Why is the TextBox not stretching?

Question

WPF

Please tell me why the TextBox stubbornly does not want to stretch?

<DockPanel LastChildFill="True" Height="25">
    <TextBox HorizontalAlignment="Stretch" Margin="0,0,5,0"/>
    <Button DockPanel.Dock="Right" Content="Send" MaxWidth="50"/>
</DockPanel>

Answer 1

<DockPanel LastChildFill="True" Height="25">

            <Button DockPanel.Dock="Right" Content="Send" MaxWidth="50"/>
            <TextBox HorizontalAlignment="Stretch" Margin="0,0,5,0" Background="Blue"/>
        </DockPanel>