Question

This is quite obscure, I may just be missing something extremely simple.

Scenario 1

Lets say I create a gradient brush, like this in my <Window.Resources> section:

<LinearGradientBrush x:Key="GridRowSelectedBackBrushGradient" StartPoint="0,0" EndPoint="0,1">
    <GradientStop Color="#404040" Offset="0.0" />
    <GradientStop Color="#404040" Offset="0.5" />
    <GradientStop Color="#000000" Offset="0.6" />
    <GradientStop Color="#000000" Offset="1.0" />
</LinearGradientBrush>

Then much later on, I want to override the HighlightBrushKey for a DataGrid. I have basically done it like this (horrible);

<LinearGradientBrush x:Key="{x:Static SystemColors.HighlightBrushKey}"
                     GradientStops="{Binding Source={StaticResource GridRowSelectedBackBrushGradient}, Path=GradientStops}"
                     StartPoint="{Binding Source={StaticResource GridRowSelectedBackBrushGradient}, Path=StartPoint}"
                     EndPoint="{Binding Source={StaticResource GridRowSelectedBackBrushGradient}, Path=EndPoint}" />

This is obviously not the most slick way of referencing a resource. I also came up with the following problem, which is almost identical.

Scenario 2

Say I created two colors in my <Window.Resources> markup, like so:

<SolidColorBrush x:Key="DataGridRowBackgroundBrush" Color="#EAF2FB" />
<SolidColorBrush x:Key="DataGridRowBackgroundAltBrush" Color="#FFFFFF" />

Then later on, I want to supply them in an Array, which feeds the ConverterParameter on a Binding so I can supply the custom Converter with my static resource instances:

<Setter Property="Background">
    <Setter.Value>
        <Binding RelativeSource="{RelativeSource Mode=Self}" 
                 Converter="{StaticResource BackgroundBrushConverter}">
            <Binding.ConverterParameter>
                <x:Array Type="{x:Type Brush}">
                    <SolidColorBrush Color="{Binding Source={StaticResource DataGridRowBackgroundBrush}, Path=Color}" />
                    <SolidColorBrush Color="{Binding Source={StaticResource DataGridRowBackgroundAltBrush}, Path=Color}" />
                </x:Array>
            </Binding.ConverterParameter>
        </Binding>
    </Setter.Value>
</Setter>

What I’ve done is attempt to rereference an existing resource, but in my efforts I’ve actually recreated the resource, and bound the properties so they match. Again, this is not ideal.

Because I’ve now hit this problem at least twice, is there a better way?

Thanks, Tom

Answer 1

I was wondering when someone was going to ask about this.

What you want to do in Scenario 1 is to effectively give a single resource an “alias.” This is easily done by markup that seems obvious only after you see it. Suppose we have this in our App.xaml or somewhere:

<ResourceDictionary>
  <LinearGradientBrush x:Key="GridRowSelectedBackBrushGradient" ... />
</ResourceDictionary>

To include an alias in another ResourceDictionary, just:

<ResourceDictionary>
  <StaticResourceExtension x:Key="{x:Static SystemColors.HighlightBrushKey}"
                           ResourceKey="GridRowSelectedBackBrushGradient" />
</ResourceDictionary>

This looks up the brush object in the first ResourceDictionary and adds the same object to the second ResourceDictionary under a new key. This also works within a single ResourceDictionary.

For your Scenario 2 the solution is just as simple:

<Binding.ConverterParameter>
  <x:Array Type="{x:Type Brush}">
    <StaticResourceExtension ResourceKey="DataGridRowBackgroundBrush" />
    <StaticResourceExtension ResourceKey="DataGridRowBackgroundAltBrush" />
  </x:Array>
</Binding.ConverterParameter>

Again, the actual Brush objects found via the ResourceKey are added directly to the Brush[] array. No new Brush is created.

I think we’re all so used to using StaticResourceExtension with markup extension syntax (eg {StaticResource Xyz}) that it’s easy to forget that it can also be used with regular element syntax as well.