Added Single method and tests.

ShittyLINQ/Single.cs

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+using System;
+using System.Collections.Generic;
+using System.Text;
+
+namespace ShittyLINQ
+{
+  public static partial class Extensions
+  {
+    public static T Single<T>(this IEnumerable<T> self)
+    {
+      if (self == null) throw new ArgumentNullException();
+      using (IEnumerator<T> e = self.GetEnumerator())
+      {
+        if (!e.MoveNext())
+        {
+          throw new InvalidOperationException();
+        }
+
+        T result = e.Current;
+        if (!e.MoveNext())
+        {
+          return result;
+        }
+        throw new InvalidOperationException();
+      }
+    }
+
+    public static T Single<T>(this IEnumerable<T> self, Func<T, bool> predicate)
+    {
+      if (self == null || predicate == null) throw new ArgumentNullException();
+
+      return self.Where(predicate).Single();
+    }
+  }
+}

ShittyLinqTests/SingleTests.cs

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+using Microsoft.VisualStudio.TestTools.UnitTesting;
+using ShittyLINQ;
+using System;
+using System.Collections.Generic;
+using System.Text;
+
+namespace ShittyTests
+{
+  [TestClass]
+  public class SingleTests
+  {
+    [TestMethod]
+    public void Single_GetSingleElement()
+    {
+      int[] numbers = new int[] { 1 };
+      int expectedResult = 1;
+
+      int result = numbers.Single();
+
+      Assert.AreEqual(expectedResult, result);
+    }
+
+    [TestMethod]
+    public void Single_GetMatchingSingleElement()
+    {
+      int[] numbers = new int[] { 1, 2 };
+      bool predicate(int i) => i == 1;
+      int expectedResult = 1;
+
+      int result = numbers.Single(predicate);
+
+      Assert.AreEqual(expectedResult, result);
+    }
+
+    [TestMethod]
+    [ExpectedException(typeof(ArgumentNullException))]
+    public void Single_SourceIsNull()
+    {
+      int[] numbers = null;
+
+      int result = numbers.Single();
+    }
+
+    [TestMethod]
+    [ExpectedException(typeof(ArgumentNullException))]
+    public void Single_PredicateIsNull()
+    {
+      int[] numbers = new int[] { 1 };
+      Func<int, bool> predicate = null;
+
+      int result = numbers.Single(predicate);
+    }
+
+    [TestMethod]
+    [ExpectedException(typeof(InvalidOperationException))]
+    public void Single_SourceIsEmpty()
+    {
+      int[] numbers = new int[] { };
+
+      int result = numbers.Single();
+    }
+
+    [TestMethod]
+    [ExpectedException(typeof(InvalidOperationException))]
+    public void Single_SourceIsEmptyWithPredicate()
+    {
+      int[] numbers = new int[] { };
+      bool predicate(int i) => i == 1;
+
+      int result = numbers.Single(predicate);
+    }
+
+    [TestMethod]
+    [ExpectedException(typeof(InvalidOperationException))]
+    public void Single_SourceHasMoreThanOneElement()
+    {
+      int[] numbers = new int[] { 1, 2 };
+      int result = numbers.Single();
+    }
+
+    [TestMethod]
+    [ExpectedException(typeof(InvalidOperationException))]
+    public void Single_MoreThanOneMatchingElement()
+    {
+      int[] numbers = new int[] { 1, 2, 1 };
+      bool predicate(int i) => i == 1;
+
+      int result = numbers.Single(predicate);
+    }
+
+    [TestMethod]
+    [ExpectedException(typeof(InvalidOperationException))]
+    public void Single_NoMatchingElement()
+    {
+      int[] numbers = new int[] { 1, 2, 3 };
+      bool predicate(int i) => i == 4;
+
+      int result = numbers.Single(predicate);
+    }
+  }
+}