Question

I have a SplitButton in my usercontrol, the usercontrol‘s datacontext is a view which defines command I want the splitbutton binding to.

As my brief xaml code showing below, the first binding works, but the second (button in the DropDownContent) doesn’t with the output:

Cannot find source for binding with reference ‘RelativeSource
FindAncestor, AncestorType=’System.Windows.Controls.UserControl….

or (if I replace binding expression using ElementName instead of ReleativeSource)

Cannot find source for binding with reference ‘ElementName=uc’…

<UserControl x:Name="uc"
             xmlns:extToolkit="http://schemas.microsoft.com/winfx/2006/xaml/presentation/toolkit/extended"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
             >
      <Grid>
        <ItemsControl ItemsSource="{Binding ItemList, IsAsync=True}">
          <ItemsControl.ItemTemplate>
            <DataTemplate>
              <extToolkit:SplitButton Command="{Binding 
                                                  RelativeSource={RelativeSource FindAncestor, AncestorType={x:Type UserControl}}, 
                                                  Path=DataContext.OpenCommand, 
                                                  Mode=OneWay}"
                                      CommandParameter="{Binding}"
                                      Content="{Binding ID}">
                <extToolkit:SplitButton.DropDownContent>
                  <Button  Command="{Binding 
                                         RelativeSource={RelativeSource FindAncestor, AncestorType={x:Type UserControl}}, 
                                         Path=DataContext.OpenCommand, 
                                         Mode=OneWay}"
                           CommandParameter="{Binding}"
                           Content="{Binding ID}"/>
                </extToolkit:SplitButton.DropDownContent>
              </extToolkit:SplitButton>
            </DataTemplate>
          </ItemsControl.ItemTemplate>
        </ItemsControl>
      </Grid>
    </UserControl>

Answer 1

In the end I found out

the DropDownContent and it content in different visual tree
than the one that holds the SplitButton. This way the Binding to
RelativeSource cannot work, it does’t find the desired relative source
since the they are in different visual trees.

REF (although it’s for another control, but I think they are based on the same thing)

So I have to add my VM as a property in the item object, and binding vm’s command from there.