I’m working on an user interface demo application in WPF.
I have a group of expander on the ui, and I want the expander show some descriptive text besides its caption while the mouse over it.
I used a TextBlock element to display the descriptive text, which is a child element in the control’s visual tree.

At first, I tried to use Style and Trigger to achieve this effect, but failed. Since Style Trigger apply its modification to the exactly same element that fires the trigger, which means if I would like to modify the property TextBlock, I can only fire the trigger by moving mouse over the TextBlock it self but the control that contains the TextBlock.

I tried some other ways, but all failed. Finally, I found I can achieve the effect by Data Binding!
Since TextBlock’s Visibility Property is a Dependency Property, and the TextBlock is contained by the Expander Control!
So we can Bind TextBlock’s Visibility Property to Expander Control’s IsMouseOver Property!
To achieve this, we need Relative Binding Source to locate TextBlock’s ancestor element, Expander Control.
Then we need BooleanToVisibilityConverter to convert the bool type value to Visibility value.

Here is the Xaml Code:

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<BooleanToVisibilityConverter x:Key="BooleanToVisibilityConverter" />
<HierarchicalDataTemplate DataType="{x:Type m:CategoryItem}"
                          ItemsSource="{Binding Properties}">
    <Border>
        <Expander ToolTip="{Binding Tooltip}" IsExpanded="True">
            <Expander.Header>
                <StackPanel Orientation="Horizontal" >
                    <TextBlock Text="{Binding Name}"/>
                    <TextBlock Name="HeaderDescription"
                               Margin="10,0,0,0"
                               Text="{Binding Description}"
                               Foreground="Green"
                               Visibility="{Binding RelativeSource={RelativeSource FindAncestor, AncestorType={x:Type Expander}}, Path=IsMouseOver, Converter={StaticResource BooleanToVisibilityConverter}}"/>
                </StackPanel>
            </Expander.Header>
            <ListBox
                DataContext="{Binding Properties}"
                ItemsSource="{Binding}"
                IsSynchronizedWithCurrentItem="True"/>
        </Expander>
    </Border>
</HierarchicalDataTemplate>

I met a strange problem while I was evaluating the Xaml Services that introduced in .net 4.
I created 2 types for Xaml Serialization Test:

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public class Foo
{
    public Foo() { }
    public Foo(int a)
    {
        Random RND = new Random();
        Value = RND.Next();
        MyProperty = RND.Next();
        Complex.Init(); // Complex is value type, which is automatically instantiated by default constructor.
    }
    [Ambient]
    public int Value { get; private set; }
    [Ambient]
    public int MyProperty { get;  private set; }
    [Ambient]
    public Nested Complex;
}
public struct Nested
{
    public void Init()
    {
        this.Temp = Guid.NewGuid();
    }
    [Ambient]
    public Guid Temp { get; set; }
}

So I can create a test instance of Foo by calling Foo(int a), e.g:

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Foo foo = new Foo(1);

But since Xaml doesn’t support serialize fields, I cannot apply AmbientAttribute to field, so the compilation fails. So I changed the field Complex of Class Foo to a property:

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[Ambient]
// public Nested Complex;
public Nested Complex { get; set;}

Then the code compiles correct. But when I executed the code, I found the Temp property of Complex is empty guid. But I found I do called the Init method in the constructor of Foo. What’s the problem?

I believe that the problem must be special behavior related to ValueType. So I omitted to call Init(), I try to assign the Complex in Foo’s Constructor as following code

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//Complex.Init();
this.Complex.Temp = Guid.NewGuid();

Compiles fails! Visual Studio tells me “Cannot modify the return value of ‘structTest.Foo.Complex’ because it is not a variable”

Still now, everything is explainable:

1. It is because this.Complex is pointed to the variable itself that Code Complex.Init() works when Complex is field.
2. It is because this.Complex is a return value, a copy of the original variable, instead of the original variable that the Code Complex.Init() doesn’t work when Complex is a property. this.Complex.Init() do changed the value of the Temp, but it modified the new copy instead of the original one.

So we need to be careful while change a value type field into a property, which may modify the program behavior a lot, and really difficult to debug while the code is complicated.