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Question

I am really in trouble 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 worked the last time I checked.

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

This does not lead me to where the error occurs. This leads me to a page that says “There are no available sources,” and nothing more. I tried to find the error by setting breakpoints in different places, but it seems to crash at different points during each run. InnerException is null.

I saw this question , as well as various articles from Google. I can’t understand what’s going on, and I don’t know how to fix the problem here. The output from Visual Studio does not seem to provide more details, but I will embed it on request. Please any help is appreciated.

Answer 1

I want you to have the 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 ComponentResourceKey to create this exception:

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

There are a few things that are causing the problem. Typically, you will get a compiler error indicating that the resource does not exist when using StaticResource . For example, in this case:

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

If instead we did:

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

Then you will get another exception (XamlParseException) by saying:

Provide a value in "System.Windows.StaticResourceExtension". Line number "6" and line position "22".

With an internal exception:

Cannot find a resource with the name "TargetType = System.Windows.FrameworkElement ID = NoSuchResourceKey". Resource names are case sensitive.

All this leads us to a real problem (a missing resource). The reason the first two examples do not give us a useful exception is because we do not set the Foreground property. We set the Value property to the Setter object. Therefore, when the resource is not found, DependencyProperty.UnsetValue used. This is great for the Setter.Value property.

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

To fix this problem, I would search your entire solution for Property="Foreground" and look for any instances that use a resource that does not exist.

I must add that you are not getting an exception when using DynamicResource , because this value passed to the Button.Foreground property has a "special value" (which allows you to delay the search). This "special value" will not assign the specified property if the resource is not found.

Answer 2

Have you rebuilt your project? Your PDB files may not match your DLLs, so VS may not load them. As for the error itself, most likely the binding to the Foreground property Foreground blame.