How to have button stretched over all window width: XAML:

<Window x:Class="DIexamples.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="MainWindow" Height="350" Width="525">
    <Grid>
        <Grid.ColumnDefinitions>
            <ColumnDefinition Width="*" />
        </Grid.ColumnDefinitions>
        <Button Content="Button" Margin="0,23,0,0" VerticalAlignment="Top" />
        <Button Content="Button" Margin="0,0,0,0" VerticalAlignment="Top" />
    </Grid>
</Window>

Important lines to notice are:

<Grid.ColumnDefinitions>
	<ColumnDefinition Width="*" />
</Grid.ColumnDefinitions>

And attribute: Margin="0,23,0,0"