Anonymous IComparer implementation

Question

Is it possible to define an anonymous implementation of IComparer?

I believe Java allows anonymous classes to be defined inline - does C#?

Looking at this code I want to define a custom IComparer inline

public static IOrderedEnumerable<TSource> OrderBy<TSource, TKey>(
    this IEnumerable<TSource> source,
    Func<TSource, TKey> keySelector,
    IComparer<TKey> comparer
)

This is my workaround: https://github.com/smartcaveman/anonymous — smartcaveman, Aug 01 '13 at 19:48
You have an answer in a related question: http://stackoverflow.com/a/1071637/661933 — nawfal, Oct 23 '13 at 10:49

David Clarke · Answer 1 · 2017-04-30T21:12:57.260

59

As indicated in one of the comments below, .Net 4.5 allows this via a static method on the Comparer<> class, e.g. comparing two objects based on the value of a property in the class:

var comparer = Comparer<KilowattSnapshot>.Create( 
        (k1, k2) => k1.Kilowatt.CompareTo(k2.Kilowatt) );

Obviously this can be used inline rather than assigned to a variable.

edited Apr 30 '17 at 21:12

answered Feb 16 '15 at 02:57

David Clarke

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score 16 · Answer 2 · answered Mar 13 '11 at 11:32

16

Even though you can't create anonymous classes that implement interfaces, you can usually use the Comparison Delegate instead of the IComparer Interface in most cases (like sorting, etc.):

Array.Sort(arr, (x, y) => 1);

Also there are some built-in implementations of IComparer like the Comparer Class or the StringComparer Class...

answered Mar 13 '11 at 11:32

Jaroslav Jandek

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With the upcoming .NET4.5 (Visual Studio 2012) it will be possible to make an `IComparer<>` from a `Comparison<>` lambda arrow by using the new factory method `Comparer<>.Create`. – Jeppe Stig Nielsen Aug 08 '12 at 16:36
Only thing is `Comparer<>.Create` is extremely ugly to use, because it can't infer arguments (contrast this with how `Tuple.Create` and `Tuple<>` interact). – skolima Feb 05 '13 at 16:42

score 14 · Answer 3 · answered Mar 20 '13 at 10:08

The .NET framework version 4.5 provides the method Comparer.Create(Comparison) to create comparers based on a specified comparison delegate (which can be a lambda function). However people who are working with earlier versions of .NET will probably need to implement something similar themselves.

score 8 · Accepted Answer · edited Sep 29 '17 at 19:34

8

No, C# does not currently allow inline interface implementations; although it does allow you to create delegates inline through lambda expressions and anonymous methods.

In your case, I would suggest using a ProjectionComparer that makes it easy to use this feature, such as the one listed here.

edited Sep 29 '17 at 19:34

Brian

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answered Mar 13 '11 at 11:08

Ani

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@ChloeRadshaw: Java does allow anonymous inner classes that implement interfaces, but it currently has no first-class function concept (such as delegates). – Ani Mar 13 '11 at 11:16

score 3 · Answer 5 · answered Mar 13 '11 at 11:08

3

No, this is not possible. However, you can get the default implementation of IComparer<TKey> by Comparer<TKey>.Default. Otherwise you'll need to create a parameterized implementation and use an instance of that.

answered Mar 13 '11 at 11:08

Femaref

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score 2 · Answer 6 · edited Nov 24 '13 at 13:34

C# does not allow implementing interfaces using anonymous inner classes inline, unlike Java. For simple comparisons (i.e. comparing on a single key), there is a better way to do this in C#. You can simply use the .OrderBy() method and pass in a lambda expression specifying the key.

using System;
using System.Collections;
using System.Collections.Generic;
using System.Linq;


namespace Test{
    public class Test{
        public static void Main(){
            IList<int> mylist = new List<int>();
            for(int i=0; i<10; i++) mylist.Add(i);
            var sorted = mylist.OrderBy( x => -x );
            foreach(int x in sorted)
                Console.WriteLine(x);
        }
    }
}

score 1 · Answer 7 · edited Aug 28 '20 at 11:36

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Array.Sort(arrayName, (x,y) => string.Compare(x.Name,y.Name,StringComparison.CurrentCulture));

edited Aug 28 '20 at 11:36

Neuron

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answered May 29 '17 at 11:02

user3452906

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score 1 · Answer 8 · edited May 23 '17 at 11:54

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Take a look at these 2 SO questions, they tackle essentially the same problem

Use of Distinct with list of Custom Object

Wrap a delegate in an IEqualityComparer

If you go this way, you should pay special attention to Slaks' comments and Dan Tao's answer about the hashcode implementation

edited May 23 '17 at 11:54

Community

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answered Mar 13 '11 at 11:34

bottlenecked

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Anonymous IComparer implementation

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