Sort ArrayList by Object field value

Question

So my ArrayList looks like this:

        using System.Collections;
        [...]
        //in main:
        Student s01 = new Student
        {
            name = "John",
            surname = "Doa"
        };

        Student s02 = new Student
        {
            name = "John",
            surname = "Doe"
        };

        Student s03 = new Student
        {
            name = "John",
            surname = "Doi"
        };

        Student s04 = new Student
        {
            name = "John",
            surname = "Doo"
        };

        Student s05 = new Student
        {
            name = "John",
            surname = "Dou"
        };

        ArrayList studentsList = new ArrayList();
        studentsList.Add(s03);
        studentsList.Add(s04);
        studentsList.Add(s05);
        studentsList.Add(s01);
        studentsList.Add(s02);

Student.cs:

class Student : IPerson
{
    public string name;
    public string surname;

    public void Describe()
    {
        Console.WriteLine("{0} {1}", name, surname);
    }

I'd like to sort my objects in studentsList by surname and write down both name + surname in foreach loop. I know that studentsList.Sort(); won't work, because it doesn't know in what way it should compare each object to other - and that's where I'm stuck, because I don't really know how to write new comparer that's using value of field of object in arrayList. I don't even know how to start.

I have to use ArrayList, but I don't have to sort it directly (I can use casting I guess) - in the end it must look like:

1. John Doa
2. John Doe
3. John Doi
4. John Doo
5. John Dou

Would be great to Sort() studentsList, because I'd be able to do something like this:

int i = 0;
foreach (Student s in studentsList)
{
   i++;
   Console.Write("{0}. ", i);
   s.Describe();
}

You will also need to add using System.Linq for it to work. — Siavas
– Siavas, Commented Jan 25, 2017 at 20:33
Looks like you missed the OfType<Student>() call. But please be aware that ArrayList is very rarely used in real code these days - it's been mostly-obsolete since 2005. — Jon Skeet
– Jon Skeet, Commented Jan 25, 2017 at 20:34

karen · Accepted Answer · 2017-01-25 20:41:00Z

1

You can also define a compareTo method in Student to be able to use studentList.Sort() . Student class should also extend :IComparable

  public int CompareTo(object obj)
  {
        if (obj == null) return 1;
        Student other = obj as Student;
        if (other != null)
        {
            // Compare last names first, then by firstName
            int value = this.lastName.CompareTo(other.lastName);
            if (value == 0)
                value = this.firstName.CompareTo(other.firstName);
            return value;
        }
        else
            throw new ArgumentException("Object is not a Student");
  }

edited Jan 25, 2017 at 20:41

answered Jan 25, 2017 at 20:34

karen

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1 Comment

Piotr Malczyk Over a year ago

Yeah, works great - not as simple as linq expression, but .Sort() works ;)

Tarek Abdel Maqsoud · Accepted Answer · 2017-01-25 20:38:15Z

0

You can do it as following

        ArrayList studentsList = new ArrayList();
        studentsList.Add(s03);
        studentsList.Add(s04);
        studentsList.Add(s05);
        studentsList.Add(s01);
        studentsList.Add(s02);

        var studentsList2 = studentsList.Cast<Student>().OrderBy(r => r.surname).ToList();

        int i = 0;
        foreach (Student s in studentsList2)
        {
            i++;
            Console.Write("{0}. ", i);
            s.Describe();
        }

edited Jan 25, 2017 at 20:38

answered Jan 25, 2017 at 20:30

Tarek Abdel Maqsoud

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2 Comments

Jon Skeet Over a year ago

That statement won't achieve anything - you'd need to use the result of the call.

Piotr Malczyk Over a year ago

Works too, but the one that suggested @Will is better, because I don't have to use indirect sorting (using the list).

Community · Accepted Answer · 2017-05-23 12:01:29Z

0

foreach (var student in studentsList.OfType<Student>().OrderBy(x => x.surname).ThenBy(x => x.name))
        {
            i++;
            Console.Write("{0}. ", i);
            student.Describe();
        }

works as it should (thanks to @Will, https://stackoverflow.com/users/1228/will), I didn't realize that linq expression would be that easy. That being said, it's only academic example because ArrayList is mostly obsolete since 2005 (@JonSkeet).

edited May 23, 2017 at 12:01

CommunityBot

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answered Jan 25, 2017 at 20:46

Piotr Malczyk

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