when does a method return if a new thread is started inside it

Question

Suppose I have some code that looks like this:

public SomeMethod() {

   foreach (x...)
   {
     if (SomeCondition)
     {
        var SomeVariable = x;
        Task.Factory.StartNew(() => SomeOtherMethod(SomeVariable));
     }
   }

   return SomeValue;
}

If SomeOtherMethod is called and started in a new thread, does a) SomeMethod wait until that thread is finished running before returning or b) the method return and then the threads of SomeOtherMethod just continue on their own even after SomeMethod returned?

Reason I ask is that I need to wait until all SomeOtherMethods finish before exit SomeMethod.

Thanks.

It's a situation in my code. The threads are database calls. — frenchie, Feb 22 '13 at 19:07
All you're really asking for is how to wait until another thread completes. That has been [answered quite comprehensively already](http://stackoverflow.com/a/2228508/716118). — voithos, Feb 22 '13 at 19:08
Then I guess the question should be "how to wait for another thread to finish?" — John Dvorak, Feb 22 '13 at 19:08
I dummied down the answer to just 2 options so can I get an a) or b) instead of links?? Thanks. — frenchie, Feb 22 '13 at 19:10

Jon Skeet · Accepted Answer · 2013-02-22T19:24:07.117

5

Collect all the tasks in a collection, then call Task.WaitAll on it:

List<Task> tasks = new List<Task>();
foreach (x...)
{
    if (SomeCondition)
    {
        var someVariable = x;
        tasks.Add(Task.Factory.StartNew(() => SomeOtherMethod(someVariable));
     }
}
Task.WaitAll(tasks.ToArray());

Note that in .NET 4.5 when writing an async method, you would use:

await Task.WhenAll(tasks);

as Task.WhenAll itself doesn't block, but returns a task which completes when all the others have. (Note the difference between WhenAll and WaitAll here - very different methods!)

(Additionally, with .NET 4.5 you can use Task.Run instead of Task.Factory.StartNew, just for convenience.)

edited Feb 22 '13 at 19:24

answered Feb 22 '13 at 19:13

Jon Skeet

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lol...you just had to one up me with your fancy 4.5 knowledge!! – P.Brian.Mackey Feb 22 '13 at 19:18
I just love Jon's answers! Thanks!! – frenchie Feb 22 '13 at 19:29

P.Brian.Mackey · Answer 2 · 2013-02-22T19:19:29.723

Task.WaitAll can be used to track and wait for all Tasks to complete. StartNew returns a Task. Track all returned values in an array and pass it to WaitAll.

E.G.

        List<Task> tasks = new List<Task>();
        while (condition)
        {
            if (true)
            {                    
                Task temp = Task.Factory.StartNew(() => 0);
                tasks.Add(temp);
            }
            //...
        }

        Task.WaitAll(tasks.ToArray());

when does a method return if a new thread is started inside it

2 Answers2