WPF DataGrid ContextMenu CommandParameter Relative x:Type ContextMenu ,Path=PlacementTarget

//xaml
<DataGrid.ContextMenu>
    <ContextMenu>
        <MenuItem Header="Serialize Binary" 
                  Command="{Binding BinSerializeCmd}"
                  CommandParameter="{Binding RelativeSource={RelativeSource Mode=FindAncestor,AncestorType={x:Type ContextMenu}},Path=PlacementTarget}"/>
    </ContextMenu>
</DataGrid.ContextMenu>

// cs

 private void BinSerializeCmdExecuted(object obj)
 {
     var dg = obj as DataGrid;
     if(dg!=null)
     {
         SaveFileDialog dialog = new SaveFileDialog();
         dialog.Filter = "All Files|*.*|Bin Files|*.bin";
         dialog.FileName = $"{DateTime.Now.ToString("yyyyMMddHHmmssffff")}_{Guid.NewGuid().ToString("N")}.bin";
         if(dialog.ShowDialog()==true)
         {
             using(System.IO.FileStream fs=new System.IO.FileStream(dialog.FileName,System.IO.FileMode.OpenOrCreate,FileAccess.ReadWrite))
             {
                 BinaryFormatter binFormatter = new BinaryFormatter();
                 binFormatter.Serialize(fs,dg.Items.Cast<Book>().ToList());
                 MessageBox.Show($"Saved in {dialog.FileName}", "Binary Formatter Serialize", MessageBoxButton.OK);
             }
         }
     }
 }