In WPF how do I implement ICommandSource to give my custom control ability to use Command from xaml?

Here's an example :

public partial class MyUserControl : UserControl, ICommandSource{    public MyUserControl()    {        InitializeComponent();    }    public ICommand Command    {        get { return (ICommand)GetValue(CommandProperty); }        set { SetValue(CommandProperty, value); }    }    public static readonly DependencyProperty CommandProperty =        DependencyProperty.Register("Command", typeof(ICommand), typeof(MyUserControl), new UIPropertyMetadata(null));    public object CommandParameter    {        get { return (object)GetValue(CommandParameterProperty); }        set { SetValue(CommandParameterProperty, value); }    }    // Using a DependencyProperty as the backing store for CommandParameter.  This enables animation, styling, binding, etc...    public static readonly DependencyProperty CommandParameterProperty =        DependencyProperty.Register("CommandParameter", typeof(object), typeof(MyUserControl), new UIPropertyMetadata(null));    public IInputElement CommandTarget    {        get { return (IInputElement)GetValue(CommandTargetProperty); }        set { SetValue(CommandTargetProperty, value); }    }    // Using a DependencyProperty as the backing store for CommandTarget.  This enables animation, styling, binding, etc...    public static readonly DependencyProperty CommandTargetProperty =        DependencyProperty.Register("CommandTarget", typeof(IInputElement), typeof(MyUserControl), new UIPropertyMetadata(null));    protected override void OnMouseLeftButtonUp(MouseButtonEventArgs e)    {        base.OnMouseLeftButtonUp(e);        var command = Command;        var parameter = CommandParameter;        var target = CommandTarget;        var routedCmd = command as RoutedCommand;        if (routedCmd != null && routedCmd.CanExecute(parameter, target))        {            routedCmd.Execute(parameter, target);        }        else if (command != null && command.CanExecute(parameter))        {            command.Execute(parameter);        }    }}

Note that the CommandTarget property is only used for RoutedCommands

Your UserControl will have a code behind file cs or vb, you have to implement the interface ICommandSource, and once you implement that, in some event you will have to actually invoke the command and also check CanExecute.