I have custom collection and I want to add wrapper to allow concurrent access. p

Question

I have custom collection and I want to add wrapper to allow concurrent access.

public class MyConcurrentCollection<T>
{
    private MyCollection _collection; // passed in constructor

    public void Add(T item)
    {
        //modifies and may temporarily "break" the collection while executing this method
    }

    public bool Contains(T item)
    {
        //only reads
    }

    // other read and write methods
}
At this moment, I have object private member that acts as lock in every method, allowing only one thread at a time to access the collection, so every method looks like this:

public bool Contains(T item)
{
    lock(_lock)
    {
        return _collection.Contains(item);
    }
}
However this seems really inefficient. Since Contains() only reads from the collection, should I allow multiple threads into it?

Of course, I need to lock access to Add() and other methods while there are threads in Contains() and I need to block access to Contains() if there is a thread wishing to modify the collection.

Is there any disadvantage of allowing multiple threads into read only methods, or should I stick with my basic solution?

Explanation / Answer

You can do it without locks, but the cost is that your add method will be more expensive. If you have a lot more reads than writes then your add method can just create a new and updated MyCollection object:

public void Add(T item)
{
MyCollection newCollection = new MyCollection();
newCollection.Append(_collection); // fill with old values
newCollection.Add(item); // add the new item
_collection = newCollection; // switch the internal pointer to the new object
}
This works because threads accessing the read method only ever see the old or the new object, so your collection is never in an in-between state for other threads. If you have more than one writer thread you must of course add a lock to the add method (but not to the read methods).

Despite all this, you should use collections from system.collections.concurrent if possible, because they are a lot less error-prone.

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