WPF TextBox won't fill in StackPanel

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 >

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" />

Old question by actual topic:

HorizontalAlignment="Stretch"

is the required thing. Juste be sure that you remove the width.