Why my WPF Rectangle control is not filling all the empty space of StackPanel?

Question

Learning basic concepts of WPF before moving to UWP. Following XAML in my WPF project is showing the windows as below.

I'm trying to display the Rectangle and Button on the right side of the StackPanel and need the Rectangle (not the Button) control to auto fill the StackPanel.

I tried the HorizontalAlignment="Stretch" with no Width attribute but without Width attribute the entire rectangle shrinks to 0 width. Don't want to hard code the width value (if possible) so that window of the app adjust itself depending on the device it's on (screen resolution). But if that scenario is still possible with hard coded width value as well please let me know that approach as well.

Window:

XAML:

Remark: I don't think ListBox is playing any role (related to this post). Only controls inside the ListItemsControl on above ListBox probably need proper adjustment. but I may be wrong.

<Window x:Class="WPFProject.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="MainWindow"
        Height="376"
        Width="337">
    <Grid>
        <ItemsControl>
            <StackPanel Orientation="Horizontal" HorizontalAlignment="Stretch" Height="10">
                <Rectangle x:Name="myRectangle" Fill="#FFF4F4F5" HorizontalAlignment="Right" Height="9" Margin="0,0,0,0" Stroke="Black" VerticalAlignment="Top" Width="100"  RenderTransformOrigin="0.533,0.6"/>
                <Button Content="&#xE1FD;" HorizontalAlignment="Right" Height="10" VerticalAlignment="Top" FontSize="5" FontWeight="Bold"/>
            </StackPanel>
        </ItemsControl>
        <ListBox HorizontalAlignment="Stretch" VerticalAlignment="Stretch" Margin="0,11,0,81" ScrollViewer.HorizontalScrollBarVisibility="Disabled" x:Name="myList" SelectionChanged="myList_ContextMenuClosing">
            <ListBox.ItemsPanel>
                <ItemsPanelTemplate>
                    <WrapPanel IsItemsHost="True" Orientation="Horizontal"/>
                </ItemsPanelTemplate>
            </ListBox.ItemsPanel>
            <ListBox.ItemTemplate>
                <DataTemplate>
                    <Rectangle Fill="{Binding FirstName}" ToolTip="{Binding FullName}" Width="20" Height="20" Stroke="#FF211E1E" OpacityMask="Black" StrokeThickness="1" />
                </DataTemplate>
            </ListBox.ItemTemplate>
        </ListBox>
        <Button x:Name="btnTest" Content="Test" HorizontalAlignment="Left" Margin="250,298,0,0" VerticalAlignment="Top" Width="75" Click="BtnTest_Click"/>
    </Grid>
</Window>

Answer 1

Two things here:

1. When you use Stackpanel with horizontal orientation, horizontalalingment="stretch" can't be used. That is because all of the elements are being Stacked with their designed width.

2. You are specifying a fixed width of 100 for your rectangle. If you do that it will not stretch anymore even if you use stretch for alignment. Also the horizontalalingment="stretch" needs to be placed on the element you are expecting to stretch, not the Panel.

For things like this use DockPanelor a Grid instead. Read more about WPF panels here: https://wpf-tutorial.com/panels/introduction-to-wpf-panels/

Here is an example for Grid:

<Grid>
    <Grid.ColumnDefinitions>
        <ColumnDefinition Width="*"/>
        <ColumnDefinition Width="Auto"/>
    </Grid.ColumnDefinitions>
    <Rectangle x:Name="myRectangle" Fill="#FFF4F4F5" HorizontalAlignment="Stretch" Height="9" Margin="0,0,0,0" 
            Stroke="Black" VerticalAlignment="Top"  RenderTransformOrigin="0.533,0.6" Grid.Row="0"/>
    <Button Content="&#xE1FD;" HorizontalAlignment="Right" Height="10" VerticalAlignment="Top" FontSize="5" FontWeight="Bold" Grid.Column="1"/>
</Grid>

Notice the width="*" attribute means the cell will use all the remaining space. If you have multiple rows/columsn defined with * the space will be divided between them.