CompareTo() method doesn't work

Question

I have a Class named Person which implement IComparable<int> generic Interface . I have a generic list which contain Person Object and I assign my list to an array and I'm sorting the list but I'm taking the follwing error.

error: {"Failed to compare two elements in the array."}

this is my Person class

public class Person : IComparable<int> 
    {
        public int Age { get; set; }

        public int CompareTo(int other)
        {
            return Age.CompareTo(other);
        }
    }

and this program cs

class Program
{
    static void Main(string[] args)
    {
        List<Person> list2 = new List<Person>();

        list2.Add(new Person() { Age = 80 });
        list2.Add(new Person() { Age = 45 });
        list2.Add(new Person() { Age = 3 });
        list2.Add(new Person() { Age = 77 });
        list2.Add(new Person() { Age = 45 });

        Person[] array = list2.ToArray();
        Array.Sort(array);

        foreach (Person item in array)
        {
            Console.WriteLine(item.Age);
        }

        Console.ReadKey();
    }
}

You have to implement `IComparable` to use it on an array of `Person`. — Keith Payne, Nov 05 '14 at 15:59
Why do you convert to an Array? Just iterate over the List while using [OrderBy](http://msdn.microsoft.com/en-us/library/vstudio/bb534966(v=vs.100).aspx). — BCdotWEB, Nov 05 '14 at 16:00
You could implement IComparer if you wanted, http://msdn.microsoft.com/en-us/library/6tf1f0bc(v=vs.110).aspx?cs-save-lang=1&cs-lang=csharp#code-snippet-2 — Mike Miller, Nov 05 '14 at 16:01

Paweł Reszka · Accepted Answer · 2014-11-05T16:05:33.443

10

Change your class to this:

public class Person : IComparable<Person> 
{
    public int Age { get; set; }

    public int CompareTo(Person other)
    {
        return Age.CompareTo(other.Age);
    }
}

If you create class with IComperable<int> you are able to compare it with int, not with the same class. You have to pass to the template the same class/struct as one you are comparing with.

edited Nov 05 '14 at 16:05

answered Nov 05 '14 at 15:59

Paweł Reszka

1,557
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4

@downvoters, This was one of those times where it might have been appropriate to edit some code and add that space between Person and other – Sam I am says Reinstate Monica Nov 05 '14 at 16:02
1

You should probably explain *why* this works and what the OP was doing wrong for this to be a complete answer – DGibbs Nov 05 '14 at 16:03

score 4 · Answer 2 · answered Nov 05 '14 at 16:03

You need to change your generic to Person since you are comparing Person to Person, and not Person to int:

public class Person : IComparable<Person>
    {
        public int Age { get; set; }


        public int CompareTo(Person other)
        {
            return Age.CompareTo(other.Age);
        }
    }

Also, you do not need to cast to an Array afterwards, you can keep it as a list:

List<Person> list2 = new List<Person>();

            list2.Add(new Person() { Age = 80 });
            list2.Add(new Person() { Age = 45 });
            list2.Add(new Person() { Age = 3 });
            list2.Add(new Person() { Age = 77 });
            list2.Add(new Person() { Age = 45 });

            list2.Sort();

            foreach (Person item in list2)
            {
                Console.WriteLine(item.Age);
            }

Rahul Singh · Answer 3 · 2014-11-05T16:17:29.250

Use this:-

public class Person : IComparable<Person>
    {
        public int Age { get; set; }

        public int CompareTo(Person other)
        {
            return this.Age.CompareTo(other.Age);
        }
    }

Your custom comparer is wrong! CompareTo() method returns an integer which indicates:-

 0 -> Current instance is equal to the object being compared.
>0 -> Current instance is greater than the object being compared.
<0 -> Current instance is less than the object being compared.

When you call Array.Sort(array); you are actually passing an array object but the problem is you had implemented IComparable of type int.

score 1 · Answer 4 · answered Nov 05 '14 at 16:03

You can implement Icompareable<Person> as the other answer states or you can implement IComparable like:

public class Person : IComparable
{
    public int Age { get; set; }

    public int CompareTo(int other)
    {
        return Age.CompareTo(other);
    }

    public int CompareTo(object obj)
    {
        Person otherPerson = obj as Person;
        if (obj == null)
            return 0;
        else
        {

            return Age.CompareTo(otherPerson.Age);
        }
    }
}

You can also use LINQ to order your List like:

foreach (Person item in list2.OrderBy(r=> r.Age))

thanks @user2711965 but I want to implement generic interface — erhan urun, Nov 05 '14 at 16:51

CompareTo() method doesn't work

4 Answers4