gistfile1.cs

public ICommand Prueba
{
	get
	{
		if (_prueba == null)
			_prueba = new AsyncRelayCommand<object, int>(o => Proceso())
			          	{
							Preview = o => Descripcion = "procesando...",
			          		Completed = (o, result) => Descripcion = "terminado.." + result,
							BlockInteraction = true
			          	};
		return _prueba;
	}
}

private static int Proceso()
{
	Thread.Sleep(TimeSpan.FromSeconds(10));
	return 45;
}


[TestFixture]
public class Algo
{
	[Test]
	public void proceso_funciona()
	{
		var vm = new ViewModel();
		var result = ((AsyncRelayCommand<object, int>) vm.Prueba).ExecuteSync(null);
		Assert.AreEqual(45, result);
	}

	[Test]
	public void preview_funciona_ok()
	{
		var vm = new ViewModel();
		((AsyncRelayCommand<object, int>)vm.Prueba).Preview(null);
		Assert.AreEqual(vm.Descripcion, "procesando...");
	}

	[Test]
	public void post_action_funciona_ok()
	{
		var vm = new ViewModel();
		((AsyncRelayCommand<object, int>)vm.Prueba).Completed(null, 45);
		Assert.AreEqual(vm.Descripcion, "terminado..45");
	}

}