WPF combining multiple DrawingBrush

Question

Is it possible to combine multiple DrawingBrushes into one or, alternatively, is there a way to assign multiple Brushes to my Grid?

<Grid Name="gridContainer" Background="{StaticResource GridTile, RectangeGridTile}">

obviously the above code would not work. Source code:

<DrawingBrush x:Key="GridTile" Viewport="0,0,4,16"  ViewportUnits="Absolute" TileMode="Tile">
    <DrawingBrush.Drawing>
        <DrawingGroup>
            <GeometryDrawing Geometry="M0,0 L1,0 1,0.05, 0,0.05Z"   Brush="Black" />
            <GeometryDrawing Geometry="M0,0 L0,1 0.1,1, 0.1,0Z"     Brush="Black" />
        </DrawingGroup>
    </DrawingBrush.Drawing>
</DrawingBrush>

<DrawingBrush x:Key="RectangeGridTile" Viewport="0,0,120,48" ViewportUnits="Absolute" TileMode="Tile">
    <DrawingBrush.Drawing >
        <DrawingGroup>
            <GeometryDrawing Geometry="M0,0 L1,0 1,0.05, 0,0.05Z"   Brush="Black" />
            <GeometryDrawing Geometry="M0,0 L0,1 0.1,1, 0.1,0Z"     Brush="Black" />
        </DrawingGroup>
    </DrawingBrush.Drawing>
</DrawingBrush>

Answer 1

custom brushes are not supported in WPF (brush types are internal and cannot be inherited from), so creating a brush is not possible.

You could use a MarkupExtension to simulate the behaviour of a custom brush, which allows you to use XAML syntax and provide a custom value, which allows us to use the built-in SolidColorBrush set to the valueof, lets say, two mixing colors:

/// <summary>
/// Markup extension to mix two SolidColorBrushes together to produce a new SolidColorBrush.
/// </summary>
[MarkupExtensionReturnType(typeof(SolidColorBrush))]
public class MixedColorBrush : MarkupExtension, INotifyPropertyChanged
{
    /// <summary>
    /// The foreground mix color; defaults to white.  
    /// If not changed, the result will always be white.
    /// </summary>
    private SolidColorBrush foreground = Brushes.White;

    /// <summary>
    /// The background mix color; defaults to black.  
    /// If not set, the result will be the foreground color.
    /// </summary>
    private SolidColorBrush background = Brushes.Black;

    /// <summary>
    /// PropertyChanged event for WPF binding.
    /// </summary>
    public event PropertyChangedEventHandler PropertyChanged;

    /// <summary>
    /// Gets or sets the foreground mix color.
    /// </summary>
    public SolidColorBrush Foreground
    {
        get 
        { 
            return this.foreground; 
        }
        set 
        { 
            this.foreground = value; 
            this.NotifyPropertyChanged("Foreground"); 
        }
    }

    /// <summary>
    /// Gets or sets the background mix color.
    /// </summary>
    public SolidColorBrush Background
    {
        get 
        { 
            return this.background; 
        }
        set 
        { 
            this.background = value; 
            this.NotifyPropertyChanged("Background"); 
        }
    }

    /// <summary>
    /// Returns a SolidColorBrush that is set as the value of the 
    /// target property for this markup extension.
    /// </summary>
    /// <param name="serviceProvider">Object that can provide services for the markup extension.</param>
    /// <returns>The object value to set on the property where the extension is applied.</returns>
    public override object ProvideValue(IServiceProvider serviceProvider)
    {
        if (this.foreground != null && this.background != null)
        {
            // Create a new brush as a composite of the old ones
            // This does simple non-perceptual additive color, e.g 
            // blue + red = magenta, but you can swap in a different
            // algorithm to do subtractive color (red + yellow = orange)
            return new SolidColorBrush(this.foreground.Color + this.background.Color);
        }

        // If either of the brushes was set to null, return an empty (white) brush.
        return new SolidColorBrush();
    }

    /// <summary>
    /// Raise the property changed event.
    /// </summary>
    /// <param name="propertyName">Name of the property which has changed.</param>
    protected void NotifyPropertyChanged(string propertyName)
    {
        if (this.PropertyChanged != null)
        {
            this.PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
        }
    }
}

Which can then be used from XAML as you would a normal brush:

<Grid>
    <Grid.Background>
        <local:MixedColorBrush Foreground="Blue" Background="Red"/>
    </Grid.Background>
</Grid>

Or by using the markup extension syntax:

<Grid Background="{local:MixedColorBrush Foreground=Blue, Background=Red}">

Note:

You cannot use DynamicResource or StaticResource references to bind the values to other resources in your application. MarkupExtension is not a DependencyObject, and resource binding only works on DependencyObjects; the built-in Brushes are DependencyObjects, which is why binding works with traditional brushes.