Why am I getting an exception on my dependency property?

Question

Just spent a few hours trying to get my head around dependency properties (finding a lot of valuable info here on the site). I've written my very first dependency property, but it is not behaving as I would like it to. Can anybody have a look at my code and see if he/she can spot whats wrong? When trying to run the app I get a TargetInvocationException was thrown

  <Window x:Class="TextEditorMVVM.MainWindow"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:c="clr-namespace:TextEditorMVVM.ViewModel"
    Title="MainWindow" Height="660" Width="621" ResizeMode="CanResize" Background="Gray">
<Window.Resources>
    <ResourceDictionary>
        <ResourceDictionary.MergedDictionaries>
            <ResourceDictionary Source="pack://application:,,,/Resources/Dictionary1.xaml"/>
        </ResourceDictionary.MergedDictionaries>
        <c:TextEditorViewModel x:Key="TextEditorViewModel"></c:TextEditorViewModel>
    </ResourceDictionary> 
</Window.Resources>
<Border CornerRadius="10" BorderThickness="12" Background="#FF505050">
    <Border.BorderBrush>
        <LinearGradientBrush StartPoint="0,0" EndPoint="0,1">
            <GradientStop Color="LightGray" Offset="1" />
            <GradientStop Color="Gray" Offset="0" />
        </LinearGradientBrush>
    </Border.BorderBrush>
    <StackPanel DataContext="{StaticResource TextEditorViewModel}">
    <Menu Height="auto" Background="Transparent" Foreground="White">
        <MenuItem Width=" auto" Height="auto" Header="_File" VerticalAlignment="Center">
            <MenuItem Header="_New" Command="{Binding CreateNewTabCommand}"></MenuItem>
            <MenuItem Header="_Open" Command="{Binding OpenFileCommand}"></MenuItem>
            <MenuItem Header="_Save" Command="{Binding SaveFileCommand}"></MenuItem>
            <MenuItem Header="_Close" Command="{Binding CloseTabCommand}"></MenuItem>
            <MenuItem Header="_Print" Command="{Binding PrintCommand}"></MenuItem>
            <MenuItem Header="_Exit" Command="{Binding }"></MenuItem>
        </MenuItem>
        <MenuItem Width=" auto" Height="auto" Header="_Edit" VerticalAlignment="Center">
            <MenuItem Header="_Cut" Command="ApplicationCommands.Cut"></MenuItem>
            <MenuItem Header="_Copy" Command="ApplicationCommands.Copy"></MenuItem>
            <MenuItem Header="_Paste" Command="ApplicationCommands.Paste"></MenuItem>
        </MenuItem>
        <MenuItem Width=" auto" Height="auto" Header="_Help" VerticalAlignment="Center">
            <MenuItem Header="_Call Mikael" Command="{Binding }"></MenuItem>
            <MenuItem Header="_Call Semyon" Command="{Binding }"></MenuItem>
            <MenuItem Header="_Cry" Command="{Binding }"></MenuItem>
        </MenuItem>
        <Expander Header="Autosave" VerticalAlignment="Center" Foreground="White">
            <StackPanel Orientation="Horizontal">
                <CheckBox VerticalAlignment="Center" IsChecked="{Binding IsChecked, Mode=TwoWay}"></CheckBox>
                <TextBox Width="40" Height="20" Margin="4" ></TextBox>
                    <Label VerticalAlignment="Center" Foreground="White">seconds</Label>
            </StackPanel>
        </Expander>
    </Menu>
        <c:TransparentTb IsTransparent="False" Text="Why aren't you working????">
        </c:TransparentTb>
    <TabControl x:Name="_tabControl" ItemsSource="{Binding TestTab}" SelectedItem="{Binding SelectedTab}" Background="Transparent" BorderThickness="0">
        <TabControl.ItemContainerStyle>
            <Style TargetType="TabItem">
                <Setter Property="Header" Value="{Binding Title}"/>
                    <Setter Property="Style" Value="Transparent"/>
                    <Setter Property="Content" Value="{Binding InputText}"/>
                </Style>
        </TabControl.ItemContainerStyle>
    </TabControl>
</StackPanel>
</Border>

class TransparentTb : TextBlock
{
    static TransparentTb()
    {

    }


    {
        get { return (bool) GetValue(IsTranparentProperty); }
        set { SetValue(IsTranparentProperty, value); }
    }

    // Using a DependencyProperty as the backing store for IsTranparent.  This enables animation, styling, binding, etc...
    public static readonly DependencyProperty IsTranparentProperty =
        DependencyProperty.Register("IsTransparent", typeof (bool), typeof (TransparentTb),
                                    new UIPropertyMetadata(false, TransparentTb.IsTransparentPropertyChanged,
                                                           TransparentTb.IsTransparentCoerce, false)); 


    private static void IsTransparentPropertyChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
    {
        TransparentTb inst = (TransparentTb) d;

        if ((bool)e.NewValue == true)
        {
            inst.Background = Brushes.Transparent;
        }
        else inst.Background = Brushes.Black;
    }

    private static object IsTransparentCoerce(DependencyObject d, object value)
    {
        return value;
    }
}

Answer 1

Changing this line of Xaml... < Setter Property="Style" Value="Transparent"/>

To this...

<!-- <Setter Property="Style" Value="Transparent"/> -->

(i.e., comment it out)

Will avoid the exception.

Setting a style the way you are doing would entail referencing something already defined in the object tree like {StaticResource Transparent}.

The inner exception is 'object reference not set...', which makes it clear that the constructor cannot find a reference to "Transparent" the way it has been coded. Note that the exception is thrown after the TransparentTb is already constructed. You can prove this by setting breakpoints.

Also, I checked your TransparentTb code, and it works fine. The culprit is your TabItem Style Setter.