How to set DataContext within a control's resources tag?a

Question

I have a UserControl, where my DataContext is defined as an attribute within my tag. Anywhere outside of my UserControl's Resources, I can bind to properties within my DataContext. However, when I attempt to do this within my control's Resources, I encounter the following: System.InvalidOperationException: Must have non-null value for 'Binding'.

I suspect this is because the DataContext is not being applied within the scope (the Resource tag) I'm trying to use it in, not only because the Binding is null, but because the property I'm trying to bind to never gets called before the crash as well. My question is: how can I bind to a property within my DataContext within this specific scope? I've tried a few things at this point, such as giving the UserControl an x:Name and attempting to set a Source={x:Reference the_x:Name}, but to no avail.

Below, B and C are the properties within my DataContext with the aforementioned issue. A is fine, as that is just a property of my Style's TargetType.

<UserControl... (DataContext defined here)>
  <UserControl.Resources...>
    <Style...>
      <Style.Triggers>
        <MultiDataTrigger>
          <MultiDataTrigger.Conditions>
            <Condition Property="A" Value="D"/>
              <Condition>
                <Condition.Binding>
                  <MultiBinding Converter="{StaticResource SomeIMultiValueConverter}">
                    <Binding Path="B"/>
                    <Binding Path="C"/>
                  </MultiBinding>
                </Condition.Binding>
            <Condition.Value>
              <sys:Boolean>False</sys:Boolean>
            </Condition.Value>

15ee8f99-57ff-4f92-890c-b56153 15ee8f99-57ff-4f92-890c-b56153 · Accepted Answer · 2017-11-09T16:40:04

In the Style, the default Source for any bindings is going to be the DataContext of the target of the style, not that of the outermost element in the XAML file, or that of the owner of the Resources. If you apply a style a Button, for example, that'll be the DataContext of the Button.

However, that won't cause this exception anyway. A Binding with a null or wrong source is not a null reference to a Binding.

This code reproduces your exception. The problem is that when used in a MultiDataTrigger, the Condition must have a Binding.

<TextBlock>
    <TextBlock.Style>
        <Style TargetType="TextBlock">
            <Style.Triggers>
                <MultiDataTrigger>
                    <MultiDataTrigger.Conditions>
                        <!-- TextBlock has no property named A, so I'll use Text -->
                        <Condition Property="Text" Value="D"/>
                    </MultiDataTrigger.Conditions>
                </MultiDataTrigger>
            </Style.Triggers>
        </Style>
    </TextBlock.Style>
</TextBlock>

If you want a MultiTrigger on multiple regular properties of the target of the style, use a MultiTrigger.

If A is a property of the viewmodel rather than of the control, you don't want Property="A" anyway; you want Binding="{Binding A}":

<Condition Binding="{Binding A}" Value="D"/>

If A is a property of the UserControl, you want Binding="{Binding A, RelativeSource={RelativeSource AncestorType=UserControl}}".

<Condition 
    Binding="{Binding A, RelativeSource={RelativeSource AncestorType=UserControl}}"
    Value="D"
    />

How to set DataContext within a control's resources tag?a

1 Answers