x:Name="LBLThreshold" Content="  threshold "  Grid.Column="2"  Grid.Row="1" FontSize="13" FontWeight="DemiBold" Height="30"
                 Foreground="MediumBlue"  />
        <TextBox x:Name="TBXThreshold" Text="2" TextAlignment="Center"  Width="30"  Height="20" TextChanged="thresholdChanged" />
        
        <Label  x:Name="LBLBrightLim" Content="  brightness limit  "  Grid.Column="2"  Grid.Row="1" FontSize="13" FontWeight="DemiBold" Height="30"
                 Foreground="MediumBlue"  />
        <TextBox Name="TBX_BrightLim" Text="200" TextAlignment="Center"  Width="30"  Height="20" TextChanged="thresholdChanged" />
    </StackPanel>


    private void sliderRadiusChanged(object sender, RoutedPropertyChangedEventArgs<double> e)
    {
        double newVal = e.NewValue;
        adjustSharpeningDisplay(0, newVal);
        sliderRadius.Value = Math.Round(newVal);
    }
        public void radiusChanged(object sender, TextChangedEventArgs e)
    {
        radius = TBXRadius.Text;
        radiusI = Convert.ToInt32(radius);
        sliderRadius.Value = radiusI;
    }

Code behind section:

    private void sliderRadiusChanged(object sender, RoutedPropertyChangedEventArgs<double> e)
    {
        double newVal = e.NewValue;
        adjustSharpeningDisplay(0, newVal);
        sliderRadius.Value = Math.Round(newVal);
    }
    public void radiusChanged(object sender, TextChangedEventArgs e)
    {
        radius = TBXRadius.Text;
        radiusI = Convert.ToInt32(radius);
        sliderRadius.Value = radiusI;
    }

        private void adjustSharpeningDisplay(int aPos, double aValue)
    {
        switch (aPos)
        {
            case 0:
                radius = Convert.ToString(Math.Truncate(aValue));
                radiusI = Convert.ToInt32(radius);
                TBXRadius.TextChanged -= radiusChanged;
                TBXRadius.Text = radius;
                TBXRadius.TextChanged += new System.Windows.Controls.TextChangedEventHandler(this.radiusChanged);
                ProjectParms.SetAsString("radius", radius);
                break;
      .......
            default:
                break;
        }
    }

and this, although it looks very the same for me, does not work.

I do get an System.NullReferenceException: "Object reference not set to an instance of an object." saying, that TBXRadius is null. (and this I do not understand)

Has anyone an idea why this two pieces of code behave different and why TBXRadius is not in the visible view.

The attached Image shows how the window should look !

Any recommendation is welcome.

Greetings Wolfgang

That is the line in the "adjustSharpeningDisplay" function (rather at the end of the note) reading "TBXRadius.TextChanged -= radiusChanged;" — Wolfgang Kurz, Aug 30 '21 at 16:50
None of those line refer to a `TBXRadius`, though. We'll probably need to see a self-contained example. — 500 - Internal Server Error, Aug 30 '21 at 16:55
The idea is, that you can change the radius value either with the slider of via the entry of a value in the textbox - which also updates the slider settinges. — Wolfgang Kurz, Aug 30 '21 at 16:58
In line 7 from top of Stackpanel "SharpenValues" TBXRadius is defined ! — Wolfgang Kurz, Aug 30 '21 at 17:02
This has nothing to do with the visual tree. Most certainly a TextChanged event handler is called during the execution of InitializeComponent, when not all generated fields are already initialized. Add a null check before accessing those fields. — Clemens, Aug 30 '21 at 17:03
The project is very complex, so it is very difficult to generate a subset of the code. But I can upload the whole project to may Domain, so that it can be downloaded for a test. — Wolfgang Kurz, Aug 30 '21 at 17:03
I will add this null check and I will try to delay the program after InitializeComponent to give it some time to complete. — Wolfgang Kurz, Aug 30 '21 at 17:07
"*delay the program*" is pointless. The problem occurs *during* the execution of InitializeComponent. You may however attach the event handlers not in XAML, but in code behind after InitializeComponent. — Clemens, Aug 30 '21 at 17:12
Hello Clemens :found th teason. The Event TextChanges is the wrong approach. it is better to usen a Binding refencing the vlaue of the slider. This code works fine and does exactly what I want it to do - many thanks for your assistance! — Wolfgang Kurz, Aug 31 '21 at 13:35

WPF XAML C# - A TextBox defined in XAML is null

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