using System;

using System.Collections.Generic;

​

public class Program

{

    public static void Main()

    {

        var array = new[]{"Hello"};

        var world = array.Insert("World"); // this will crash at run time

        Console.WriteLine(array.Length);

    }

}

​

public static class DummyExtension

{

    public static T Insert<T>(this IList<T> list, T insertValue)

    {

        Console.WriteLine("WrongInsert");

        list.Add(insertValue);

        return insertValue;

    }

    [Obsolete("Don't use this", true)]

    public static T Insert<T>(this T[] list, T insertValue)

    {

        Console.WriteLine("RightInsert");

        return insertValue;

    }

}

​