In C# how can I safely exit a lock with a try catch block inside?

Question

Here is an example of an exception happening inside a lock, with a try-catch block.

int zero = 0;
int j = 10;

lock (sharedResource.SyncRoot)
{
    try
    {
        j = j / zero;
    }
    catch (DivideByZeroException e)
    {
        // exception caught but lock not released
    }
}

How do I safely release this lock in the catch?

Answer 1

The lock won't be released until you pass out of the scope of the lock(sharedResource.SyncRoot) block. lock (sharedResource.SyncRoot) {} is basically identical to:

Monitor.Enter(sharedResource.SyncRoot);
try
{
}
finally
{
    Monitor.Exit(sharedResource.SyncRoot);
}

You can either do the Enter/Exit yourself if you want more control, or just rescope the lock to what you want, like:

try
{
    lock(sharedResource.SyncRoot)
    {
        int bad = 2 / 0;
    }
}
catch (DivideByZeroException e)
{
   // Lock released by this point.
}

Answer 2

Won't it be released automatically?

From the MSDN lock means

System.Threading.Monitor.Enter(x);
try {
   ...
}
finally {
   System.Threading.Monitor.Exit(x);
}

So you don't have to bother.