Analyzer + codefix: Mark objects used for locking as readonly (new or RCS1169 derivative)

[IanKemp]

trafficstars

Consider:

object locker = new object();

void LockedAction()
{
    lock (locker)
    {
        // ... do something requiring locking...
    }
}

But any code can reassign the value of locker:

void SomethingElse()
{
    locker = new object();
}

The end result is that LockedAction is no longer thread-safe and Bad Things™ will happen when it is invoked concurrently.

To prevent this bug, locker should be declared as readonly so that it can never be reassigned:

readonly object locker = new object();

void SomethingElse()
{
    locker = new object(); // compile error
}

I'm aware this is already covered by RSC1169, but this is a special and IMO more important case.