How to get button content to fill width in WPF

You should set Button.Template. Also Grid.ColumnDefinitions can be used to set the position and width of elements inside properly.

        <Button Height="30" Content="Smaple text">            <Button.Template>                <ControlTemplate TargetType="{x:Type Button}">                    <Grid>                        <Border Background="{TemplateBinding Background}"                         BorderBrush="{TemplateBinding BorderBrush}"                         BorderThickness="{TemplateBinding BorderThickness}">                            <Grid>                                <Grid.ColumnDefinitions>                                    <ColumnDefinition Width="Auto"/>                                    <ColumnDefinition Width="*"/>                                </Grid.ColumnDefinitions>                                <ContentPresenter HorizontalAlignment="Left"  Grid.Column="0"                                          VerticalAlignment="Center"/>                                <Canvas Background="AliceBlue" Grid.Column="1" />                            </Grid>                        </Border>                    </Grid>                </ControlTemplate>            </Button.Template>        </Button>

You could also set the HorizontalContentAlignment to Stretch on the Button itself.

This will tell the content to fill the horizontal space available on the button.

<Window    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"    x:Name="Window"    Title="test"    Width="640" Height="480">    <Grid x:Name="LayoutRoot">        <Button Height="30" HorizontalContentAlignment="Stretch">            <Grid HorizontalAlignment="Stretch">                <TextBlock Text="Sample Text" HorizontalAlignment="Stretch" TextAlignment="Left"></TextBlock>                <Canvas Width="40" Background="AliceBlue" HorizontalAlignment="Right"></Canvas>            </Grid>        </Button>    </Grid></Window>