C# 用 GDI+ 繪製驗證碼 (1) — ASCII 碼產生隨機字串演算法

隨機字串產生策略

數字與字母皆有 ASCII 編碼。除預先備妥字元陣列外，可直接用 ASCII 範圍產生。本文採簡單字元來源字串法：

public static string Generate(RandomGeneratorStyle style, int length) {
    string strValidateString = "";
    Random rnd = new Random();
    string strValidateStringSource;
    switch (style) {
        case RandomGeneratorStyle.Number:
            strValidateStringSource = "0123456789"; break;
        case RandomGeneratorStyle.NumberAndChar:
            strValidateStringSource = "abcdefghijklmnopqrstuvwxyz0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ"; break;
        case RandomGeneratorStyle.NumberAndCharIgnoreCase:
            strValidateStringSource = "ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789"; break;
        default:
            strValidateStringSource = "abcdefghijklmnopqrstuvwxyz0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ"; break;
    }
    for (int i = 0; i < length; i++)
        strValidateString += strValidateStringSource[rnd.Next(strValidateStringSource.Length - 1)];
    return strValidateString;
}

簡單高效，可依需求擴充字元集（如特殊符號）。