C# - Lock question using EnterWriteLock

Question

The following code is from MSDN:

private ReaderWriterLockSlim cacheLock = new ReaderWriterLockSlim();
private Dictionary<int, string> innerCache = new Dictionary<int, string>();

public void Add(int key, string value)
{
    cacheLock.EnterWriteLock();
    try
    {
        innerCache.Add(key, value);
    }
    finally
    {
        cacheLock.ExitWriteLock();
    }
}

I've seen code like this in other places.The EnterWriteLock() is always outside the try block. Does anyone know why it's not inside the try block?

Answer 1

Suppose the EnterWriteLock() fails. For whatever reason.

Then the one thing you shouldn't do is to Exit a lock you never Entered.

It's a very basic pattern that also holds for example for streams, but not seen as often thanks to the using() {} statement.

var s = File.Create(...);
// (only) if the previous line succeeded, 
// we gain the responsibility to close s, no matter what
try
{
   // do some I/O
}
finally
{
   s.Dispose();
}