using System;

using System.Linq;

​

public class Program

{

    public static void Main()

    {

        var testStrings = new string[]{"", "test", " test ", "test 1", "test  2", " test   3  4 5 "};

        foreach (var str in testStrings)

            Console.WriteLine($"'{Sanitize(str)}'");

    }

​

    private static string Sanitize(string str)

    {

        return string.Join(" ", str.Split(' ').Where(s => !string.IsNullOrWhiteSpace(s)));

    }

}