CommandHandler pattern

public interface IHandleCommands<TCommand> where TCommand : IAmACommand
{
    void Handle(TCommand command);
}

public interface IAmACommand
{
    Guid AggregateId { get; }
}

public class AddLineToInvoiceCommandHandler : IHandleCommands<AddLineToInvoiceCommand>
{
    public void Handle(AddLineToInvoiceCommand command)
    {
        // ... Do Something here to handle the command, for anything else
        // E.g. transactions, logging, etc... use the decorator pattern
    }
}

public class AddLineToInvoiceCommand
{
    Guid _aggregateId;
    long _quantity;
    Guid _itemId;

    public AddLineToInvoiceCommand(Guid invoiceAggregateId, long quantity, Guid itemId)
    {
        _aggregateId = invoiceAggregateId;
        _quantity = quantity;
        _itemId = itemId;
    }

    public Guid AggregateId
    { get { return _aggregateId; } }

    public long Quantity
    { get { return _quantity; } }

    public Guid ItemId
    { get { return _itemId; } }
    
}