C# How do I pass more than just IAsyncResult into AsyncCallback?

Question

How do I pass more than just the IAsyncResult into AsyncCallback?

Example code:

//Usage
var req = (HttpWebRequest)iAreq;
req.BeginGetResponse(new AsyncCallback(iEndGetResponse), req);

//Method
private void iEndGetResponse(IAsyncResult iA, bool iWantInToo) { /*...*/ }

I would like to pass in example variable bool iWantInToo. I don't know how to add that to new AsyncCallback(iEndGetResponse).

score 12 · Accepted Answer · answered Apr 11 '11 at 18:08

12

You have to use the object state to pass it in. Right now, you're passing in the req parameter - but you can, instead, pass in an object containing both it and the boolean value.

For example (using .NET 4's Tuple - if you're in .NET <=3.5, you can use a custom class or KeyValuePair, or similar):

var req = (HttpWebRequest)iAreq;
bool iWantInToo = true;
req.BeginGetResponse(new AsyncCallback(iEndGetResponse), Tuple.Create(req, iWantInToo));

//Method
private void iEndGetResponse(IAsyncResult iA) 
{
    Tuple<HttpWebRequest, bool> state = (Tuple<HttpWebRequest, bool>)iA.AsyncState;
    bool iWantInToo = state.Item2;

    // use values..
}

answered Apr 11 '11 at 18:08

Reed Copsey

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Very very interesting. Testing this now. I've never seen this Tuple deal before, looks very useful! – PiZzL3 Apr 11 '11 at 18:29
Nobody sees this method ugly? There is supposed to be a better to pass parameters. – derek Sep 02 '16 at 05:49
@derek this pattern, in general, has been replaced for async work. most newer apis work against Task – Reed Copsey Sep 02 '16 at 05:52
@ReedCopsey So modern way is to use "await" to let the callback finish and then do the job what previously "callback" did? – derek Sep 02 '16 at 05:56
@derek Yes. `await` makes callbacks unnecessary – Reed Copsey Sep 02 '16 at 22:22

C# How do I pass more than just IAsyncResult into AsyncCallback?

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