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Equals () and == in a common function

I am making a comparator for the installation operation for different types.

So I have a general class

 public class Comparer<T, Tid>
...
     public bool Equals(T x, T y)
       {
          var xid = m_idfunc(x);
           var yid = m_idfunc(y);
           return (Tid)xid == (Tid)yid;
       }

Where m_idfunc is the lambda passed to the Comparer constructor, this is

Func<T,Tid>

I am creating a comparator with Tid = string. I get in the equals function xid = string1, yid = string2

If line 1 and line 2 are the same (for example, "foo" and "foo")

xid == yid

gives false

(Tid)xid == (Tid)yid

also gives false (it should not be necessary - I just despaired)

heres my closest window - paused on return xid == yid line

yid.GetType() == typeof(string)
true
xid.GetType() == typeof(string)
true
xid==yid
false
(string)xid==(string)yid
true
xid.Equals(yid)
true

What's happening?

+5
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4 answers

What is interesting is that it can work the way you want it to. Here is an example:

using System;
using System.Text;

namespace ConsoleApplication1 {

    class Program {

        public static void Main()  {
            string myString = "1";
            object objectString = "1";
            string myCopiedString = string.Copy(myString);
            string internedString = string.Intern(myCopiedString);

            Console.WriteLine(myString); //1
            Console.WriteLine(objectString); //1
            Console.WriteLine(myCopiedString); //1
            Console.WriteLine(internedString); //1

            Console.Write(objectString == myString); //true
            Console.Write(objectString == "1"); //true
            Console.Write(objectString == myCopiedString); //!!!FALSE!!!!
            Console.Write(objectString == internedString); //true
            Console.Write(objectString == SomeMethod()); //!!!FALSE!!!
            Console.Write(objectString == SomeOtherMethod()); //true
        }

        public static string SomeMethod() {
            StringBuilder sb = new StringBuilder();
            return sb.Append("1").ToString();
        }

        public static string SomeOtherMethod() {
            return "1".ToString();
        }        
    }
}

, , . , , , , .

, "". == - , , , , . , EqualityComparer<T>.Default.Equals .

VS, : " : , " string ". , , , , , .

+3

, , , , . , .:) , - , 100%. (, , )

-, ,

class Comparer<T, TId>
{
    private readonly Func<T, TId> m_idfunc;
    public Comparer(Func<T, TId> idFunc)
    {
        m_idfunc = idFunc;
    }

    public bool Equals(T x, T y)
    {
        var xid = m_idfunc(x);
        var yid = m_idfunc(y);
        return (TId)xid == (TId)yid;
    }
}

fooobar.com/questions/8454/...

class Comparer<T, TId> where TId : class

. 1.

Equals

public bool Equals(T x, T y)
{
    var xid = m_idfunc(x);
    var yid = m_idfunc(y);
    return (TId)xid == (TId)yid;
}

False (. xid | yid). , Generics . , , , "" .

Comparer

class Comparer<T>
{
    private readonly Func<T, string> m_idfunc;
    public Comparer(Func<T, string> idFunc)
    {
        m_idfunc = idFunc;
    }

    public bool Equals(T x, T y)
    {
        var xid = m_idfunc(x);
        var yid = m_idfunc(y);
        return xid == yid;
    }
}

True.

100%, , == string . , , ( IL, , ), , false. ( , , , , , )

.

using System;
namespace ConsoleApplication1
{
    internal class Program
    {
        private static void Main(string[] args)
        {
            var compare = new Comparer<Example, string>(example => example.id(example));
            var ex1 = new Example();
            var ex2 = new Example();
            Console.WriteLine(compare.Equals(ex1, ex2));
            Console.ReadLine();
        }
        class Example
        {
            public string id(Example example)
            {
                return new string(new [] {'f', 'o', 'o'});
            }
        }
        class Comparer<T, TId> where TId : class 
        {
            private readonly Func<T, TId> m_idfunc;
            public Comparer(Func<T, TId> idFunc)
            {
                m_idfunc = idFunc;
            }

            public bool Equals(T x, T y)
            {
                var xid = m_idfunc(x);
                var yid = m_idfunc(y);
                return (TId)xid == (TId)yid;
            }
        }
    }
}

, ... .:)

+1

, EqualityComparer<TId> Comparer<T, Tid>. , :

interface IObjectWithId<T>
{
    T Id { get; }
}

class IdEqualityComparer<T, TId> : EqualityComparer<T>
    where T : IObjectWithId<TId>
{
    public override bool Equals(T x, T y)
    {
        return EqualityComparer<TId>.Default.Equals(x.Id, y.Id);
    }

    public override int GetHashCode(T obj)
    {
        return EqualityComparer<TId>.Default.GetHashCode(obj.Id);
    }
}

class A : IObjectWithId<string>
{
    public string Id { get; set; }
}

:

var a = new A { Id = "foo" };
var b = new A { Id = "foo" };
var c = new A { Id = "bar" };

var comparer = new IdEqualityComparer<A, string>();

Console.WriteLine(comparer.Equals(a, b)); // true
Console.WriteLine(comparer.Equals(a, c)); // false
0

C "==" . -, , , , , , , , . ; , . , : System.String.

vb.net , = , . Is. - vb.net, = . Is, , =.

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0

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