WPF TextBox won't fill in StackPanel

I have a TextBox control within a StackPanel whose Orientation is set to Horizontal, but can't get the TextBox to fill the remaining StackPanel space.

XAML:

<Window x:Class="WpfApplication1.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Height="180" Width="324">

    <StackPanel Background="Orange" Orientation="Horizontal" >
        <TextBlock Text="a label" Margin="5" VerticalAlignment="Center"/>
        <TextBox Height="25" HorizontalAlignment="Stretch" Width="Auto"/>
    </StackPanel>
</Window>

And this is what it looks like:

Why is that TextBox not filling the StackPanel?

I know I can have more control by using a Grid control, I'm just confused about the layout.

Solution 1:

I've had the same problem with StackPanel, and the behavior is "by design". StackPanel is meant for "stacking" things even outside the visible region, so it won't allow you to fill remaining space in the stacking dimension.

You can use a DockPanel with LastChildFill set to true and dock all the non-filling controls to the Left to simulate the effect you want.

<DockPanel Background="Orange" LastChildFill="True">
    <TextBlock Text="a label" Margin="5" 
        DockPanel.Dock="Left" VerticalAlignment="Center"/>
    <TextBox Height="25" Width="Auto"/>
</DockPanel >

Solution 2:

I would recommend using a Grid instead:

<Window x:Class="WpfApplication1.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Height="180" Width="324">

    <Grid Background="Orange">
        <Grid.ColumnDefinitions>
            <ColumnDefinition Width="Auto"/>
            <ColumnDefinition Width="*"/>
        </Grid.ColumnDefinitions>

        <TextBlock Grid.Column="0" Text="a label" 
           VerticalAlignment="Center"/>
        <TextBox  Grid.Column="1"/>
    </Grid>
</Window>

The other way to get around this problem is to stack the label on top instead of to the right. I noticed that UWP has a built in header property you can use for that, not sure if the header property exists for WPF.

<TextBox Header="MyLabel" />