Why doesn't a textbox stretch inside stackpanel WPF?

Question

<Grid>
    <Grid.ColumnDefinitions>
        <ColumnDefinition Width="25*" />
        <ColumnDefinition Width="49*" />
        <ColumnDefinition Width="25*" />
    </Grid.ColumnDefinitions>
    <StackPanel Grid.Column="1" Margin="30" VerticalAlignment="Center">
        <StackPanel Orientation="Horizontal">
            <TextBlock Margin="0,0,5,0" Text="Username:" />
            <TextBox HorizontalAlignment="Stretch"/>
        </StackPanel>
    </StackPanel>

--Result image--

Im trying to make it so that the textbox will fill the rest of the space inside the current StackPanel. however the "Stretch" propety doesn't seem to work - why is that?

Is there a different way do it or what am I doing wrong?

Answer 1

A StackPanel always tries to achieve the minimum possible height/width, depending on orientation; therefore, Stretch has no effect. You might want to use a DockPanel instead, which allows children to stretch:

<Grid>
    <Grid.ColumnDefinitions>
        <ColumnDefinition Width="25*" />
        <ColumnDefinition Width="49*" />
        <ColumnDefinition Width="25*" />
    </Grid.ColumnDefinitions>
    <StackPanel Grid.Column="1"
                Margin="30"
                VerticalAlignment="Center">
        <DockPanel>
            <TextBlock DockPanel.Dock="Left" Margin="0,0,5,0"
                       Text="Username:" />
            <TextBox HorizontalAlignment="Stretch"/>
        </DockPanel>
    </StackPanel>
</Grid>