SplitButton该控件除了本身Button 的功能外，还具有下拉菜单的功能，可以在按键右侧添加下拉菜单控件：

            <SplitButton Content="..." Width="80" Height="22"  Grid.Column="2"  Margin="10,0,2,0"                      
                         Command="{Binding SelectFileCommand}"
                         VerticalAlignment="Center" HorizontalAlignment="Center">
                <SplitButton.DropDownContent>
                    <Button Content="DownLoad" Command="{Binding DownLoadCommand}" />
                </SplitButton.DropDownContent>