C# custom Iterator implementation - prevent collection modification during foreach loop

Question

I created a custom collection that implements IEnumerable(T) and a custom IEnumerator(T).

I also added an Add() method to the custom collection which looks like this:

public void Add(T item)
{
    T[] tempArray = new T[_array.Length + 1];

    for (int i = 0; i < _array.Length; i++)
    {
        tempArray[i] = _array[i];
    }

    tempArray[_array.Length] = item;

    _array = tempArray;
    tempArray = null;
}

The implementation is based on this example http://msdn.microsoft.com/en-us/library/system.collections.ienumerator.aspx.

When I do a foreach loop with my array I would like to prevent collection modification (like calling Add() inside the loop) and throw a new InvalidOperationException. How would I be able to do that?

Answer 1

You'd need to have a version ID within your class. Increment it on entry to Add. When you create an iterator (in the GetEnumerator() call) you would remember the version number - and on each iteration, you'd check whether the version number is still what it was to start with, throwing otherwise.