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<ListBox Name="listBoxIDs" DisplayMemberPath="IDName" >   
</ListBox>                    

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listBoxIDs.DisplayMemberPath ="IDNumber";                    

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<ListBox Name="listBoxIDs">
    <ListBox.ItemTemplate>
        <DataTemplate>
            <StackPanel Margin="0,5,0,0" Orientation="Horizontal">
                <Image ItemImage" Width="15" Height="15" Source="/Image.png"></Image>
                <TextBlock Margin="5,0,0,0" Text="{Binding IDName}"></TextBlock>  <---------nun müsste der Path hier verändert werden.
            </StackPanel>            
        </DataTemplate>
    </ListBox.ItemTemplate>
</ListBox>