Question

I have List<string> collection called List<string> list.

I have two threads.
One thread is enumerating through all list elements and adding to collection.
Second thread is enumerating through all list elements and removing from it.

How can make it thread safe?
I tried creating global Object “MyLock” and using lock(MyLock) block in each thread function but it didn’t work.

Can you help me?

Answer 1

If you have access to .NET 4.0 you can use the class ConcurrentQueue or a BlockingCollection with a ConcurrentQueue backing it. It does exactly what you are trying to do and does not require any locking. The BlockingCollection will make your thread wait if there is no items available in the list.

A example of removing from the ConcurrentQueue you do something like

ConcurrentQueue<MyClass> cq = new ConcurrentQueue<MyClass>();

void GetStuff()
{
    MyClass item;
    if(cq.TryDeqeue(out item))
    {
        //Work with item
    }
}

This will try to remove a item, but if there are none available it does nothing.

BlockingCollection<MyClass> bc = BlockingCollection<MyClass>(new ConcurrentQueue<MyClass>());

void GetStuff()
{
    if(!bc.IsCompleated) //check to see if CompleatedAdding() was called and the list is empty.
    {
        try
        {
            MyClass item = bc.Take();
            //Work with item
        }
        catch (InvalidOpperationExecption)
        {
            //Take is marked as completed and is empty so there will be nothing to take
        }
    }
}

This will block and wait on the Take till there is something available to take from the list. Once you are done you can call CompleteAdding() and Take will throw a execption when the list becomes empty instead of blocking.