MBR IT/.NET 247 : Asynchronous Delegate w/ Callback Parameters on microsoft.public.dotnet.languages.csharp

Asynchronous Delegate w/ Callback Parameters

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Michael C

Hi all,

Is there an easy way to get the parameters of an asynchronous delegate call
from the callback function? Here's an example of what I'm trying to do:

private delegate ArrayList AsyncDelegate (string server);

private void GetServerInfo (string server)
{
AsyncDelegate ad = new AsyncDelegate(GetServerArray);
IAsyncResult iar = ad.BeginInvoke(server, new
AsyncCallback(EndGetServerInfo), ad);
}

private void EndGetServerInfo(IAsyncResult iar)
{
AsyncDelegate ad = (AsyncDelegate)iar.AsyncState;
ArrayList a = ad.EndInvoke(iar);
}

private ArrayList GetServerArray(string server)
{
ArrayList a = new ArrayList();
// this code establishes a SQL connection, executes a
// few queries and .Adds the results to the ArrayList
return (a);
}

In this code, GetServerInfo executes the "GetServerArray" routine
asynchronously. This routine basically connects to a SQL Server, executes a
couple of SELECT statements and returns the results in an ArrayList. After
completion, the EndGetServerInfo is called, and I retrieve the results via
..EndInvoke.

Everything works great, and I get my ArrayList of information, but there's
one small problem: in the EndGetServerInfo callback routine, I need to
retrieve not just the returned ArrayList, but also the initial parameter I
passed into the "GetServerInfo" routine (i.e., the "string server"
variable).

I imagine there's an easy way to do this via the IAsyncResult, but I'm not
sure how. Any ideas? Or would it be easier just to have my
"GetServerArray" routine .Add the value of the "server" variable as the
first item on the returned ArrayList? Seems kind of hokey, and would like
to avoid that if possible.

Thanks,
Michael C., MCDBA

Reply to this message...

Richard Blewett [DevelopMentor] (VIP)

There is no automatic way to do this AFAIK but there is a mechanism available.

ad.BeginInvoke(server, new AsyncCallback(EndGetServerInfo), ad);

This last parameter is the AsyncState - you can pass anything in here so pack up your parameters in an object and pass it this way. Now the issue is "how do I get the delegate instance to call EndInvoke?"

Well it is documented (and therefore you can rely on it) that the IAsyncResult you get in a AsyncCallback is actually an AsyncResult instance. This class is in the System.Runtime.Remoting.Messaging namespace (where else ;-) ). So you can write your callback as follows:

Void Callback( IAsyncResult iar )
{
    AsyncResult ar = (AsyncResult)iar;

    AsyncDelegate mydel = (AsyncDelegate)ar.AsyncDelegate; // AsyncDelegate is a property on the AsyncResult

    MyParamsObject mp = (MyParamObject)ar.AsyncState;

    ...
}

HTH

Regards

Richard Blewett - DevelopMentor
http://staff.develop.com/richardb/weblog
?
nntp://news.microsoft.com/microsoft.public.dotnet.languages.csharp/
Hi all,

Is there an easy way to get the parameters of an asynchronous delegate call from the callback function? Here's an example of what I'm trying to do:

private delegate ArrayList AsyncDelegate (string server);

private void GetServerInfo (string server) { AsyncDelegate ad = new AsyncDelegate(GetServerArray); IAsyncResult iar = ad.BeginInvoke(server, new AsyncCallback(EndGetServerInfo), ad); }

private void EndGetServerInfo(IAsyncResult iar) { AsyncDelegate ad = (AsyncDelegate)iar.AsyncState; ArrayList a = ad.EndInvoke(iar); }

private ArrayList GetServerArray(string server) { ArrayList a = new ArrayList(); // this code establishes a SQL connection, executes a // few queries and .Adds the results to the ArrayList return (a); }

In this code, GetServerInfo executes the "GetServerArray" routine asynchronously. This routine basically connects to a SQL Server, executes a couple of SELECT statements and returns the results in an ArrayList. After completion, the EndGetServerInfo is called, and I retrieve the results via .EndInvoke.

