Store App Combobox DataTemplate Problem

The Standard way to solve a Problem is often the fastest and best way:

"Reduce your Problem until it disapears":)

After some thoughts about the possible Combobox process I suspected the ContentPresenter.

I assume that, if the selection is made from the open list, the Combobox needs a copy of the selected item.
Next assumption: The Combobox copied only the DataTemplate, not the binded value.
And here is the problem:

The binded value is a framework element object (Grid or ComboboxItem, I tried both) and a framework element object can only be content or child of one framework parent element:

public void AListeFüllen()
{
    // Im tatsächlichen Projekt sind diese Strukturen umfangreicher.
    ComboBoxItem cbi1 = new ComboBoxItem(); 
    Grid         gr1  = new Grid(); 
    TextBlock    ta1  = new TextBlock(); 
    ta1.Text = "Wert 1"; 
    gr1.Children.Add(ta1); 
    cbi1.Content = gr1; 
    AuswahlListe.Add(new Element(1, cbi1));
    ...
}

And while the combobox didn't made a copy of the binded value my binded value was content of two parents: the shown selected value and the combobox-list.

After I Change the design from

public class Element : INotifyPropertyChanged
{
    internal void RaisePropertyChanged(string prop)
    {
        if (PropertyChanged != null) { PropertyChanged(this, new PropertyChangedEventArgs(prop)); }
    }
    public event PropertyChangedEventHandler PropertyChanged;

    private int _Id;
    public int Id
    {
        ...
    }
    public FrameworkElement FElement { get; set; }
    public Element(int id, FrameworkElement felement)
    {
        FElement = felement;
        Id = id;
    }
    public Element()
    {
    }
}

to

public class Element : INotifyPropertyChanged
{
    internal void RaisePropertyChanged(string prop)
    {
        if (PropertyChanged != null) { PropertyChanged(this, new PropertyChangedEventArgs(prop)); }
    }
    public event PropertyChangedEventHandler PropertyChanged;

    private int _Id;
    public int Id
    {
        ...
    }
    public Visibility ASichtbar { get; set; }
    public Visibility BSichtbar { get; set; }
    public Visibility CSichtbar { get; set; }
    public string A { get; set; }
    public string B { get; set; }
    public string C { get; set; }
    public Element(int id, Visibility asichtbar, string a, Visibility bsichtbar, string b, Visibility csichtbar, string c)
    {
        ASichtbar = asichtbar;
        BSichtbar = bsichtbar;
        CSichtbar = csichtbar;
        A = a;
        B = b;
        C = c;
        Id = id;
    }
    public Element()
    {
    }
}

So I can use this values in the DataTemplate:

<datatemplate x:key="Schablone" xmlns:x="#unknown">
        <grid>
            <textblock foreground="Red" visibility="{Binding ASichtbar}" text="{Binding A}" />
            <textblock foreground="Orange" visibility="{Binding BSichtbar}" text="{Binding B}" />
            <textblock foreground="Green" visibility="{Binding CSichtbar}" text="{Binding C}" />
        </grid>
</datatemplate>

And now it works:):
Solution, solved
I hope, that this will prevent other to make this "mistake".

Greetings from Germany
Ralf