Comparison

This .NET type enables custom sorting. In C# it is often used with Array.Sort or List.Sort. We implement Comparison using its constructor.

The target is a method that receives 2 parameters of the appropriate type and returns an int. It determines sorting order of the parameters.

Lambda example

We do not need to use a static method for the Comparison implementation. We can instead use a lambda expression. Here we sort on string lengths with a lambda Comparison.

using System;

string[] array = { "4444", "1", "22", "333" };

// Use lambda to sort on length.
Array.Sort(array, (a, b) => a.Length.CompareTo(b.Length));

// Write result.
Console.WriteLine("RESULT: {0}", string.Join(";", array));
RESULT: 1;22;333;4444

Delegate example

In this program we create an array of strings. We create a new Comparison object using the CompareLength method as the target.

using System;

class Program
{
    static int CompareLength(string a, string b)
    {
        // Return result of CompareTo with lengths of both strings.
        return a.Length.CompareTo(b.Length);
    }

    static void Main()
    {
        // Source array.
        string[] array =
        {
            "bird",
            "elephant",
            "dog",
            "cat",
            "mouse"
        };

        // Display array.
        foreach (string element in array)
        {
            Console.WriteLine(element);
        }
        Console.WriteLine();

        // Create Comparison instance and use it.
        Comparison<string> comparison = new Comparison<string>(CompareLength);
        Array.Sort(array, comparison);

        // Display array.
        foreach (string element in array)
        {
            Console.WriteLine(element);
        }
    }
}
bird
elephant
dog
cat
mouse

cat
dog
bird
mouse
elephant

Requirements

The Comparison type requires that you specify a method that receives two parameters of type T and then returns an int. You pass the method name to the constructor.

Notes, target method

How do you implement the code inside the target method (like CompareLength)? Use the CompareTo method on a property, field, or other value from each type.

Discussion

There are many other examples of using Comparison. You can use Comparison without specifying the constructor explicitly. It is compatible with the lambda expression syntax.

Summary

We looked at the Comparison type. We provided an example of the Comparison constructor. And we pointed out that it is not necessary to provide in all cases.