Everything works great, and I get my ArrayList of information, but there's one small problem: in the EndGetServerInfo callback routine, I need to retrieve not just the returned ArrayList, but also the initial parameter I passed into the "GetServerInfo" routine (i.e., the "string server"
variable).

I imagine there's an easy way to do this via the IAsyncResult, but I'm not sure how. Any ideas? Or would it be easier just to have my "GetServerArray" routine .Add the value of the "server" variable as the first item on the returned ArrayList? Seems kind of hokey, and would like to avoid that if possible.

Thanks,
Michael C., MCDBA

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Reply to this message...

Michael C

The only problem is that currently my AsyncState is "ad", which returns my
ArrayList results from the procedure I'm calling. If I replace the "ad"
with the server name, I lose my actual results. I thought maybe there was a
simple solution, like a property I was missing or something... Maybe the
correct solution is to return an array of Objects, like this:

o[0] = (string) "servername"
o[1] = (ArrayList) results

That, or just Add the server name onto the ArrayList as the first item as
soon as I enter the GetServerArray function.

Thanks,
Michael C., MCDBA

"Richard Blewett [DevelopMentor]" <Click here to reveal e-mail address> wrote in message
news:%23gWW%Click here to reveal e-mail address...
> There is no automatic way to do this AFAIK but there is a mechanism
available.
[Original message clipped]

"how do I get the delegate instance to call EndInvoke?"
[Original message clipped]

This class is in the System.Runtime.Remoting.Messaging namespace (where else
;-) ). So you can write your callback as follows:
[Original message clipped]

AsyncCallback(EndGetServerInfo), ad); }
[Original message clipped]

queries and .Adds the results to the ArrayList return (a); }
[Original message clipped]

couple of SELECT statements and returns the results in an ArrayList. After
completion, the EndGetServerInfo is called, and I retrieve the results via
..EndInvoke.
[Original message clipped]

retrieve not just the returned ArrayList, but also the initial parameter I
passed into the "GetServerInfo" routine (i.e., the "string server"
[Original message clipped]

routine .Add the value of the "server" variable as the first item on the
returned ArrayList? Seems kind of hokey, and would like to avoid that if
possible.
[Original message clipped]

Reply to this message...

Richard Blewett [DevelopMentor] (VIP)

I already explained how to get hold of your async delegate instance without passing it via the AsyncState. Look for the section of my reply about the System.Runtime.Remoting.Messaging.AsyncResult class

Regards

Richard Blewett - DevelopMentor
http://staff.develop.com/richardb/weblog

nntp://news.microsoft.com/microsoft.public.dotnet.languages.csharp/
The only problem is that currently my AsyncState is "ad", which returns my ArrayList results from the procedure I'm calling. If I replace the "ad"
with the server name, I lose my actual results. I thought maybe there was a simple solution, like a property I was missing or something... Maybe the correct solution is to return an array of Objects, like this:

o[0] = (string) "servername"
o[1] = (ArrayList) results

That, or just Add the server name onto the ArrayList as the first item as soon as I enter the GetServerArray function.

Thanks,
Michael C., MCDBA

"Richard Blewett [DevelopMentor]" wrote in message news:%23gWW%Click here to reveal e-mail address...
> There is no automatic way to do this AFAIK but there is a mechanism
available.
[Original message clipped]

This class is in the System.Runtime.Remoting.Messaging namespace (where else
;-) ). So you can write your callback as follows:
[Original message clipped]

Reply to this message...

Michael C

Yeah, thanks for that. So I went ahead and inserted the server name at the
top of the ArrayList that's returned.

Thanks,
Michael C., MCDBA

"Richard Blewett [DevelopMentor]" <Click here to reveal e-mail address> wrote in message
news:Click here to reveal e-mail address...
> I already explained how to get hold of your async delegate instance
without passing it via the AsyncState. Look for the section of my reply
about the System.Runtime.Remoting.Messaging.AsyncResult class
[Original message clipped]

correct solution is to return an array of Objects, like this:
[Original message clipped]

Reply to this message...

	System.AsyncCallback
	System.Collections.ArrayList
	System.IAsyncResult
	System.Runtime.Remoting.Messaging.AsyncResult

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