Question

I am really at a loss here, and have been trying to find a solution to this for several hours. I am lost. I get the following exception during an operation that was working the last time I checked.

'{DependencyProperty.UnsetValue}' is not a valid value for property 'Foreground'.

It does not take me to where the error is occuring. It takes me to a page that says “No Source Available”, and nothing else. I have tried locating the error by placing breakpoints in various places, but it seems to fail at different points during each run through. The InnerException is null.

I have seen this question, as well as various articles from Google. I cannot figure out what is going on, and I do not know how to troubleshoot from here. The Visual Studio output doesn’t appear to give any more detailed information, but I will paste it on request. Please, any help is appreciated.

Answer 1

I’m willing to be you have a missing resource. If you do something like:

<Window x:Name="window" x:Class="WpfApplication4.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:sys="clr-namespace:System;assembly=mscorlib"
        xmlns:local="clr-namespace:WpfApplication4"
        Title="MainWindow" Height="350" Width="525" >
    <Window.Resources>
    <Style TargetType="Button">
        <Setter Property="Foreground" Value="{StaticResource NoSuchResourceKey}" />
    </Style>
    </Window.Resources>
    <StackPanel>
        <Button Content="Click Me" />
    </StackPanel>
</Window>

Then you will get such an exception. We can even use a ComponentResourceKey to produce this exception:

<Style TargetType="Button">
    <Setter Property="Foreground" Value="{StaticResource {ComponentResourceKey TypeInTargetAssembly={x:Type FrameworkElement}, ResourceId=NoSuchResourceKey}}" />
</Style>

There are few things here that cause the issue. Normally, you would get a compiler error saying a resource doesn’t exist when using StaticResource. Such as in this case:

<Button Content="Click Me" Foreground="{StaticResource NoSuchResourceKey}" />

If instead, we had done:

<Button Content="Click Me" Foreground="{StaticResource {ComponentResourceKey TypeInTargetAssembly={x:Type FrameworkElement}, ResourceId=NoSuchResourceKey}}" />

Then you would get a different exception (XamlParseException), saying:

Provide value on ‘System.Windows.StaticResourceExtension’ threw an exception.’ Line number ‘6’ and line position ’22’.

With an internal exception of:

Cannot find resource named ‘TargetType=System.Windows.FrameworkElement ID=NoSuchResourceKey’. Resource names are case sensitive.

Which all leads us to the real problem (a missing resource). The reason the first two examples don’t give us a useful exception, is that we are not setting the Foreground property. We are setting the Value property on a Setter object. So when the resource is not found, DependencyProperty.UnsetValue is used. Which is perfectly valid for the Setter.Value property.

Later, when the Style is applied to the Button we get the exception, because that’s when the DependencyProperty.UnsetValue is actually assigned to the Button.Foreground property.

To fix this issue, I’d search over your entire solution for Property="Foreground" and look for any instances that use a resource that doesn’t exist.

I should add that you don’t get an exception when using DynamicResource, because in that value passed to the Button.Foreground property is a “special value” (which allows deferred lookup). This “special value” won’t assign the given property unless the resource is found.