A Study In Equality

using System;
using System.Collections.Generic;

// A program to test and demonstrate the difference between reference and value types when used in comparisons
// and as keys to dictionaries.

namespace refvaluetests
{
	public struct AStruct
	{
		private readonly int i;

		public int I
		{
			get { return i; }
		}

		public AStruct(int i)
		{
			this.i = i;
		}
	}

	public struct AStructWithOperatorEquals
	{
		private readonly int i;

		public AStructWithOperatorEquals(int i)
		{
			this.i = i;
		}

		public static bool operator ==(AStructWithOperatorEquals s1, AStructWithOperatorEquals s2)
		{
			bool ret = false;

			if (((object)s1 != null) && ((object)s2 != null))
			{
				ret = s1.i == s2.i;
			}

			return ret;
		}

		/// <summary>
		/// If one is defined, the other is required.
		/// </summary>
		public static bool operator !=(AStructWithOperatorEquals s1, AStructWithOperatorEquals s2)
		{
			return !(s1 == s2);
		}

		/// <summary>
		/// Also this is required!
		/// </summary>
		public override bool Equals(object obj)
		{
			bool ret = false;

			if (!(obj is AStructWithOperatorEquals))
			{
				ret = false;
			}
			else
			{
				ret = i == ((AStructWithOperatorEquals)obj).i;
			}

			return ret;
		}

		/// <summary>
		/// Avoid the compiler warning by implementing this method.
		/// </summary>
		public override int GetHashCode()
		{
			return base.GetHashCode();
		}
	}

	public class AClass
	{
		private readonly int i;

		public int I
		{
			get { return i; }
		}

		private AClass() { }

		public AClass(int i)
		{
			this.i = i;
		}
	}

	public class AnEqualsClass
	{
		public readonly int i;

		private AnEqualsClass() { }

		public AnEqualsClass(int i)
		{
			this.i = i;
		}

		public override bool Equals(object obj)
		{
			bool ret = false;
			AnEqualsClass s = obj as AnEqualsClass;

			if (s == null)
			{
				ret = false;
			}
			else
			{
				ret = i == s.i;
			}

			return ret;
		}

		/// <summary>
		/// Avoid the compiler warning by implementing this method.
		/// We need this method for Equals to work with generics like lists and Dictionaries.
		/// </summary>
		public override int GetHashCode()
		{
			// This is very important!
			// We return the hash code of our field, not the base algorithm.
			// The base algorithm returns different values for different instances.
			// RIGHT:
			return i.GetHashCode();
			// WRONG:
			// return base.GetHashCode();
		}
	}

	public class AnOperatorEqualClass
	{
		private readonly int i;

		public int I
		{
			get { return i; }
		}

		private AnOperatorEqualClass() { }

		public AnOperatorEqualClass(int i)
		{
			this.i = i;
		}

		public static bool operator ==(AnOperatorEqualClass s1, AnOperatorEqualClass s2)
		{
			bool ret = false;

			if (((object)s1 != null) && ((object)s2 != null))
			{
				ret = s1.i == s2.i;
			}

			return ret;
		}

		/// <summary>
		/// If one is defined, the other is required.
		/// </summary>
		public static bool operator !=(AnOperatorEqualClass s1, AnOperatorEqualClass s2)
		{
			return !(s1 == s2);
		}

		/// <summary>
		/// Also this is required!		 
		/// </summary>
		public override bool Equals(object obj)
		{
			bool ret = false;
			AnOperatorEqualClass s = obj as AnOperatorEqualClass;

			if (s==null)
			{
				ret = false;
			}
			else
			{
				ret = i == s.i;
			}

			return ret;
		}

		/// <summary>
		/// Avoid the compiler warning by implementing this method.
		/// </summary>
		public override int GetHashCode()
		{
			// This is very important!
			// We return the hash code of our field, not the base algorithm.
			// The base algorithm returns different values for different instances.
			return i.GetHashCode();
		}
	}

	class Program
	{
		static void Main(string[] args)
		{
			CompareStructs();
			CompareStructsWithOperatorEquals();
			CompareClasses();
			CompareEqualsClasses();
			CompareOperatorEqualClasses();
			IndexStructs();
			IndexClasses();
			IndexEqualsClasses();
			DictionaryStruct();
			DictionaryClasses();
			DictionaryEqualsClasses();
			Console.WriteLine("\r\n");
		}

