Comments on: The Yield Statement in C# http://www.alteridem.net/2007/08/22/the-yield-statement-in-c/ Software by Design Fri, 04 Jul 2008 03:18:00 +0000 http://wordpress.org/?v=2.5.1 By: Lars Corneliussen http://www.alteridem.net/2007/08/22/the-yield-statement-in-c/#comment-2103 Lars Corneliussen Tue, 10 Jun 2008 21:40:00 +0000 http://www.alteridem.net/2007/08/22/the-yield-statement-in-c/#comment-2103 If you're interested in how the yield keyword actually works, read this: http://startbigthinksmall.wordpress.com/2008/06/09/behind-the-scenes-of-the-c-yield-keyword/ If you’re interested in how the yield keyword actually works, read this:

http://startbigthinksmall.wordpress.com/2008/06/09/behind-the-scenes-of-the-c-yield-keyword/

]]> By: David Brenchley http://www.alteridem.net/2007/08/22/the-yield-statement-in-c/#comment-1158 David Brenchley Fri, 24 Aug 2007 14:44:44 +0000 http://www.alteridem.net/2007/08/22/the-yield-statement-in-c/#comment-1158 Is the yield threadsafe? What happens when you have 2 threads hitting the same static method? Is the state maintained for each separate thread? Is the yield threadsafe? What happens when you have 2 threads hitting the same static method? Is the state maintained for each separate thread?

]]> By: Judah http://www.alteridem.net/2007/08/22/the-yield-statement-in-c/#comment-1157 Judah Thu, 23 Aug 2007 16:08:54 +0000 http://www.alteridem.net/2007/08/22/the-yield-statement-in-c/#comment-1157 One key point to add is that execution of yield methods are deferred: they aren't evaluated until you foreach over them. Thus, the following code, even though it looks like an infinite loop, is perfectly fine: while(true) { yield return 1; } It will simply keep yielding 1 until you stop foreach-ing over it. Another point to consider is that since yield methods are delay-executed, any argument validation won't get applied until you foreach over it. For example, IEnumerable GetEvens(IEnumerable input) { if (input == null ) throw new ArgumentNullException(); ... yield return ...; } Calling that method like this: IEnumerable evens = GetEvents(null); It won't result in an exception! Not until you foreach over evens. For this reason, argument validation should be done first, then return the result of another method that does the actual yielding. One key point to add is that execution of yield methods are deferred: they aren’t evaluated until you foreach over them. Thus, the following code, even though it looks like an infinite loop, is perfectly fine:

while(true)
{
yield return 1;
}

It will simply keep yielding 1 until you stop foreach-ing over it.

Another point to consider is that since yield methods are delay-executed, any argument validation won’t get applied until you foreach over it. For example,

IEnumerable GetEvens(IEnumerable input)
{
if (input == null ) throw new ArgumentNullException();
…

yield return …;
}

Calling that method like this:
IEnumerable evens = GetEvents(null);

It won’t result in an exception! Not until you foreach over evens.

For this reason, argument validation should be done first, then return the result of another method that does the actual yielding.

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