MainWindow.xaml

<Window x:Class="Z12_Commands.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        xmlns:local="clr-namespace:Z12_Commands"
        mc:Ignorable="d"
        Title="MainWindow" Height="350" Width="525">
    <DockPanel LastChildFill="True">
        <Menu IsMainMenu="True" DockPanel.Dock="Top">
            <MenuItem Header="XXX" Command="{x:Static local:Commands.TestCommand}" />
        </Menu>

        <Grid>
            <Grid.RowDefinitions>
                <RowDefinition Height="*" />
                <RowDefinition Height="Auto" />
                <RowDefinition Height="*" />
            </Grid.RowDefinitions>

            <Grid.ColumnDefinitions>
                <ColumnDefinition Width="*"/>
                <ColumnDefinition Width="Auto"/>
                <ColumnDefinition Width="*"/>
            </Grid.ColumnDefinitions>
            
            <Button Grid.Row="1" Grid.Column="1" Click="Button_Click">Flip CanExecute</Button>
        </Grid>
    </DockPanel>
</Window>