MVVM DataGrid SelectedItem始终为null

Question

我无法获得SelectedItem，行选择有效，但已重试SelectedItem为null。

查看型号：

// Binding DataGrid ItemSource   
public ObservableCollection<Message> TableDataGrid_ItemSource { get; set; }

// Binding DataGrid SelectedItem
private Message _tableDataGrid_SelectedItem;
public Message TableDataGrid_SelectedItem
        {
            get { return _tableDataGrid_SelectedItem; }
            set { _tableDataGrid_SelectedItem = value;  
                  MessageRemovetButton_IsEnabled = true;
                  OnPropertyChanged(); }
         } 

XAML：

  <DataGrid 
        Name="TableDataGrid" 
        ItemsSource="{Binding TableDataGrid_ItemSource}" 
        AutoGenerateColumns="False" 
        SelectedItem="{Binding TableDataGrid_SelectedItem, Mode=TwoWay}" 
        IsReadOnly="True" SelectionUnit="FullRow"
    >

代码：

MessageBox.Show(_messageManagerViewModel.TableDataGrid_SelectedItem);