C# 生成随机数

/// <summary>
/// 创建随机数
/// </summary>
/// <param name="num"></param>
/// <returns></returns>
private string CreateRandom(int num)
{
string str = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz";
StringBuilder sb = new StringBuilder();
for (int i = 0; i < num; i++)
{
sb.Append(str[new Random(Guid.NewGuid().GetHashCode()).Next(0, str.Length - 1)]);
}
return sb.ToString();
}