Lock scope in C#: is returned object still "locked"?

Question

Assuming I have an object A containing

// ...

private List<double> someList = new List<double>();

// ... 

public List<double> SomeList
{
    get { lock (this) { return someList; } }
}

// ...

would it be thread safe to perform operation on the list as in the code below. Knowing that several operations could be executed simultaneously by different threads.

A.SomeList.Add(2.0);

or

A.SomeList.RemoveAt(0);

In other words, when is the lock released?

Answer 1

There is no thread safety here.

The lock is released as soon as the block it protects is finished, just before the property returns, so the calls to Add ad RemoveAt are not protected by the lock.

Answer 2

The lock you shown in the question isn't of much use.

To make list operations thread safe you need to implement your own Add/Remove/etc methods wrapping those of the list.

public void Add(double item)
{
    lock(_list)
    {
        _list.Add(item);
    }
}

Also, it's a good idea to hide the list itself from the consumers of your class, i.e. make the field private.