为什么CommandParameter在datagrid中绑定ElementName = null

Question

我不明白为什么CommandParameter在Datagrid中绑定ElementName = null Cat.Wpf

<Window x:Class="MyCat.Cat"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        xmlns:local="clr-namespace:MyCat"
        xmlns:vm ="clr-namespace:MyCat.ViewModel"
        mc:Ignorable="d" WindowStartupLocation="CenterScreen"
        Title="MyCat •" Height="650" Width="800">
<Window.DataContext>
  <vm:MyFriends/>
</Window.DataContext>

<Grid>
      <Grid.RowDefinitions>
        <RowDefinition Height="25"/>
        <RowDefinition/>
      </Grid.RowDefinitions>

      <Menu>
         <MenuItem Grid.Row="0" Grid.Column="0"
              Header="Show Name" x:Name="mimi" 
              Command="{Binding GetNameCat}"
              CommandParameter="{Binding ElementName=mimi}"/>
      </Menu>
      <DataGrid CanUserAddRows="False" CanUserDeleteRows="False" 
              Grid.Row="1" Grid.Column="0"
              SelectionUnit="FullRow">
              <DataGrid.ContextMenu>
                <ContextMenu>
                    <MenuItem Header="List cats" x:Name="cats"
                              Command="{Binding GetCats}"
                              CommandParameter="{Binding ElementName=cats}"/>
                </ContextMenu>
            </DataGrid.ContextMenu>
      </DataGrid>

CommandParameter="{Binding ElementName=mimi}"有效，但是CommandParameter="{Binding ElementName=cats}不起作用。它总是返回null。我找出解决方案 CommandParameter="{Binding RelativeSource={RelativeSource Self}}"。

有些身体，请告诉我为什么？