Add an extra comparer interface to the default comparer in C#

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Suppose I have a list of student objects where a single student has the properties Name, Age and Company properties. I would like to implement extra Comparer interfaces that will allow me to sort a list of students using the Age and company properties. I have already implemented a default comparer that is based on the Name of the student. The names are compared lexicographically and the sort is executed on the students list. Now I would like to implement additional sort interfaces that let me sort the students list based on the age or the company names lexicographically.
How do I do that in C#?

What I have tried:

C#

public interface IPerson
{
    string Name { get; set; }
    int Age { get; set; }
    string Company { get; set; }
}
//implementation details for my Student class
public class Student : IPerson, IComparable<Student>  
{
    private string _name;
    private string _company;
    private int _age;   
    public string Name { 
        get => _name;
        set => _name = value; }
    public string Company { get => _company; set => _company= value; 
    
    }
    public int Age { get => _age; set => _age = value; }
    //define the constructor
    public Student(string name, int age, string company)
    {
        _name = name;
        _company = company; 
        _age = age; 
    }
    //the default comparer is name based
    public int CompareTo(Student other)
    {
        return _name.CompareTo(other. Name);    
    }
   //how do I implement additional interfaces for age and company and how do 
   //tell the compiler in Main method to use which type to do it

Posted 15-Jun-23 2:33am

Tim the Gamer

Updated 15-Jun-23 3:07am

v2

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Richard Deeming · Accepted Answer · 2023-06-15T02:40:00

Solution 1

Rather than having the class implement the comparison, use external classes implementing IComparer<IPerson>.

C#

public abstract class PersonComparer : IComparer<IPerson>
{
    public int Compare(IPerson left, IPerson right)
    {
        if (left is null) return right is null ? 0 : -1;
        if (right is null) return 1;
        return CompareCore(left, right);
    }
    
    protected abstract int CompareCore(IPerson left, IPerson right);
}

public class PersonNameComparer : PersonComparer
{
    protected override int CompareCore(IPerson left, IPerson right) => StringComparer.CurrentCulture.Compare(left.Name, right.Name);
}

public class PersonAgeComparer : PersonComparer
{
    protected override int CompareCore(IPerson left, IPerson right) => left.Age.CompareTo(right.Age);
}


public class PersonCompanyComparer : PersonComparer
{
    protected override int CompareCore(IPerson left, IPerson right) => StringComparer.CurrentCulture.Compare(left.Company, right.Company);
}

IComparer<T> Interface (System.Collections.Generic) | Microsoft Learn[^]

Posted 15-Jun-23 2:40am

Richard Deeming

Comments

Tim the Gamer 15-Jun-23 8:54am

This will make the code complex and a pain to the eye, where is the constructor of this external class and how do I use it to sort a list of Student objects?

Richard Deeming 15-Jun-23 9:03am

"where is the constructor"
Since there are no constructors defined, the compiler will generate a default constructor.

"how do I use it"
Pass an instance of the class to the List<T>.Sort(IComarer<T>) method[^].

Richard Deeming 15-Jun-23 9:05am

The other alternative, assuming you only care about List<T> sorting, would be to use a Comparison<T> delegate:

listOfStudents.Sort((x, y) => x is null ? (y is null ? 0 : -1) : (y is null ? 1 : x.Age.CompareTo(y.Age)));

If your list will never contain null values, you can simplify that to:

listOfStudents.Sort((x, y) => x.Age.CompareTo(y.Age));

List<T>.Sort Method (System.Collections.Generic) | Microsoft Learn[^]

Richard Deeming 15-Jun-23 9:07am

Or, if you're not worried about memory usage, use LINQ:

listOfStudents = listOfStudents.OrderBy(x => x.Age).ToList();

Tim the Gamer 15-Jun-23 9:15am

_listOfStudents.Sort((x, y) => x.Age.CompareTo(y.Age));_ worked without implementing your class.

Richard Deeming 15-Jun-23 9:19am

As I said, that will work for the List<T> class.

If you never need to sort anything else, then great. Otherwise, you'll almost certainly need an IComparer<T> instance.