Question

I must use a self-coded comparer to sort a list of objects.

myObjectList.Sort(new MyComparer(_sortExpression, _sortDirection));

How can I append a 2nd expression and direction to sort by if values on 1st expression are equal?

I mean e.g.: Sort by last name and then if equal by first name. But I must use my own comparer.

myObjectList.Sort(
   new MyComparer(new string[]{_exp1, _exp2}, new string[]{_dir1, _dir2}));

Thanks in advance!
Lord Vader

Answer 1

You could define an extension method that allows to chain comparers as follows:

public static class ComparerExtensions
{
    public static IComparer<T> ThenBy<T>(this IComparer<T> comparer1,
                                         IComparer<T> comparer2)
    {
        return new ChainedComparer<T>(comparer1, comparer2);
    }

    private class ChainedComparer<T> : IComparer<T>
    {
        private readonly IComparer<T> comparer1;
        private readonly IComparer<T> comparer2;

        public ChainedComparer(IComparer<T> comparer1,
                               IComparer<T> comparer2)
        {
            this.comparer1 = comparer1;
            this.comparer2 = comparer2;
        }

        public int Compare(T x, T y)
        {
            int result = this.comparer1.Compare(x, y);
            if (result == 0) result = this.comparer2.Compare(x, y);
            return result;
        }
    }
}

Usage:

var comparer = new MyComparer(_sortExpression1, _sortDirection1).ThenBy(
               new MyComparer(_sortExpression2, _sortDirection2));

myObjectList.Sort(comparer);

This sorts the list by the first comparer and, if two items are equal, then by the second comparer.

---

Alternative solution:

class ChainComparer<T> : IComparer<T>
{
    private readonly IEnumerable<IComparer<T>> comparers;

    public ChainComparer(params IComparer<T>[] comparers)
    {
        this.comparers = comparers;
    }

    public int Compare(T x, T y)
    {
        int result = 0;
        foreach (var comparer in this.comparers)
        {
            result = comparer.Compare(x, y);
            if (result != 0) break;
        }
        return result;
    }
}

Usage:

myObjectList.Sort(new ChainComparer<MyClass>(
    new MyComparer(_sortExpression1, _sortDirection1),
    new MyComparer(_sortExpression2, _sortDirection2)));