Question

I’ve just started programming in C#, and I’ve stumbled upon a small problem – lock doesn’t seem to work and I get a “Collection was modified after the enumerator was instantiated.” exception.

My code basically works like this:

private static object myLock = new object();

private Stack<MyObject> myObjects;

....

// Method that throws an exception
public void Update()
{
    lock(myLock)
    {
        foreach (MyObject ob in myObjects)
        {
            ob.Update(); // has a foreach loop of it's own
        }
    }

}

// Method invoked by a click event
public void InvokedMethod()
{
    lock(myLock)
    {
        myObjects.Push(new MyObject());
    }

}

What am I doing wrong?

Thank you !

Answer 1

This is most likely to do with the code that you haven’t included inside the foreach block – what are you doing in the code you’ve missed out? If you’re modifying the screens collection (add or remove), this will cause the exception you have seen.