#1,200 – Overriding a Default Style

In WPF, you often set up default styles to set a series of properties on all instances of a certain type of element. Below, we set a default style that applies to all TextBlock elements, using the TargetType attribute. All properties in the style will be set for all instances of the TextBlock element.

<Window.Resources>
    <ResourceDictionary>
        <Style TargetType="TextBlock">
            <Setter Property="Foreground" Value="Purple"/>
            <Setter Property="FontStyle" Value="Italic"/>
        </Style>
    </ResourceDictionary>
</Window.Resources>

<StackPanel Margin="10">
    <TextBlock Text="Fafnir"/>
    <TextBlock Text="Siegfried"/>
    <TextBlock Text="Brynhildr"/>
</StackPanel>

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Sometimes you’ll be working in an environment where these default styles have been defined for you, but you want to override them.

The first way to override a default style is to define one of your own. Below, we define a named style and use it on the second TextBlock. (The style is added to the ResourceDictionary that we showed above).

<Style x:Key="BlueBig" TargetType="TextBlock">
   <Setter Property="Foreground" Value="Blue"/>
   <Setter Property="FontSize" Value="20"/>
</Style>
<TextBlock Text="Siegfried" Style="{StaticResource BlueBig}"/>

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The second way to override a default style is to revert the element to having no style at all using the x:Null markup extension. Below, the default style exists in our application but the first and third elements indicate that that they don’t want to use it. Notice that these TextBlocks appear normally.

<TextBlock Text="Fafnir" Style="{x:Null}"/>
<TextBlock Text="Siegfried" Style="{StaticResource BlueBig}"/>
<TextBlock Text="Brynhildr" Style="{x:Null}"/>

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#1,199 – Complete WPF Command Example

Below is a cheat sheet for creating a custom command in WPF.

Use a static property to expose a command object from the ViewModel or code-behind. RoutedUICommand allows for associating control text with the command itself.

private static RoutedUICommand _pressMeCommand = 
    new RoutedUICommand("Press Me", "PressMe", typeof(MainWindow));
public static RoutedUICommand PressMeCommand
{
    get { return _pressMeCommand; }
}

Add command handlers (in code-behind or ViewModel).

private void PressMe_CanExecute(object sender, CanExecuteRoutedEventArgs e)
{
    e.CanExecute = CowboyCanTalk;
}
 
private void PressMe_Executed(object sender, ExecutedRoutedEventArgs e)
{
    MessageBox.Show("Howdy howdy I'm a cowboy");
}

Bind the command to its handlers, done here in XAML for a window.

<Window.CommandBindings>
    <CommandBinding Command="local:MainWindow.PressMeCommand" 
                    CanExecute="PressMe_CanExecute" 
                    Executed="PressMe_Executed"/>
</Window.CommandBindings>

Wire a button up to the command. Note use of command’s Text property for the button’s Content (text).

<Button Command="local:MainWindow.PressMeCommand" 
        Content="{Binding RelativeSource={RelativeSource Self}, Path=Command.Text}" />

We can also wire up the command to other GUI elements, e.g. a menu item in a context menu. The menu item’s text is set up automatically.

<Window.ContextMenu>
    <ContextMenu>
        <MenuItem Command="{x:Static local:MainWindow.PressMeCommand}" />
    </ContextMenu>
</Window.ContextMenu>

#1,198 – Selectively Enabling Child Elements in a Disabled Panel

When you set IsEnabled to false in a panel, all child elements in that panel are disabled. You cannot selectively enabled child elements in the panel.

You may, however, want to selectively enable child elements in a panel. (E.g. Disable entire panel, then set IsEnabled=True, IsReadOnly=True on TextBox controls so that you can copy text).

One possible solution is to define a new control that inherits from TextBox and does not coerce the value of IsEnabled.

    public class CanEnableTextBox : TextBox
    {
        static CanEnableTextBox()
        {
            CanEnableTextBox.IsEnabledProperty.OverrideMetadata(typeof(CanEnableTextBox),
                new System.Windows.UIPropertyMetadata(true,
                    new PropertyChangedCallback(IsEnabledPropertyChanged),
                    new CoerceValueCallback(CoerceIsEnabled)));

        }

        private static void IsEnabledPropertyChanged(DependencyObject source, DependencyPropertyChangedEventArgs args)
        {
            // Overriding PropertyChanged results in merged metadata, which is what we want--
            // the PropertyChanged logic in UIElement.IsEnabled will still get invoked.
        }

        private static object CoerceIsEnabled(DependencyObject source, object value)
        {
            return value;
        }
    }

You can now use this control in a panel that has IsEnabled set to false and you’ll be able to set IsEnabled on the child TextBox.

#1,197 – Autosizing in a Grid with Maximum Size

You can use autosizing for grid rows or columns to have the row or column automatically size to fit its contents. There are times, however, when you want the row or column to fit its contents, but only up to a limit. E.g. Autosize a row height, but only to a maximum of 20% of the size of the entire grid.

