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

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.