如何在WPF中编辑旧模板

Question

我尝试创建从TextBox继承的新自定义控件，名为OltholderTextbox。 So I added a TextBlock directly above the TextBox using Template :

<!-- PlaceHolderTextBox template -->

<ControlTemplate TargetType="TextBox">
    <Grid>
        <TextBlock x:Name=""holder"" />
        <ScrollViewer  x:Name=""PART_ContentHost"" Margin=""0"" />
    </Grid>
</ControlTemplate>

All are ok for now, the real TextBox will appear in the ScrollViewer automatically.现在，我想得出此类来创建我的AutoCompleteTextBox Control，从OlacholderTextbox继承了继承，因此我必须再次编辑模板，但是如何像上面的ScrollViewer一样将基本控制量载箱添加到模板中？ !!

<!-- AutocompleteTextBox template -->

<Grid>
    <PlaceHolderTextBox Name=""tbTextBox""/>

    <Popup x:Name=""pop""    
                           Visibility=""Visible""
                           StaysOpen=""False""    
                           Placement=""Bottom"">

        <ListBox x:Name=""lbSuggestions""       
                 Height=""135""    
                 />
    </Popup>
</Grid>

在模板中显示的OptrolderTextBox控件中的问题将被视为新控件（不绑定到继承的属性（如文本）），而不像第一类中的scrollviewer一样。

I tried to use PART_XXX mechanism in the base class to solve this issue, but I failed to edit incoming template -like ScrollViewer did- before use :

Public Class PlaceHolderTextBox
Inherits TextBox

Public Overrides Sub OnApplyTemplate()

    Dim host = Me.GetTemplateChild("PART_PlaceHolderContentHost")
    If host IsNot Nothing AndAlso TypeOf host Is Grid Then
        Dim xaml = "<Grid>
                        <TextBlock x:Name=""holder"" />
                        <Grid x:Name=""PART_ContentHost"" Margin=""0"" />
                    </Grid>"
        Dim strReader = New StringReader(xaml)
        Dim xmlReader = Xml.XmlReader.Create(strReader)
        Dim grid As Grid = XamlReader.Load(xmlReader)
        CType(host, Grid).Children.Add(grid)
    End If

    MyBase.OnApplyTemplate()
End Sub
End Class

So I added new controls to the grid (PART_PlaceHolderContentHost) but ScrollViewer here will not work correctly because it添加到真实网格（不是模板）中。因此，如何编辑传入的模板以使ScrollViewer视为TextBoxBase。