使用哈希码概念困境进行对象比较

Question

I have written a piece of code to check for Object Equality. I took refrence from one of the question in stack overflow itself. Now this code is giving true even if we have two different objects. Can someone explain why?

using System;
namespace ConsolePractice
{
    public class Test
    {
        public int Value { get; set; }
        public string String1 { get; set; }
        public string String2 { get; set; }



        public override int GetHashCode()
        {
            int hash = 19;
            hash = hash * 31 + Value;
            hash = hash * 31 + String1.SafeGetHashCode();
            hash = hash * 31 + String2.SafeGetHashCode();
            return hash;
        }
        public override bool Equals(object obj)
        {
            Test test = obj as Test;
            if (obj == null)
            {
                return false;
            }
            return Value == test.Value &&
                String1 == test.String1 &&
                String2 == test.String2;
        }
    }

    class Demo
    {
        static void Main()
        {
            Test p1 = new Test
            {
                Value = 10,
                String1 = "Test1",
                String2 = "Test2"
            };
            Test p2 = new Test
            {
                Value = 10,
                String1 = "Test1",
                String2 = "Test2"
            };
            bool areEqual = p1.Equals(p2);

            Console.WriteLine(areEqual.ToString());
            Console.ReadLine();

        }
    }
}

and in my UtilityClass

 static class utility
    {
        public static int SafeGetHashCode<T>(this T value) where T : class
        {
            return value == null ? 0 : value.GetHashCode();
        }
    }

After no success,i tried below code which also return true. What blunder am I doing here?Please help

using System;

using System.Collections.Generic;


class ThingEqualityComparer : IEqualityComparer<Thing>
{
    public bool Equals(Thing x, Thing y)
    {
        if (x == null || y == null)
            return false;

        return (x.Id == y.Id && x.Name == y.Name);
    }

    public int GetHashCode(Thing obj)
    {
        return obj.GetHashCode();
    }
}


public class Thing
{
    public int Id { get; set; }
    public string Name { get; set; }

}
class Demo
{
    static void Main()
    {
        Thing p1 = new Thing
        {
            Id = 10,
            Name = "Test1",

        };
        Thing p2 = new Thing
        {
            Id = 10,
            Name = "Test1",

        };

        var comparer = new ThingEqualityComparer();
        Console.WriteLine(comparer.Equals(p1, p2));


        Console.ReadLine();

    }
}

Answer 1

You override Equals() and GetHashCode() to define what "equals" means in a given context.

Your Equals of:

Test test = obj as Test;
if (obj == null)
{
  return false;
}
return Value == test.Value &&
  String1 == test.String1 &&
  String2 == test.String2;

Has a bug in that it should be if(test == null) return false; but otherwise it says "two Test objects are the same if they have the same Value , String1 and String2 , otherwise they are not. Your GetHashCode() is consistent with that.

As such, it's not a bug to have the code return true :

Now this code is giving true even if we have two different objects.

Yes, two different equal objects.

If you want Equals and GetHashCode() to tell you whether they are the same object or not (that is, for an object of a class to be equal only to itself) then don't override Equals and GetHashCode at all; stay with the default behaviour.