String excerption in JavaScript and C# for shortening messages or other strings

excerpt.cs

/// <summary>
/// Create excerpt string with n characters
/// </summary>
/// <param name="longString">The long string</param>
/// <param name="delimiter">How many chars there should be</param>
/// <returns>Shortened string</returns>
public static string Excerpt(string longString, int delimiter = 30) {
    string shortened = ""; int i = 0;
    if (String.IsNullOrEmpty(longString) == false) {
        if (longString.Length > delimiter) {
            foreach (var c in longLink) {
                if (i < delimiter) shortened += c;
                if (i == delimiter) shortened += "...";
                i++;
            }
        } else {
            return longString;
        }
    }
    return shortened;
}

excerpt.js

/**
 * Excerpts a string with <delimiter> chars
 * @param  {int} delimiter      How many chars'd be cutted
 * @return {string}             Excerpted or full string
 */
String.prototype.excerpt = function (delimiter) {
    delimiter = delimiter || 30;
    if (this.length > delimiter) {
        var cutted = [], i = 0;
        delimiter = delimiter == 'undefined' || delimiter == null ? 25 : delimiter;
        while (i < delimiter) {
            cutted.push(this[i]);
            i++;
        }
        return cutted.join("") + "...";
    } else {
        return this;
    }
};

your-app.cs

using Excerpt;

var message = Console.ReadLine(); // Hello my friend, how are you today?
Console.WriteLine(Excerpt(message, 20)); // Returns "Hello my friend, how..."

your-app.js

var message = "Hello my friend, how are you today?";
alert(message.excerpt(20)); // Returns "Hello my friend, how..."