using System;

using System.Collections.Generic;

using System.Linq;

public class Program

{

    public static void Main()

    {

        var pairs = ((IEnumerable<int>)new[] { 1, 2, 3, 4 }).Pairs();

        Console.WriteLine(string.Join($",{Environment.NewLine}", pairs.Select(p => $"{{first:{p.First}, second:{p.Second}}}")));

    }

}

​

public static class Ext

{

    public static IEnumerable<Pair<T>> Pairs<T>(

        this IEnumerable<T> source)

    {

        return source.SelectMany(

            (value, index) => source.Skip(index + 1),

            (first, second) => new Pair<T>(first, second));

    }

    public sealed class Pair<T>

    {

        internal Pair(T first, T second)

        {

            this.First = first;

            this.Second = second;

        }

        public T First { get; }

        public T Second { get; }

    }

}