设置Label.Content以控制DataTrigger

Question

recently I've tried to make label style which would allow to display image or textblock depending on property being set or not. I've bound proper objects to labels' DataContext and prepared reusable style for these labels. Default content is textblock with Name as its text but if IsIconSet property is true, then content would change into image with corresponding IconPath as source.

Similar approach works perfectly with label's properties like background or cursor but in described scenario it breaks up when IsIconSet has the same value in both instances. Then it displays nothing for first label and correct textblock/image for second label.

I've tried to attach converter to Name and IconPath bindings in style in order to check what value is being passed but it seems that it isn't even invoked on first label.

Has anyone managed to do something similar? Am I missing something fundamental? Or maybe there is another approach for such behaviour? Any help will be appreciate.

Simplified code:

MainWindow

<StackPanel DataContext="{Binding First}">
    <Label Style="{StaticResource LabelStyle}" />
</StackPanel>
<StackPanel DataContext="{Binding Second}">
    <Label Style="{StaticResource LabelStyle}" />
</StackPanel>

Style

<Style x:Key="LabelStyle" TargetType="Label">
    <Setter Property="Content">
        <Setter.Value>
            <TextBlock Text="{Binding Name}"/>
        </Setter.Value>
    </Setter>
    <Style.Triggers>
        <DataTrigger Binding="{Binding IsIconSet}" Value="True">
            <Setter Property="Content">
                <Setter.Value>
                    <Image Source="{Binding IconPath}"/>
                </Setter.Value>
            </Setter> 
        </DataTrigger>
    </Style.Triggers>
</Style>

Classes

public class ViewModel : INotifyPropertyChanged
{
    private LabelClass _first;
    private LabelClass _second;

    public LabelClass First
    {
        get => _first;
        set
        {
            _first = value;
            OnPropertyChanged();
        }
    }
    public LabelClass Second
    {
        get => _second;
        set
        {
            _second = value;
            OnPropertyChanged();
        }
    }

    public ViewModel()
    {
        First = new LabelClass("First", "Resources/first.png");
        Second = new LabelClass("Second", "Resources/second.png");
    }

    public event PropertyChangedEventHandler PropertyChanged;

    private void OnPropertyChanged([CallerMemberName]string propertyName = null)
    {
        PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
    }
}

public class LabelClass : INotifyPropertyChanged
{
    private string _name;
    private string _iconPath;

    public string Name
    {
        get => _name;
        set
        {
            _name = value;
            OnPropertyChanged();
        }
    }
    public string IconPath
    {
        get => _iconPath;
        set
        {
            _iconPath = value;
            OnPropertyChanged();
            OnPropertyChanged("IsIconSet");
        }
    }
    public bool IsIconSet => !string.IsNullOrEmpty(IconPath);

    public LabelClass(string name, string iconPath = null)
    {
        Name = name;
        IconPath = iconPath;
    }

    public event PropertyChangedEventHandler PropertyChanged;

    private void OnPropertyChanged([CallerMemberName]string propertyName = null)
    {
        PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
    }
}

Answer 1

LabelStyle can be used by multiple Labels, but TextBlock and Image from Content setter are created just once, there is only one instance for all labels, but it cannot be displayed in multiple places. so it is displayed in only one.

to fix the issue use ContentTemplate, like shown below.

<Setter Property="Content" Value="{Binding}"/> line means that entire DataContext is considered as Label Content. Is is necessary for bindings in ContentTemplate

<Style x:Key="LabelStyle" TargetType="Label">
    <Setter Property="Content" Value="{Binding}"/>
    <Setter Property="ContentTemplate">
        <Setter.Value>
            <DataTemplate>
                <TextBlock Text="{Binding Name}"/>
            </DataTemplate>
        </Setter.Value>
    </Setter>
    <Style.Triggers>
        <DataTrigger Binding="{Binding IsIconSet}" Value="True">
            <Setter Property="ContentTemplate">
                <Setter.Value>
                    <DataTemplate>
                        <Image Source="{Binding IconPath}"/>
                    </DataTemplate>
                </Setter.Value>
            </Setter>
        </DataTrigger>
    </Style.Triggers>
</Style>

---

alternatively, convert TextBlock and Image to non-shared resources:

<Image Source="{Binding IconPath}" x:Key="Img" x:Shared="False"/>
<TextBlock Text="{Binding Name}" x:Key="Txt" x:Shared="False"/>

<Style x:Key="LabelStyle" TargetType="Label">
    <Setter Property="Content" Value="{StaticResource Txt}"/>
    <Style.Triggers>
        <DataTrigger Binding="{Binding IsIconSet}" Value="True">
            <Setter Property="Content" Value="{StaticResource Img}"/>
        </DataTrigger>
    </Style.Triggers>
</Style>