SynchronizedEnumerator.cs

public class SynchronizedEnumerator<T> : IEnumerator<T>
{
    bool isDisposed;
    readonly object gate;
    readonly bool lockTaken;
    readonly IEnumerator<T> enumerator;

    public SynchronizedEnumerator(object gate, IEnumerator<T> enumerator)
    {
        this.gate = gate;
        this.enumerator = enumerator;
        Monitor.Enter(gate, ref lockTaken);
    }

    public T Current => enumerator.Current;

    object IEnumerator.Current => Current!;
    public bool MoveNext() => enumerator.MoveNext();
    public void Reset() => enumerator.Reset();

    public void Dispose()
    {
        if (!isDisposed)
        {
            isDisposed = true;
            try
            {
                enumerator.Dispose();
            }
            finally
            {
                if (lockTaken)
                {
                    Monitor.Exit(gate);
                }
            }
        }
    }
}