ソースコード

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

public static class IEnumerableExt
{
    public static IEnumerable<(T item1, T item2)> Combine<T>
    (
        this IEnumerable<T> first,
        IEnumerable<T>      second
    )
    {
        return first.SelectMany( i1 => second.Select( i2 => ( i1, i2 ) ) );
    }
}

通常

var list1 = new[] { 1, 2, 3 };
var list2 = new[] { 1, 2, 3 };

foreach ( var i1 in list1 )
{
    foreach ( var i2 in list2 )
    {
        Console.WriteLine( i1 + ", " + i2 );
    }
}

拡張メソッド

var list1 = new[] { 1, 2, 3 };
var list2 = new[] { 1, 2, 3 };

foreach ( var ( x, y ) in list1.Combine( list2 ) )
{
    Console.WriteLine( x + ", " + y );
}