Sample

Sample.cs

public class SubmitOrder 
{
    public SubmitOrder(int orderId)
    {
        OrderId = orderId;
    }
 
    public int OrderId { get; private set; }
}

public class SubmitOrderHandler : IHandler<SubmitOrder>
{
    private IOrderRepository _orderRepository;

    public SubmitOrderHandler(IOrderRepository orderRepository)
    {
        _orderRepository = orderRepository;
    }

    public void Execute(SubmitOrder command) 
    {
        var order = orderRepository.Get(command.OrderId);
        order.Submit();
        orderRepository.Update(order);
    }
}

public class Order 
{
    public int Id { get; private set; }

    public string Supplier { get; private set; }

    public string Status { get; private set; }

    public void Submit()
    {
        if (Status == "Not Submitted" && Supplier != null) 
        {
            Status = "Submitted";
        }
    }
}

public class Program
{
    private static dispatcher = new MessageDispatcher();

    public static void Main() 
    {
        var command = new SubmitOrder(999);

        // Command dispatcher that binds it to the correct handler
        dispatcher.Dispatch(command);

    }
}