Question

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Limiting the number of threads executing a method at a single time

In a single app we need to limit executing a section of code to 4 threads. This section is not the start/end of the thread, it is a section of code inside the thread.

How to do this with one is easy – create an EventWaitHandle and wait/signal on it. But every way I’ve thought of to do 4 leaves the possibility of determining I need to wait, another thread exists the section and signals, and I then wait – forever.

So what is the most light-weight way to do this when all threads are in a single app?

Background: We are changing our licensing model to allow a maximum of N (4 in this example) threads calling our library consecutively. Because we’re a library, not a server, we don’t control the thread usage (like databases can). In the case of a web app, the calling app also doesn’t control the number of threads and so we need to pause thread 5 calling us until one the first 4 exit that code.

Answer 1

Semaphore is the threading primitive that would fit you perfectly. It will allow up to the specified number of concurrent users; an additional user will block.

private static Semaphore _semaphore = new Semaphore(4, 4);

Then for the code you want to share:

_semaphore.WaitOne();
try
{
    // protected code goes here
}
finally
{
    _semaphore.Release();
}

The nice thing about this in terms of overhead is that WaitOne is efficient: it blocks your thread properly, and you get unblocked efficiently when someone calls a Release.