C# multicast delegate method return type

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Hi All,
I have some confusion on Multicast Delegate(in C#),

when the multicast delegate is called, it invokes the

method and should the method must void? I can't find

any official document.

Please give me a hand and thanks a lot.

You can check the example the link as below.

http://msdn.microsoft.com/zh-cn/library/ms173175.aspx

Posted 23-Apr-11 5:33am

jkhuang

Updated 23-Apr-11 5:34am

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2 solutions

Solution 2

You can only use return type of the delegate if you do not invoke the delegate, but manually call each handler from the invocation list of the delegate invocation list. Here is the example on how to do it:

C#

delegate int NonVoidDelegate(string parameter);

//...


NonVoidDelegate delegateInstance = delegate(string parameter) { return 1; };
delegateInstance += delegate(string parameter) { return 2; };
delegateInstance += delegate(string parameter) { return 3; };

var list = delegateInstance.GetInvocationList();

foreach (var handler in list) {
   NonVoidDelegate method = (NonVoidDelegate)handler;
   int retValue = method("some parameter");
   //do something with retValue...
}

—SA

Posted 23-Apr-11 16:19pm

Sergey Alexandrovich Kryukov

Updated 19-Dec-13 6:15am

v2

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Nish Nishant · Accepted Answer · 2011-04-23T05:35:00

Solution 1

In general, it's a good practice to use delegates with void return types. Otherwise when you have a multicast delegate the return value is unpredictable because the order of delegate invocation is not guaranteed.

Posted 23-Apr-11 5:35am

Nish Nishant

Comments

Richard MacCutchan 23-Apr-11 11:38am

A nice, clear, simple answer that tells us everything we need to know: +5.

Nish Nishant 23-Apr-11 11:38am

Thanks Richard!

Sergey Alexandrovich Kryukov 23-Apr-11 22:22pm

Nishant, this is a prudent advice for those who wants to do just the basics -- my 5.
For crazy people like myself there is an advanced use (I used it for quite a practical purposes).
For this advanced use, return value does make sense.
Please see my answer and code sample.
--SA

ThatsAlok 18-Dec-13 1:54am

better to put parameter as reference, if you want to return values from multicast delegate

Sergey Alexandrovich Kryukov 18-Dec-13 3:11am

Why?! I don't see any benefits of it. Can you explain in detail?
—SA

ThatsAlok 19-Dec-13 4:52am

Here :-
public delegate int delegateReturnInt();
event delegateReturnInt evntdelegateReturnInt;

public delegate void delegateReturnIntRef(ref int value);
event delegateReturnIntRef evntdelegateReturnIntRef;
evntdelegateReturnInt += new delegateReturnInt(Form1_evntdelegateReturnInt10);
evntdelegateReturnInt += new delegateReturnInt(Form1_evntdelegateReturnInt20);

evntdelegateReturnIntRef += new delegateReturnIntRef(Form1_evntdelegateReturnIntRef);
evntdelegateReturnIntRef += new delegateReturnIntRef(Form1_evntdelegateReturnIntRef2);

One Way - by using invocation list !-
var list = evntdelegateReturnInt.GetInvocationList();
foreach (var handler in list)
{
delegateReturnInt method = (delegateReturnInt)handler;
int retValue = method();
//do something with retValue...
}

Second way:- not good, however

int value = 20;
evntdelegateReturnIntRef.Invoke(ref value);

Sergey Alexandrovich Kryukov 19-Dec-13 12:18pm

First sample looks like you copied it from my answer. In second way, you are not returning anything. I would say this approach is acceptable in some cases, but this is not how a problem is posed — how to deal with multiple return values?
—SA

Sergey Alexandrovich Kryukov 19-Dec-13 12:18pm

First sample looks like you copied it from my answer. In second way, you are not returning anything. I would say this approach is acceptable in some cases, but this is not how a problem is posed — how to deal with multiple return values?
—SA

ThatsAlok 19-Dec-13 12:21pm

agree its similar to you... but frankly i hav'nt copied from you :-)...

Sergey Alexandrovich Kryukov 19-Dec-13 12:30pm

Okay, no problem. I just thought you would have to see my answer before commenting, say, to avoid redundancy...
—SA

ThatsAlok 19-Dec-13 12:39pm

yeah i agree.. i missed that...apologies for any misconduct :-)

Sergey Alexandrovich Kryukov 19-Dec-13 12:43pm

Please, no problem at all. I also often miss a lot and write redundant posts, just trying to make it a bit better...
It's good to see that you understand the issue...
Cheers,
—SA