Add first or default method tests

ShittyLINQ/FirstOrDefault.cs

@@ -0,0 +1,18 @@
+using System;
+using System.Collections.Generic;
+using System.Text;
+
+namespace ShittyLINQ
+{
+    public static partial class Extensions
+    {
+
+        public static T FirstOrDefault<T>(this IEnumerable<T> self)
+        {
+            if (self == null) throw new ArgumentNullException();
+
+            var iterator = self.GetEnumerator();
+            return !iterator.MoveNext() ? default(T) : iterator.Current;
+        }
+    }
+}

ShittyLinqTests/FirstOrDefaultTests.cs

@@ -0,0 +1,40 @@
+using Microsoft.VisualStudio.TestTools.UnitTesting;
+using ShittyLINQ;
+using System;
+
+namespace ShittyTests
+{
+
+    [TestClass]
+    public class FirstOrDefaultTests
+    {
+        [TestMethod]
+        public void FirstOrDefault_GetFirstNumber()
+        {
+            int[] numbers = new int[] { 1, 2, 3, 4, 5 };
+            int expectedResult = 1;
+
+            int result = numbers.First();
+
+            Assert.AreEqual(expectedResult, result);
+        }
+
+        public void FirstOrDefaultTests_SourceIsEmpty()
+        {
+            int[] numbers = new int[] { };
+            int expectedResult = 0;
+
+            int result = numbers.First();
+            Assert.AreEqual(expectedResult, result);
+        }
+
+        [TestMethod]
+        [ExpectedException(typeof(ArgumentNullException))]
+        public void First_SourceIsNull()
+        {
+            int[] numbers = null;
+
+            int result = numbers.First();
+        }
+    }
+}