ReaderWriterLock blocks when trying to read resource

Question

I am trying to use a ReaderWriterLock for a shared resource between two Tasks.For some reason it starts waiting indefinitely:

class State {

        private const int TIMEOUT = 5000;
        private ReaderWriterLock lck = new ReaderWriterLock();
        private TimeSpan lastIssuedAt;

        public TimeSpan LastIssuedAt {
            get {

                this.lck.AcquireReaderLock(TIMEOUT);
                return this.lastIssuedAt;
            }
            set {
                this.lck.AcquireWriterLock(TIMEOUT);
                this.lastIssuedAt = value;
            }
        }
}

When a task tries to get the property LastIssuedAt it just blocks and I do not understand why.

I thought that it gets released when the method exits.Also do i need to release the `Readerlock` too ?Also for the `get` where do i release it ? before `return` ? — Bercovici Adrian, May 06 '19 at 14:34
Do i need an additional variable for the `get`? `get { lock.AcquireReader(); var value=... ; lock.ReleaseReader(); return val ; }` — Bercovici Adrian, May 06 '19 at 14:37

score 1 · Accepted Answer · answered May 06 '19 at 16:18

1

Take a look at the example on MSDN: https://learn.microsoft.com/en-us/dotnet/api/system.threading.readerwriterlock?view=netframework-4.8

You forgot to release the lock. A common pattern for doing so is try/finally:

ReaderWriterLock lck = new ReaderWriterLock();

lck.AcquireReaderLock(timeOut);
try
{
    // Do what needs to be done under the lock
}
finally 
{
    // Ensure that the lock is released.
    lck.ReleaseReaderLock();
}

Also, check out ReaderWriterLockSlim: https://learn.microsoft.com/en-us/dotnet/api/system.threading.readerwriterlockslim?view=netframework-4.8 Which MSDN recommends for new development.

answered May 06 '19 at 16:18

Peter Sulucz

116
4

But in my case i need to release the lock in the `getter`. – Bercovici Adrian May 06 '19 at 16:40
1

That's the magic of 'finally'. https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/keywords/try-finally Anything in the finally block will run when you leave the 'try' block. So you're lock will get released even if you return out of 'try'. – Peter Sulucz May 06 '19 at 18:29
I didn't want to use a `try-catch` inside the `getter`. – Bercovici Adrian May 06 '19 at 19:00
1

You don't need a catch. Just try/finally – Peter Sulucz May 06 '19 at 19:11

ReaderWriterLock blocks when trying to read resource

1 Answers1