SingleOrNull tests

namespace CompanyName.Linq.Test
{
    using CompanyName.Linq;
    using Microsoft.VisualStudio.TestTools.UnitTesting;
    using System;
    using System.Collections;
    using System.Collections.Generic;
    using System.Diagnostics;

    [TestClass]
    public class EnumerableExtensionsTester
    {
        [TestMethod]
        [ExpectedException(typeof(ArgumentNullException))]
        public void TestSingleOrNull_NullSource()
        {
            IEnumerable<int> enumerable = null;
            enumerable.SingleOrNull();
        }

        [TestMethod]
        public void TestSingleOrNull_EmptyList()
        {
            var enumerable = new int[0];
            var result = enumerable.SingleOrNull();
            Assert.IsNull(result);
        }

        [TestMethod]
        public void TestSingleOrNull_SingleElementList()
        {
            var enumerable = new[] { 42 };
            var result = enumerable.SingleOrNull();
            Assert.AreEqual(42, result);
        }

        [TestMethod]
        public void TestSingleOrNull_MultiElementList()
        {
            var enumerable = new[] { 42, 43 };
            var result = enumerable.SingleOrNull();
            Assert.IsNull(result);
        }

        private sealed class EnumerableFake<T> : IEnumerable<T>
        {
            private readonly IEnumerable<T> source;

            public EnumerableFake(IEnumerable<T> source)
            {
                Debug.Assert(source != null);
                this.source = source;
            }

            public IEnumerator<T> GetEnumerator()
            {
                return source.GetEnumerator();
            }

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

        [TestMethod]
        public void TestSingleOrNull_Empty()
        {
            var enumerable = new EnumerableFake<int>(new int[0]);
            var result = enumerable.SingleOrNull();
            Assert.IsNull(result);
        }

        [TestMethod]
        public void TestSingleOrNull_SingleElement()
        {
            var enumerable = new EnumerableFake<int>(new[] { 42 });
            var result = enumerable.SingleOrNull();
            Assert.AreEqual(42, result);
        }

        [TestMethod]
        public void TestSingleOrNull_MultiElement()
        {
            var enumerable = new EnumerableFake<int>(new[] { 42, 43 });
            var result = enumerable.SingleOrNull();
            Assert.IsNull(result);
        }
    }
}