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Why does the following piece of XAML give me a XamlParseException with the (meaningless) message "Expression type is not a valid Style value." at runtime?

<Control x:Class="TestApp.Max.MyControl" xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation" xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" xmlns:d="http://schemas.microsoft.com/expression/blend/2008" xmlns:Max="clr-namespace:TestApp.Max" mc:Ignorable="d" d:DesignHeight="300" d:DesignWidth="300"> <Control.Template> <ControlTemplate> <TextBlock Name="txt" Text="{TemplateBinding Max:MyControl.Foo}" /> <ControlTemplate.Triggers> <Trigger Property="Control.IsMouseOver" Value="True"> <Setter TargetName="txt" Property="Text" Value="{TemplateBinding Max:MyControl.Bar}" /> </Trigger> </ControlTemplate.Triggers> </ControlTemplate> </Control.Template> </Control>

The offending line is

<Setter TargetName="txt" Property="Text" Value="{TemplateBinding Max:MyControl.Bar}" />

If I replace the TemplateBinding with a normal Binding it starts to work:

{Binding RelativeSource={RelativeSource TemplatedParent}, Path=Text2}

Shouldn't I be able to use a TemplateBinding since I am within a ControlTemplate? And what does the exception message really mean?

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    I just thought I would chip in that I am having a similar issue, Completely forgot about being able to use a regular binding, thanks for the work around. Commented Dec 4, 2013 at 21:28

2 Answers 2

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Binding TemplatedParent: In this line the value of the path2 is going to apply for the Text property of the TextBlock, so it runs fine.

In TemplateBinding : Have a close look at this, The resolved value of the Max:MyControl.Bar is going to act as the resource key for the Template binding [Here the value of Bar is not an actual value, instead it s a property key name ] which doesnt exists and so it throws the error "The given key was not present in the dictionary."

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Triggers work best when defined in stand-alone styles, not in-place content. Try defining your trigger in a style resource, then reference the style resource from your template.

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sorry, doesn't answer my question. i know how to work around the problem, but would like to know why it happens.

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