WPF从DataTemplate样式绑定到ViewModel属性

Question

I am trying to bind ForeGround color of all TextBlock items to a ViewModel property. The TextBlock elements locate under a Grid that itself is defined under DataTemplate . This whole code is defined under a UserControl .

I am trying to use RelativeSource binding to find the UserControl 's DataContext and get the property I need.

XAML:

<my:MapControl>
    <my:MapControl.Resources>
        <ResourceDictionary>
            <DataTemplate x:Key="SomeTemplate">
                <Grid>
                     <Grid.RowDefinitions>
                          <RowDefinition />
                          <RowDefinition />
                     </Grid.RowDefinitions>
                     <Grid.Style>
                         <Style TargetType="Grid">
                              <Setter Property="TextElement.Foreground" Value="{Binding RelativeSource={RelativeSource AncestorType={x:Type UserControl}}, Path=DataContext.TextColor}" />
                         </Style>
                     </Grid.Style>
                     <TextBlock Grid.Column="0" />
                     <TextBlock Grid.Column="1" />
                </Grid>
            </DataTemplate>
        </ResourceDictionary>
    </my:MapControl.Resources>
</my:MapControl>

ViewModel:

public class MapViewModel
{
    public virtual string TextColor
    {
        get { return _textColor; }
        set
        {
            _textColor = value;
            this.RaisePropertyChanged("TextColor");
        }
    }
    private string _textColor = "Black";
}

The above binding doesn't work. If I change the Value binding to a hard-coded value, like "Red" for example, the Foreground color on those TextBlocks are showing correctly.

How to get the binding to work with this setup?

Answer 1

Analysis

It seems the root cause — binding to an instance of the string type instead of an instance of the Brush type.

Some of the possible solutions:

1. Change the type of the TextColor property of the MapViewModel class to from the string type to the SolidColorBrush type and update the implementation of the MapViewModel class appropriately.
2. Create custom implementation of the IValueConverter interface which takes the string as the input and outputs an instance of the SolidColorBrush type.

Answer 2

What version of .NET are you using? Works fine with 4.5 but IIRC it didn't with earlier versions and you had to declare a solidcolorbrush explicitly:

<Style TargetType="Grid">
    <Setter Property="TextElement.Foreground">
        <Setter.Value>
            <SolidColorBrush Color="{Binding RelativeSource={RelativeSource AncestorType={x:Type Window}}, Path=DataContext.TextColor}" />
        </Setter.Value>
    </Setter>
</Style>

Whatever you do don't create a brush or any other UI resources in your viewmodel, it's a violation of MVVM.