		static void CompareStructs()
		{
			Console.WriteLine("CompareStructs:");
			AStruct s1 = new AStruct(10);
			AStruct s2 = new AStruct(10);
			Console.WriteLine("AStruct.Equals(AStruct) ? " + ((s1.Equals(s2)) ? "Yes" : "No"));
			// operator == cannot be applied to operands AStruct because operator == is not implemented.
			// Console.WriteLine("s1 == s2 ? " + ((s1 == s2) ? "Yes" : "No"));
		}

		static void CompareStructsWithOperatorEquals()
		{
			Console.WriteLine("\r\nCompareStructsWithOperatorEquals:");
			AStructWithOperatorEquals s1 = new AStructWithOperatorEquals(10);
			AStructWithOperatorEquals s2 = new AStructWithOperatorEquals(10);
			Console.WriteLine("s1 == s2 ? " + ((s1 == s2) ? "Yes" : "No"));
		}

		static void CompareClasses()
		{
			Console.WriteLine("\r\nCompareClasses:");
			AClass s1 = new AClass(10);
			AClass s2 = new AClass(10);
			Console.WriteLine("AClass.Equals(AClass) ? " + ((s1.Equals(s2)) ? "Yes" : "No"));
			Console.WriteLine("s1 == s2 ? " + ((s1 == s2) ? "Yes" : "No"));
		}

		static void CompareEqualsClasses()
		{
			Console.WriteLine("\r\nCompareEqualsClasses:");
			AnEqualsClass s1 = new AnEqualsClass(10);
			AnEqualsClass s2 = new AnEqualsClass(10);
			Console.WriteLine("AnEqualsClass.Equals(AnEqualsClass) ? " + ((s1.Equals(s2)) ? "Yes" : "No"));
			Console.WriteLine("s1 == s2 ? " + ((s1 == s2) ? "Yes" : "No"));
		}

		static void CompareOperatorEqualClasses()
		{
			Console.WriteLine("\r\nCompareOperatorEqualClasses:");
			AnOperatorEqualClass s1 = new AnOperatorEqualClass(10);
			AnOperatorEqualClass s2 = new AnOperatorEqualClass(10);
			Console.WriteLine("AnOperatorEqualsClass.Equals(AnOperatorEqualsClass) ? " + ((s1.Equals(s2)) ? "Yes" : "No"));
			Console.WriteLine("s1 == s2 ? " + ((s1 == s2) ? "Yes" : "No"));
		}

		static void IndexStructs()
		{
			Console.WriteLine("\r\nIndexStructs:");
			List<AStruct> list = new List<AStruct>();
			AStruct s1 = new AStruct(10);
			list.Add(s1);
			AStruct s2 = new AStruct(10);
			Console.WriteLine("List contains s2 : " + list.Contains(s2));
		}

		static void IndexClasses()
		{
			Console.WriteLine("\r\nIndexClasses:");
			List<AClass> list = new List<AClass>();
			AClass s1 = new AClass(10);
			list.Add(s1);
			AClass s2 = new AClass(10);
			Console.WriteLine("List contains s2 : " + list.Contains(s2));
		}

		static void IndexEqualsClasses()
		{
			Console.WriteLine("\r\nIndexEqualsClasses:");
			List<AnEqualsClass> list = new List<AnEqualsClass>();
			AnEqualsClass s1 = new AnEqualsClass(10);
			list.Add(s1);
			AnEqualsClass s2 = new AnEqualsClass(10);
			Console.WriteLine("List contains s2 : " + list.Contains(s2));
		}

		static void DictionaryStruct()
		{
			Console.WriteLine("\r\nDictionaryStruct:");
			Dictionary<AStruct, int> dict = new Dictionary<AStruct, int>();
			AStruct s1 = new AStruct(10);
			dict[s1] = 1;
			AStruct s2 = new AStruct(10);
			Console.WriteLine("Dictionary contains s2 : " + dict.ContainsKey(s2));
		}

		static void DictionaryClasses()
		{
			Console.WriteLine("\r\nDictionaryClasses:");
			Dictionary<AClass, int> dict = new Dictionary<AClass, int>();
			AClass s1 = new AClass(10);
			dict[s1] = 1;
			AClass s2 = new AClass(10);
			Console.WriteLine("Dictionary contains s2 : " + dict.ContainsKey(s2));
		}

		static void DictionaryEqualsClasses()
		{
			Console.WriteLine("\r\nDictionaryEqualsClasses:");
			Dictionary<AnEqualsClass, int> dict = new Dictionary<AnEqualsClass, int>();
			AnEqualsClass s1 = new AnEqualsClass(10);
			dict[s1] = 1;
			AnEqualsClass s2 = new AnEqualsClass(10);
			Console.WriteLine("Dictionary contains s2 : " + dict.ContainsKey(s2));
		}
	}
}