using System;

using System.Linq;

using System.Text;

​

public class Program

{

    public static void Main()

    {

        string original = "Artículo 13: Cualquiera sean los sistemas de abastecimiento, el agua potable deberá " + 

            "cumplir con los requisitos físicos, químicos, radiactivos y bacteriológicos establecidos en " +

            "la reglamentación vigente sobre la materia.[...]";

        Console.WriteLine(Truncate(original, 15));

        Console.WriteLine(Truncate2(original, 15));

    }

​

    // One way to do this would be to split the original string on the space

    // character, and then use the `System.Linq` extension method `Take` to take 

    // up to the first 15 items from the resulting array, and then join those

    // items with a space again to form the final string result:

    private static string Truncate(string original, int maxWords)

    {

        return string.IsNullOrWhiteSpace(original) 

            ? original 

            : string.Join(" ", original.Split(' ').Take(maxWords));

    }

​

    // Another way is to use a loop where you walk through the original

    // word, adding characters to the result and counting spaces until 

    // we have the required number of words:

    private static string Truncate2(string original, int maxWords)

    {

        if (string.IsNullOrWhiteSpace(original)) return original;

​

        var result = new StringBuilder();

​

        for (int i = 0, spaceCount = 0; i < original.Length; i++)

        {

            if (original[i] == ' ') spaceCount++;

            if (spaceCount == maxWords) break;

            result.Append(original[i]);

        }

​

        return result.ToString();

    }

}