using System;

using System.Linq;

using System.Collections.Generic;

public class Program

{

    public static void Main()

    {

        var url = Combine("https://test.com", "", "firstRealPart", "/secondPart/");

        Console.WriteLine(url);

    }

    private static string Combine(params string[] parts)

    {

        if (parts.Length == 0)

                return "";

            if (parts.Count(x => !string.IsNullOrWhiteSpace(x)) == 1)

                return parts.First(x => !string.IsNullOrWhiteSpace(x));

​

            var toCombine = new List<string>();

            foreach (var part in parts.Where(x => !string.IsNullOrWhiteSpace(x)))

            {

                if (ReferenceEquals(part, parts.First()))

                    toCombine.Add(part.TrimEnd('/', '\\'));

                else if (ReferenceEquals(part, parts.Last()))

                    toCombine.Add(part.TrimStart('/', '\\'));

                else

                    toCombine.Add(part.Trim('/', '\\'));

            }

​

            return string.Join('/', toCombine);

    }

}