Fun with Equality

Isn’t it funny how the “easiest” concepts can be the most complicated? A reader sent me the following quiz to help us appreciate the subtlies of equality in the system. Luckily he gave me the answers as well… ;-)

Consider the following program:

object x = new object();

object y = new object();

/*Question 1*/ Console.WriteLine(x == y && !x.Equals(y));

/*Question 2*/ Console.WriteLine(x != y && x.Equals(y));

/*Question 3*/ Console.WriteLine(x == y);

/*Question 4*/ Console.WriteLine(x == y && (object)x != (object)y);

They should all print “false” right?

For each question, find a way to declare and initialize x and y such that:

            Question 1: prints true

            Question 2: prints true

            Question 3: does not even compile

            Question 4: prints true

Notice, we are not asking for a solution that works for all 4, one solution for each will be fine…

All answers should be of the form:

         public static void Question<<question number>>()

        {

            <<type>> x = <<intialize instance>>;

            <<type>> y = <<intialize instance>>;

            Console.WriteLine(<<question>>);

        }

So for example, a legal (but incorrect) answer to question1 would be:

         public static void Question1()

     {

            string x = "1";

            object y = new Object() ;

            Console.WriteLine(x == y && !x.Equals(y));

        }

The rules of the game:

1. You can only use “base” data types: object, string, Int32, double, Type, etc
2. No more than two lines of plainly formatted code (before the Console.WriteLine)
3. You can’t change anything about the question line, it has to appear exactly as I show above
4. Extra credit for getting solutions no one else thinks of and for pointing out differences between versions of the CLR

Comments

• Anonymous
  April 03, 2005
  public static void Question1() {
  int x = 1;
  long y = 1;
  Console.WriteLine(x == y && !x.Equals(y));
  }
  public static void Question2() {
  double x = double.NaN;
  double y = double.NaN;
  Console.WriteLine(x != y && x.Equals(y));
  }
  public static void Question3() {
  Char x = 'a';
  String y = "a";
  Console.WriteLine(x != y && x.Equals(y));
  }
  public static void Question4() {
  int x = 1;
  long y = 1;
  Console.WriteLine(x == y && (object)x != (object)y);
  }

• Anonymous
  April 03, 2005
  // Question 1:
  public static void Question1()
  {
  int x = 5;
  float y = 5f;
  Console.WriteLine(x == y && !x.Equals(y));
  }
  
  In 2.0, you've got the various int.Equals, float.Equals, etc. methods, whereas in previous versions these Equal methos were just calling the Object.Equals method.
  
  // Question 2:
  public static void Question2()
  {
  object x = 5.ToString();
  string y = 5.ToString();
  Console.WriteLine(x != y && x.Equals(y));
  }
  
  // Question 3:
  public static void Question3()
  {
  int x = 5;
  object y = new object();
  Console.WriteLine(x == y); // Can't compare int and object
  }
  
  // Question 4:
  public static void Question4()
  {
  int x = 5;
  int y = 5;
  Console.WriteLine(x == y && (object)x != (object)y);
  }

• Anonymous
  April 03, 2005
  //My solutions, which play with Nullable<T> and String (and Nullable<String>)
  
  //If you switch the data types (int? x and int y) then this returns false
  public static void Question1() {
  int x = 0;
  int? y = 0;
  Console.WriteLine(x == y && !x.Equals(y));
  }
  
  //Reference equality vs value equality
  public static void Question2() {
  object x = "";
  object y = String.Copy("");
  Console.WriteLine(x != y && x.Equals(y));
  }
  
  // Operator '==' cannot be applied to operands of type 'string?' and 'string'
  // I'm not sure why this is the case, but C# doesn't seem to lift operators when T is a reference type
  public static void Question3() {
  string? x = "";
  string y = "";
  Console.WriteLine(x == y && (object)x != (object)y);
  }
  
  // This one acts returns false on .NET 1.1,
  // but mscorlib in 2.0 does not intern its own strings, including String.Empty
  public static void Question4() {
  string x = String.Empty;
  string y = "";
  Console.WriteLine(x == y && (object)x != (object)y);
  }

• Anonymous
  April 03, 2005
  public static void Question1()
  {
  char x = 'X';
  int y = 88;
  Console.WriteLine(x == y && !x.Equals(y));
  }
  
  public static void Question2()
  {
  object x =true;
  object y = true;
  Console.WriteLine(x != y && x.Equals(y));
  }
  public static void Question3()
  {
  bool x =true;
  string y = "true";
  Console.WriteLine(x == y);
  }
  public static void Question4()
  {
  int x =1;
  int y = 1;
  Console.WriteLine(x == y && (object)x != (object)y);
  }

• Anonymous
  April 04, 2005
  This is a test

• Anonymous
  May 10, 2005
  public static void Question1()
  {
  int x = 10;
  long y = 10;
  Console.WriteLine(x==y && !x.Equals(y));
  }
  public static void Question2()
  {
  object x = 10;
  object y = 10;
  Console.WriteLine(x!=y && x.Equals(y));
  }
  public static void Question3()
  {
  object x = new object();
  int y = 10;
  Console.WriteLine(x==y);
  }
  public static void Question4()
  {
  int x = 10;
  int y = 10;
  Console.WriteLine(x==y && (object)x!=(object)y);
  }

• Anonymous
  July 01, 2005
  /* Personal comment: great quiz :-) */
  using System;
  
  class Quiz
  {
  public static void Main()
  {
  Question1();
  Question2();
  Question3();
  Question4();
  }
  
  public static void Question1()
  {
  int x = 1;
  double y = 1;
  Console.WriteLine(x == y && !x.Equals(y));
  }
  
  public static void Question2()
  {
  object x = 1;
  object y = 1;
  Console.WriteLine(x != y && x.Equals(y));
  }
  
  public static void Question3()
  {
  int x = 1;
  string y = "1";
  Console.WriteLine(x == y);
  }
  
  public static void Question4()
  {
  int x = 1;
  double y = 1.0;
  Console.WriteLine(x == y && (object)x != (object)y);
  }
  }

• Anonymous
  October 09, 2006
  PingBack from http://aleemkhan.wordpress.com/2005/05/11/fun-with-equality/

• Anonymous
  March 19, 2008
  PingBack from http://cityjokesblog.info/brad-abrams-fun-with-equality/

• Anonymous
  June 15, 2009
  PingBack from http://debtsolutionsnow.info/story.php?id=603

• Anonymous
  June 16, 2009
  PingBack from http://fixmycrediteasily.info/story.php?id=6249