c#生成随机字符串

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public class RandomStringBuilder {     /// <summary>     /// 生成单个随机数字     /// </summary>     private int createNum()     {         Random random = new Random(Guid.NewGuid().GetHashCode());         int num = random.Next(10);         return num;     }       /// <summary>     /// 生成单个大写随机字母     /// </summary>     private string createBigAbc()     {         //A-Z的 ASCII值为65-90         Random random = new Random(Guid.NewGuid().GetHashCode());         int num = random.Next(65, 91);         string abc = Convert.ToChar(num).ToString();         return abc;     }       /// <summary>     /// 生成单个小写随机字母     /// </summary>     private string createSmallAbc()     {         //a-z的 ASCII值为97-122         Random random = new Random(Guid.NewGuid().GetHashCode());         int num = random.Next(97, 123);         string abc = Convert.ToChar(num).ToString();         return abc;     }         /// <summary>     /// 生成随机字符串     /// </summary>     /// <param name="length">字符串的长度</param>     /// <returns></returns>     public string Create(int length)     {         // 创建一个StringBuilder对象存储密码         StringBuilder sb = new StringBuilder();         //使用for循环把单个字符填充进StringBuilder对象里面变成14位密码字符串         for (int i = 0; i < length; i++)         {             Random random = new Random(Guid.NewGuid().GetHashCode());             //随机选择里面其中的一种字符生成             switch (random.Next(3))             {                 case 0:                     //调用生成生成随机数字的方法                     sb.Append(createNum());                     break;                 case 1:                     //调用生成生成随机小写字母的方法                     sb.Append(createSmallAbc());                     break;                 case 2:                     //调用生成生成随机大写字母的方法                     sb.Append(createBigAbc());                     break;             }         }         return sb.ToString();     } }