MainWindow.xaml

<Window x:Class="delegatecommand.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:delegatecommand"
        xmlns:ViewModels="clr-namespace:delegatecommand.ViewModels"
        mc:Ignorable="d"
        Title="MainWindow"
        Height="350"
        Width="525"
        d:DataContext="{d:DesignInstance ViewModels:MainViewModel}">
    <Grid>
        <Grid.RowDefinitions>
            <RowDefinition Height="Auto" />
            <RowDefinition Height="*" />
        </Grid.RowDefinitions>

        <Button Grid.Row="0"
                Content="Update Text"
                Command="{Binding UpdateTextCommand}"
                CommandParameter="FooBar"
                Width="Auto" />
        <TextBlock Grid.Row="1"
                   Text="{Binding Text}" />
    </Grid>
</Window>