Answer: When you don't have enough change for the phone booth.
I'll be here all week, thank you very much. Bad pun notwithstanding, the answer to this question is pretty much never (see Rico's almost rule #1). The Garbage Collector in .NET is like a highly motivated and skilled employee. If you quit being a micro-manager ("You forgot to put the cover sheet on the TPS report") and stop looking over its shoulder, it's able to just do its job and perform quite well.
However, note that Rico says "Almost Rule #1". That must mean there are appropriate exceptions to the rule, no matter how few they may be. What are those situations? The reason I ask is I ran into the following code on the net (dramatization):
/// Stops the socket server and closes
/// every client connection.
public void Stop()
throw new ObjectDisposedException(
"Object is already disposed.");
This is the Stop() method of your typical Socket Server. It closes any connected socket clients and then closes the listening thread. After that, it calls GC.Collect() and GC.WaitForPendingFinalizers(), violating Rico's almost rule. Is this perhaps one of those appropriate times to call GC.Collect()?
Typically, your socket server will have been running for a long time, so it is very likely it will have been promoted to Generation 2 and contain references to a several other Generation 2 objects. Rico points out that
If your algorithm is regularly producing objects that live to gen2 and then die shortly thereafter, you're going to find that the percent time spent in GC goes way up. Forcing more of these collects is really the last thing you wanted to do (assuming you could, note again GC.Collect() doesn't promise to do a gen2 collect).
However, this is situation is different in that the server has been around a while and calling Stop() on a server typically means you're not planning to use the Server any time soon afterwards. In fact, you're most likely about to dispose of it.
Given that, It seems to me that this might be one of those cases where calling GC.Collect() is appropriate. The goal here is a one time Generation 2 collection. Of course, there's no guarantee that a Generation 2 collect will occur. Maybe this is a situation where it makes no difference either way. Any thoughts?
For more reading:
Garbage Collector Basics and Performance Hints
Programming For Garbage Collection