Just a simple class that implements IEnumerable.

Counter.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace YourNamespaceHere // this sucks for copying classes between projects
{
    public class Counter : IEnumerable<int>
    {
        private int min, max, step;

        public Counter(int min, int max)
        {
            this.min = min;
            this.max = max;
            this.step = (min > max) ? -1 : 1;
        }

        public Counter(int min, int max, int step)
        {
            this.min = min;
            this.max = max;
            this.step = step;
        }

        public IEnumerator<int> GetEnumerator()
        {
            return new Enumerator(min, max, step);
        }

        System.Collections.IEnumerator System.Collections.IEnumerable.GetEnumerator()
        {
            return GetEnumerator();
        }

        public class Enumerator : IEnumerator<int>
        {
            private int min, max, step;
            private int value;

            public Enumerator(int min, int max, int step)
            {
                this.min = min;
                this.max = max;
                this.step = step;
                Reset();
            }

            public int Current
            {
                get { return value; }
            }

            object System.Collections.IEnumerator.Current
            {
                get { return Current; }
            }

            public bool MoveNext()
            {
                value += step;
                return (value <= max);
            }

            public void Reset()
            {
                value = min - step;
            }

            public void Dispose()
            {
                // nothing to do here
            }
        }
    }
}