|WPF| Grid如何让一行自适应，有相应的XAML源码 谢谢

Question

<Window x:Class="SRQC11_5_2.MainWindow"
         xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
         xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
         Title="MainWindow" Height="350" Width="525">
     <Grid>
         <Grid.RowDefinitions>
             <RowDefinition></RowDefinition>
             <RowDefinition></RowDefinition>
             <RowDefinition Height="40"></RowDefinition>
         </Grid.RowDefinitions>
         <StackPanel Grid.Row="0" Background="LightBlue">
         </StackPanel>
         <StackPanel Grid.Row="1" Background="Green">
             <TextBox x:Name="txt1"></TextBox>
         </StackPanel>
         <StackPanel Grid.Row="2" Background="Orange"></StackPanel>
     </Grid>
 </Window>