ReSharperMultipleEnumerationWarningBug.cs

using System.Collections;
using System.Collections.Generic;
using System.Linq;
using NUnit.Framework;

namespace TestPad
{
    public class Tests
    {
        [Test]
        public void NoWarning()
        {
            var enumerable = new CustomDataSource("Miss Kaboo made possum pie");
            GivesNoWarning(enumerable);
            Assert.That(enumerable.GetEnumeratorCalls, Is.EqualTo(1));
        }

        [Test]
        public void Warning()
        {
            var enumerable = new CustomDataSource("Miss Kaboo made possum pie");
            GivesWarning(enumerable);
            Assert.That(enumerable.GetEnumeratorCalls, Is.EqualTo(1));
        }

        class CustomDataSource : IEnumerable<char>
        {
            private string _data;

            public CustomDataSource(string data)
            {
                _data = data;
            }

            public int GetEnumeratorCalls { get; private set; }

            public IEnumerator<char> GetEnumerator()
            {
                GetEnumeratorCalls++;
                return ((IEnumerable<char>)_data.ToCharArray()).GetEnumerator();
            }

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

        public IEnumerable<char> GivesWarning(IEnumerable<char> a)
        {
            var b = a.Skip(5).ToList();
            var c = a.Take(5).ToList();
            var d = new List<char>();
            d.AddRange(b);
            d.AddRange(c);

            return c;
        }

        public IEnumerable<char> GivesNoWarning(IEnumerable<char> a)
        {
            var b = new List<char>();
            var c = new List<char>();
            var d = "abcdefghuijklmnopqrstuvwxyz";
            b.AddRange(d.Where(ch => !a.Contains(ch)));
            c = a.Where(ch => !b.Contains(ch)).ToList();

            return c;
        }
    }
}