IComparer using Lambda Expression

Question

 class p {
     public string Name { get; set; }
     public int Age { get; set; }
 };

 static List<p> ll = new List<p>
 {
     new p{Name="Jabc",Age=53},new p{Name="Mdef",Age=20},
     new p{Name="Exab",Age=45},new p{Name="G123",Age=19}
 };
 protected static void SortList()
 {
     IComparer<p> mycomp = (x, y) => x.Name.CompareTo(y.Name);  <==(Line 1)
     ll.Sort((x, y) => x.Name.CompareTo(y.Name));<==(Line 2)
 }

Here the List.sort expects an IComparer<p> as parameter. And it works with the lambda as shown in Line 2. But when I try to do as in Line 1, I get this error:

Cannot convert lambda expression to type System.Collections.Generic.IComparer' because it is not a delegate type

I investigated this for quite some time but I still don't understand it.Maybe my understanding of IComparer is not quite good.Can somebody give me a hand ?

score 23 · Accepted Answer · edited Jul 25 '13 at 21:21

23

When you do ll.Sort((x, y) => x.Name.CompareTo(y.Name)); it uses the overload for Comparison<T>, not IComparer. Comparison<T> is a delegate, so you can use a lambda expression for it.

Comparison<p> mycomp = (x, y) => x.Name.CompareTo(y.Name); will work.

edited Jul 25 '13 at 21:21

BlueRaja - Danny Pflughoeft

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answered May 23 '10 at 19:52

sepp2k

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Where/how is Comparison defined? – Doguhan Uluca Apr 04 '12 at 20:53
2

@duluca It's defined in the `System` namespace as `public delegate int Comparison(T x, T y)` (minus the `in` in versions prior to .net 4). It's documented [here](http://msdn.microsoft.com/en-us/library/tfakywbh%28v=vs.100%29.aspx). – sepp2k Apr 04 '12 at 21:02
`CompareTo()` is a culture specific comparison. Something like `ll.sort((x, y) => String.CompareOrdinal(x.Name, y.Name))` may suit you better. – Ryan Kirkman Mar 04 '13 at 00:03
I like how I could lift the *exact* mycomp definition here to work in my scenario :) – nchaud Oct 22 '14 at 11:52
1

@RyanKirkman How often do you want to compare two `Name`s according to the numeric value of the underlying UTF-16 code units? Normally, when you sort "names" you use an alphabet relevant in the context, and that is indeed culture-specific. – Jeppe Stig Nielsen Aug 18 '17 at 17:17

score 9 · Answer 2 · edited May 23 '17 at 12:02

9

There's an existing solution you might refer to: https://stackoverflow.com/a/16839559/371531

This one uses Comparer<T>.Create introduced in .NET Framework 4.5.

edited May 23 '17 at 12:02

Community

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answered Sep 25 '14 at 05:30

Liu Yue

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This is a great one :) – Simon_Weaver Jan 27 '19 at 06:08

score 2 · Answer 3 · answered May 23 '10 at 19:49

2

IComparer is an interface, not a delegate.

You'll want to use the lambda expression on its .CompareTo(), not on the interface itself.

answered May 23 '10 at 19:49

devoured elysium

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score 1 · Answer 4 · answered Aug 26 '13 at 03:11

Use the following simple class:

public static class ComparerUtilities
{
    class _Comparer<T> : Comparer<T>
    {
        Comparison<T> _comparison;

        public _Comparer(Comparison<T> comparison)
        {
            _comparison = comparison;
        }

        public override int Compare(T x, T y)
        {
            return _comparison(x, y);
        }
    }

    public static IComparer<T> FromComparison<T>(Comparison<T> comparison)
    {
        return new _Comparer<T>(comparison);
    }
}

IComparer using Lambda Expression

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