Question

I have a multidimensional array with mutexes, and I use them so some critical sections. At the end is sketch of my class.

Now, I tried to add and delete characters in different threads without stopping, and at the same time getting characters from another thread to test if my server will work properly, but I get an exception when I try to get a character that says something like this: “Collection was modified during foreach..”

Anyone knows why? I have read the documentation of the lock statement in MSDN and it should work, I don’t know what I am doing wrong. Maybe new object[] can’t be used as a mutex?

If lock(objectReference) shouldn’t be used in this case, what should I use instead?

Sketch of my class:

partial class MapCharacter
{
    public MapCharacter()
    {
        mutexes = new object[5, 5];
        for (int x = 0; x < 5; x++)
        {
            for (int y = 0; y < 5; y++)
            {
                mutexes[x, y] = new object();
            }
        }
    }

    object[,] mutexes;

    public object RequestMutex(int x, int y)
    {
        return mutexes[x, y];
    }

    public void RemoveCharacter(Character characterToRemove)
    {
        lock (RequestMutex(characterToRemove.X, characterToRemove.Y))
        {
            loggedCharacters.Remove(characterToRemove);
        }
    }

    public void AddCharacter(Character characterToAdd)
    {
        lock (RequestMutex(characterToRemove.X, characterToRemove.Y))
        {
            loggedCharacters.Add(characterToAdd);
        }
    }

    public Character[] GetCharacters(int x, int y)
    {
        lock (RequestMutex(characterToRemove.X, characterToRemove.Y))
        {
            foreach (var charact in loggedCharacters)
            {
                // exception here
            }
        }
    }

    // more methods that use the mutex when trying to remove/add/modify characters
}

Answer 1

Your problem is that you are locking the individual characters but not loggedCharacters. You need to protect loggedCharacters with a lock or make it a concurrent collection in order to avoid that exception.