ToLookup method and tests

ShittyLINQ/ToLookup.cs

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+using System;
+using System.Collections.Generic;
+
+namespace ShittyLINQ
+{
+	public static partial class Extensions
+	{
+		/// <summary>
+		/// Creates a lookup from an IEnumerable 
+		/// </summary>
+		/// <param name="source">Set of elements to be converted to lookup</param>
+		/// <param name="keySelector">Key selector to determine the key of the lookup</param>
+		/// <typeparam name="TSource">Source type</typeparam>
+		/// <typeparam name="TKey">Key type</typeparam>
+		/// <returns>A lookup made from source</returns>
+		/// <exception cref="ArgumentNullException">If source, keySelector or element Selector is null</exception>
+		public static Dictionary<TKey, IEnumerable<TSource>> ToLookup<TSource, TKey>(
+			this IEnumerable<TSource> source,
+			Func<TSource, TKey> keySelector)
+		{
+			return ToLookup(source, keySelector, t => t, EqualityComparer<TKey>.Default);
+		}
+
+		/// <summary>
+		/// Creates a lookup from an IEnumerable 
+		/// </summary>
+		/// <param name="source">Set of elements to be converted to lookup</param>
+		/// <param name="keySelector">Key selector to determine the key of the lookup</param>
+		/// <param name="elementSelector">Element selector to determine the value of the lookup</param>
+		/// <typeparam name="TSource">Source type</typeparam>
+		/// <typeparam name="TKey">Key type</typeparam>
+		/// <typeparam name="TElement">Value type</typeparam>
+		/// <returns>A lookup made from source</returns>
+		/// <exception cref="ArgumentNullException">If source, keySelector or element Selector is null</exception>
+		public static Dictionary<TKey, IEnumerable<TElement>> ToLookup<TSource, TKey, TElement>(
+			this IEnumerable<TSource> source,
+			Func<TSource, TKey> keySelector,
+			Func<TSource, TElement> elementSelector)
+		{
+			return source.ToLookup(keySelector, elementSelector, EqualityComparer<TKey>.Default);
+		}
+
+		/// <summary>
+		/// Creates a lookup from an IEnumerable 
+		/// </summary>
+		/// <param name="source">Set of elements to be converted to lookup</param>
+		/// <param name="keySelector">Key selector to determine the key of the lookup</param>
+		/// <param name="comparer">Comparer to be used to determine equality between elements</param>
+		/// <typeparam name="TSource">Source type</typeparam>
+		/// <typeparam name="TKey">Key type</typeparam>
+		/// <returns>A lookup made from source</returns>
+		/// <exception cref="ArgumentNullException">If source, keySelector or element Selector is null</exception>
+		public static Dictionary<TKey, IEnumerable<TSource>> ToLookup<TSource, TKey>(
+			this IEnumerable<TSource> source,
+			Func<TSource, TKey> keySelector,
+			IEqualityComparer<TKey> comparer)
+		{
+			return ToLookup(source, keySelector, t => t, comparer);
+		}
+
+		/// <summary>
+		/// Creates a lookup from an IEnumerable 
+		/// </summary>
+		/// <param name="source">Set of elements to be converted to lookup</param>
+		/// <param name="keySelector">Key selector to determine the key of the lookup</param>
+		/// <param name="elementSelector">Element selector to determine the value of the lookup</param>
+		/// <param name="comparer">Comparer to be used to determine equality between elements</param>
+		/// <typeparam name="TSource">Source type</typeparam>
+		/// <typeparam name="TKey">Key type</typeparam>
+		/// <typeparam name="TElement">Value type</typeparam>
+		/// <returns>A lookup made from source</returns>
+		/// <exception cref="ArgumentNullException">If source, keySelector or element Selector is null</exception>
+		public static Dictionary<TKey, IEnumerable<TElement>> ToLookup<TSource, TKey, TElement>(
+			this IEnumerable<TSource> source,
+			Func<TSource, TKey> keySelector,
+			Func<TSource, TElement> elementSelector,
+			IEqualityComparer<TKey> comparer)
+		{
+			if (source == null || keySelector == null || elementSelector == null) throw new ArgumentNullException();
+
+			List<TElement> tempList;
+			IEnumerable<TElement> existingValues;
+
+			Dictionary<TKey, IEnumerable<TElement>> ret = new Dictionary<TKey, IEnumerable<TElement>>(comparer ?? EqualityComparer<TKey>.Default);
+			foreach (TSource item in source)
+			{
+				existingValues  = ret.GetValueOrDefault(keySelector(item));
+				if (existingValues == null)
+				{
+					tempList = new List<TElement>();
+				}
+				else
+				{
+					tempList = existingValues.ToList();
+					ret.Remove(keySelector(item));
+				}
+				tempList.Add(elementSelector(item));
+				ret.Add(keySelector(item), tempList);
+
+			}
+			return ret;
+		}
+	}
+
+}

ShittyLinqTests/ToLookupTests.cs

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+using System.Collections.Generic;
+using Microsoft.VisualStudio.TestTools.UnitTesting;
+using ShittyLINQ;
+using ShittyTests.TestHelpers;
+
+namespace ShittyTests
+{
+	[TestClass]
+	public class ToLookupTests
+	{
+		[TestMethod]
+		public void ToLookup_Numbers()
+		{
+			IEnumerable<int> numbers = new int[] { 1, 2, 3, 4, 5, 6, 7, 8, 9 };
+
+			var result = numbers.ToLookup(x => x, x => true);
+
+			Assert.AreEqual(result.Count, 9);
+
+			foreach (int number in numbers)
+			{
+				Assert.IsTrue(result.ContainsKey(number));
+			}
+		}
+
+		[TestMethod]
+		public void ToLookup_Person()
+		{
+			var adam = new Person("Adam", 20, "Arquitech", "Amber");
+			var brian = new Person("Brian", 45, "Arquitech", "Blue");
+			var charles = new Person("Charles", 33, "Arquitech", "Cyan");
+			var dani = new Person("Dani", 33, "Developer", "Deep Purple");
+
+			IEnumerable<Person> people = new[] { adam, brian, charles, dani };
+
+			var result = people.ToLookup(person => person.Age);
+
+			Assert.AreEqual(result.TryGetValue(brian.Age, out var expectedBrian), true);
+			Assert.IsNotNull(expectedBrian);
+		}
+	}
+}