Extension Methods Interface

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

public static class Test
{
    static void Main()
    {

        List<int> numbers = new List<int> { 1, 2, 3, 4, 5 };

        Console.WriteLine(numbers.ToString<int>());

        numbers.IncreaseWidth(5);

        Console.WriteLine(numbers.ToString<int>());

    }
}

public static class IListExtensions
{

    public static void IncreaseWidth(

          this IList<int> list,

          int amount)
    {

        for (int i = 0; i < list.Count; i++)
        {

            list[i] += amount;

        }

    }

}

public static class IEnumerableExtensions
{

    public static string ToString<T>(

          this IEnumerable<T> enumeration)
    {

        StringBuilder result = new StringBuilder();

        result.Append("[");

        foreach (var item in enumeration)
        {

            result.Append(item.ToString());

            result.Append(", ");

        }

        if (result.Length > 1)

            result.Remove(result.Length - 2, 2);

        result.Append("]");

        return result.ToString();

    }

}