WPF DataContext PropertyChanged is Null

Question

I have two WPF components, one is containing a list of the other one. I set the DataContext in the parent WPF component, so I can define a property. Unfortunately, the PropertyChanged of my model is always null, therefore my label (computed property) is not updated.

The component containing the view:

<Grid x:Name="ValidationGrid" Background="White">
    <subView:SingleStepView x:Name="Step01" Grid.Column="0" Grid.Row="0" >
        <subView:SingleStepView.DataContext>
            <subView:SingleStepModel CurrentStep="Step1"/>
        </subView:SingleStepView.DataContext>
    </subView:SingleStepView>
    <subView:SingleStepView x:Name="Step02" Grid.Column="1" Grid.Row="0" >
        <subView:SingleStepView.DataContext>
            <subView:SingleStepModel CurrentStep="Step2"/>
        </subView:SingleStepView.DataContext>
    </subView:SingleStepView>
</Grid>

The View with the label:

<UserControl x:Name="SingleStep" x:Class="MyNameSpace.Views.SubView.SingleStepView"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
             xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
             xmlns:local="clr-namespace:MyNameSpace.Views.SubView"
             mc:Ignorable="d"
             d:DesignHeight="70" d:DesignWidth="146" MouseDoubleClick="SingleStep_MouseDoubleClick">
    <UserControl.Resources>
        <local:SingleStepModel x:Key="SingleStepModel" />
    </UserControl.Resources>
    <Grid x:Name="SingleStepGrid" DataContext="{StaticResource SingleStepModel}">
        <Label x:Name="LblStepName" Content="{Binding StepName, UpdateSourceTrigger=PropertyChanged}" HorizontalAlignment="Center" VerticalAlignment="Center" Grid.Row="1" Grid.Column="0"/>
    </Grid>
</UserControl>

The code for the Model:

public partial class SingleStepModel : INotifyPropertyChanged
{
    public event PropertyChangedEventHandler PropertyChanged;
    private void OnPropertyRaised(string propertyname)
    {
        if (PropertyChanged != null)
        {
            PropertyChanged(this, new PropertyChangedEventArgs(propertyname));
        }
    }
    public String StepName
    {
        get
        {
            return IrrelevantLogic();
        }
    }
}

On an other property within this model I call:

OnPropertyRaised(nameof(StepName));

By using the debugger, I can see that the PropertyChanged is always null and therefore not called. As such, my label is not changed even tough I can see that the StepName would contain a new value.

I have the suspicion that because I am assigning the DataContext in the ValidationGrid, the INotifyPropertyChanged does not get initialized correctly, but I was unable to resolve this issue.

Any help or guidance in the right direction is appreciated.

Answer 1

The SingleStepView control has a private view model which makes the DataContext assignment in the "component containing the view" useless. The two SingleStepModel instances assigned there are simply ignored.

Remove the DataContext assignment from the Grid, so that the Label's DataContext will automatically be inherited from the UserControl.

It is also pointless to set UpdateSourceTrigger=PropertyChanged, because that would only affect a TwoWay or OneWayToSource Binding.

<UserControl x:Class="MyNameSpace.Views.SubView.SingleStepView" ...>
    <Grid x:Name="SingleStepGrid">
        <Label Content="{Binding StepName}" .../>
    </Grid>
</UserControl>

---

If you want to declare a DataContext for the UserControl that is used in the XAML Designer, set the design-time DataContext, d:DataContext.