_lock.EnterUpgradeableReadLock（）有什么好处？

Question

我看到了以下构造的许多案例：

try {}
finally { _lock.EnterUpgradeableReadLock(); }

像这样：

private ReaderWriterLockSlim _lock = new ReaderWriterLockSlim();

public void A<T>(Action<ICollection<T>> operation)
{
    try { }
    finally { _lock.EnterUpgradeableReadLock(); }
    try
    {
        B(operation);
    }
    finally
    {
        _lock.ExitUpgradeableReadLock();
    }
}

但我不明白发生了什么事？利润是多少？