What to pass to the lock keyword?

Question

What is the difference (if any) between using

void MethodName()
{
    lock(this)
    {
        // (...)
    }
}

or

private object o = new object();
void MethodName()
{
    lock(o)
    {
        // (...)
    }
}

?

Is there a difference in performance? Style? Behaviour?

Answer 1

lock(this) will lock on the "current" object.

Locking on "this" is usually a bad idea as it exposes the lock to other code; I prefer to have a readonly field, like this:

public class Foo
{
    private readonly object padlock = new object();

    public void SomeMethod()
    {
        lock(padlock)
        {
            ...
        }
    }
}

That way all calls to SomeMethod (and anything else in Foo which locks on padlock) will lock on the same monitor for the same instance of Foo, but nothing else can interfere by locking on that monitor.

In reality, unless you're dealing with "rogue" code, it's unlikely that other code will actually lock on the reference to an instance of Foo, but it's a matter of encapsulation.