Question

I am writing an address book program. I have each person’s details stored in a List<Person>. I need to be able to sort this list by last name (using first name if there are ties) or by post code.

So far I have this:

public class Person
{
    public string LastName { get; set; }
    public string FirstName { get; set; }
    public string PostCode { get; set; }
    // etc..
}

public class AddressBook
{
    public List<Person> People { get; set; }

    // asc: ascending or descending
    // column: the property to use when sorting
    //         (in my case either LastName or Postcode)
    public void Sort(bool asc, string column)
    {
        // What should I put here?
    }

    // etc...
}

I have tried using the ICompare and IComparable interfaces but I am just not getting it.

How do I write the Sort method?

Answer 1

You can use an implementation of IComparer<T>:

public class PersonComparer : IComparer<Person>
{
    private readonly bool _sortAscending;
    private readonly string _columnToSortOn;

    public PersonComparer(bool sortAscending, string columnToSortOn)
    {
        _sortAscending = sortAscending;
        _columnToSortOn = columnToSortOn;
    }

    public int Compare(Person x, Person y)
    {
        if(x == null && y == null) return 0;
        if(x == null) return ApplySortDirection(-1);
        if(y == null) return ApplySortDirection(1);

        switch(_columnToSortOn)
        {
            case "LastName":
                return ApplySortDirection(SortByName(x, y));
                break;
            case "PostCode":
                return ApplySortDirection(SortByPostCode(x, y));
                break;
            default:
                throw new ArgumentOutOfRangeException(
                    string.Format("Can't sort on column {0}",
                    _columnToSortOn));
        }
    }

    private int SortByPostCode(Person x, Person y)
    {
        return x.PostCode.CompareTo(y.PostCode);
    }

    private int SortByName(Person x, Person y)
    {
        var lastNameResult = x.LastName.CompareTo(y.LastName);
        if(lastNameResult != 0)
            return lastNameResult;
        return x.FirstName.CompareTo(y.FirstName);
    }

    private int ApplySortDirection(int result)
    {
        return _sortAscending ? result : (result * -1);
    }
}

You would use it in the Sort method of your AddressBook class like this, assuming People is a List<Person>:

public void Sort(bool asc, string column)
{
    People.Sort(new PersonComparer(asc, column));
}

This code has the benefit of using an in-place sort.