How to cast generic action delegate to concrete action delegate?

Question

I am implementing a subscription-based reading of messages from IBM MQ websphere and I am using IBM.XMS.dll which has an event-based mechanization. Following is my code for subscription:

T is the type of data accepted by the callback function onMessageReceived. I will have to parse the message body from IMessage, convert to type T and invoke onMessageReceived fromOnMQMessageCallback

    public override void Subscribe<T>(Action<T> onMessageReceived)
    {
        try
        {
            MessageListener messageListener = new MessageListener(OnMQMessageCallback);
            _consumer.MessageListener = messageListener;

            _connection.Start();
        }
        catch (Exception ex)
        {
            throw;
        }
    }

    private void OnMQMessageCallback(IMessage msg)
    {
        try
        {
            //I want to invoke onMessageReceived in this method
        }
        catch (Exception)
        {
            throw;
        }
    }

From OnMQMessageCallback, I want to invoke onMessageReceived() which is passed into the Subscribe() method from the caller. I cannot figure out how to do it.

Just to make sure, in your actual code, you do something else than catch/throw, right? Like custom logging or something? Otherwise, you should think about removing that. http://stackoverflow.com/questions/218781/is-there-a-benefit-to-just-a-throw-in-a-catch/218929 — Tipx, Apr 25 '17 at 16:09
What is `T`? how would you call `onMessageReceived` if you had a reference to it? `T` is only known at the scope of the `Subscribe` method. — Ofir Winegarten, Apr 25 '17 at 16:15
@OfirWinegarten T is the type of data accepted by the callback function onMessageReceived. I wil have to parse the message body from IMessage, convert to type T and invoke onMessageReceived — PushCode, Apr 25 '17 at 16:24

score 3 · Accepted Answer · answered Apr 25 '17 at 16:05

3

You can achieve this using anonymous delegate

   MessageListener messageListener = new MessageListener((m) =>         
    try
    {
        onMessageReceived();
    }
    catch (Exception)
    {

        throw;
    });`

answered Apr 25 '17 at 16:05

Valentin

5,380
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It's worth mentioning that it uses *closure* for a reference on `onMessageReceived` – Fabjan Apr 25 '17 at 16:09
1

`onMessageReceived` is `Action` Where is `T`? – Ofir Winegarten Apr 25 '17 at 16:11
This could be a little clearer but I think this is the answer. You take `m`, you convert it to `T` and then you pass the result of this conversion into `onMessageReceived`. – Roman Starkov Apr 25 '17 at 16:36

Ofir Winegarten · Answer 2 · 2017-04-25T16:40:54.427

There's no way to call the onMessageReceived if it's defined a Action<T> out side the scope of Subscribe. You simply don't know what T is.

Either you do this and let the caller handle the parsing later:

public override void Subscribe(Action<IMessage> onMessageReceived)
{
    MessageListener messageListener = new MessageListener(onMessageReceived);
    _consumer.MessageListener = messageListener;

    _connection.Start();
}

Or you parse it yourself (don't know how) and call it:

public override void Subscribe<T>(Action<T> onMessageReceived)
{
    MessageListener messageListener = new MessageListener((m) =>         
    {
        T result = (T) m.Body; // or some other casting
        onMessageReceived(result);
    });`

    _consumer.MessageListener = messageListener;

    _connection.Start();
}

No need for OnMQMessageCallback anymore.

Side Note: There's no point in try..catch..throw it's the same as not doing a try..catch at all.

How to cast generic action delegate to concrete action delegate?

2 Answers2