Parameter to QueueUserWorkItem with a lambda expression?

Question

In following code, what does the parameter 's' stand for? Can we not just omit 's' since its not being used in the method, so we have an anonymous method with no parameter like () => ...?

ThreadPool.QueueUserWorkItem((s)=> 
{
 Console.WriteLine("Working on a thread from threadpool");
});

UPDATE 1:

According to the accepted answer, the anonymous method is just a replacement for the normal WaitCallback delegate method like the one in ocd below, that is needed by QueueUserWorkItem as a parameter to it. Therefore, 's' should be of object type, since its a parameter to the ThreadProc method.

void ThreadProc(Object stateInfo) {
   // No state object was passed to QueueUserWorkItem, so  
   // stateInfo is null.
    Console.WriteLine("Working on a thread from threadpool");
 }

Ben Voigt · Accepted Answer · 2014-09-12 05:17:16Z

4

The C# 2.0 syntax for anonymous delegates allows omitting the parameter list, in which case it will match any set of (non-ref non-out) parameters and ignore them.

ThreadPool.QueueUserWorkItem(delegate {
   Console.WriteLine("Working on a thread from threadpool");
});

Note that delegate {} is different from delegate () {}

The lambda syntax, on the other hand, doesn't work without a parameter list provided.

answered Sep 12, 2014 at 5:17

Ben Voigt

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7 Comments

Sunil Over a year ago

When I check if 's' is null within the anonymous method, it is null. So its confusing why 's' is needed?

Ben Voigt Over a year ago

@Sunil: It is needed because it is part of the System.Threading.WaitCallback type.

Sunil Over a year ago

I can run a Task like this: Task.Run (()=> { DoThis();}), which allows a method without any parameters. Sorry its not making sense.

Sunil Over a year ago

So its the parameter of WaitCallback type that is normally passed to QueueUserWorkItem method?

Ben Voigt Over a year ago

Yes, QueueUserWorkItem requires a WaitCallback delegate. So the lambda or anonymous delegate has to be compatible with it. And WaitCallback has one parameter.

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