You can autosize within limits by using the MaxHeight attribute and a value converter.

Below, we have a grid with two rows, each containing a ListBox. The second row is set to autosize and the first to use all remaining space.

<Window.Resources>
    <ResourceDictionary>
        <local:MaxHeightConverter x:Key="MaxHeightConverter"/>
        <sys:Double x:Key="adjMaxHeightRatio">0.2</sys:Double>
    </ResourceDictionary>
</Window.Resources>

<Grid x:Name="grdRoot">
    <Grid.RowDefinitions>
        <RowDefinition Height="*"/>
        <RowDefinition Height="Auto"/>
    </Grid.RowDefinitions>

    <ListBox Grid.Row="0" ItemsSource="{Binding Dogs}" />

    <ListBox Grid.Row="1" ItemsSource="{Binding MoreDogs}" MaxHeight="{Binding ElementName=grdRoot, Path=ActualHeight, Converter={StaticResource MaxHeightConverter}, ConverterParameter={StaticResource adjMaxHeightRatio}}" />
</Grid>

The second ListBox will take as much room as it needs (grow to fit all elements), until it reaches 20% of the height of the parent grid. At that point, it will be constrained and get a vertical scrollbar.

Here’s the code for the converter:

public class MaxHeightConverter : IValueConverter
{
    public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
    {
        double pctHeight = (double)parameter;

        if ((pctHeight <= 0.0) || (pctHeight > 100.0))
            throw new Exception("MaxHeightConverter expects parameter in the range (0,100]");

        return ((double)value * pctHeight);
    }

    public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
    {
        throw new NotImplementedException();
    }
}

#1,196 – Making a Window Fully Transparent

You can make the background of a window fully transparent by setting its Background property to Transparent, settings AllowsTransparency to true and setting WindowStyle to None.

Below is an example.  Note that because WindowStyle is None, the window doesn’t have a normal border and we can’t therefore move the window around.

<Window x:Class="WpfApplication1.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="Transparent"
        Height="190" Width="268"
        Background="Transparent"
        AllowsTransparency="True"
        WindowStyle="None"
        WindowStartupLocation="CenterScreen">

    <StackPanel>
        <TextBlock Text="Topper (1937) starred Cary Grant and Constance Bennett"
                   TextWrapping="Wrap"
               HorizontalAlignment="Center" Height="40" Margin="47,0"/>
        <Button Content="Ok, Got It"
                Padding="10,5" Margin="10"
                HorizontalAlignment="Center"
                Click="Button_Click"/>
    </StackPanel>
</Window>

Here’s the window in action. (We added a handler to the button’s Click event to close the window when clicked).

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#1,195 – Making a Window Partially Transparent

You can use an alpha value when setting a background color to make the background of a control partially transparent.  (E.g. Making a tooltip partially transparent).

You can make the background of a Window transparent, or partially transparent, by doing three things:

  • Set the Background to Transparent or use a background that is partially transparent
  • Set the AllowsTransparency property to true
  • Set the WindowStyle to None

The WindowStyle must be set to None when setting AllowsTransparency to true.

Here’s an example:

<Window x:Class="WpfApplication1.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="Transparent"
        Height="190" Width="268"
        Background="#D5F0F0FF"
        AllowsTransparency="True"
        WindowStyle="None">

    <StackPanel>
        <Button Content="Click Me"
                Padding="10,5" Margin="10"
                HorizontalAlignment="Center"/>
    </StackPanel>
</Window>

We now get a window that doesn’t include a border (so we can’t move it), but whose background is partially transparent.  The controls within a transparent window are opaque and all work fine.

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#1,194 – DesiredSize of Child Elements Includes Margins

After the measure phase, during which a custom element calls the Measure method on each of its child elements, each child element will set its DesiredSize property to indicate how much space it wants.

The DesiredSize of a child element accounts for any margins that have been set on that child.  Assume that we have the following XAML (MyElement is a custom element with a single child).

    <Grid>
        <Grid.ColumnDefinitions>
            <ColumnDefinition/>
            <ColumnDefinition/>
        </Grid.ColumnDefinitions>

        <loc:MyElement x:Name="Left">
            <loc:MyElement.Child>
                <Label Content="Doowahditty"/>
            </loc:MyElement.Child>
        </loc:MyElement>

        <loc:MyElement Grid.Column="1" x:Name="Right">
            <loc:MyElement.Child>
                <Label Content="Doowahditty" Margin="10"/>
            </loc:MyElement.Child>
        </loc:MyElement>
    </Grid>

We can see that the child element uses a margin.
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We can look at the value of the DesiredSize property after Measure has been called on the child element.  We can see that in the second case, the desired size is larger, indicating that the child wants to add a margin.

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