랜덤 문자열을 얻을 수 있습니다.

RandomString.cs

using System;
using System.Linq;

[Flags]
public enum Options
{
    Upper = 1,
    Lower = 1 << 1,
    Number = 1 << 2
}

public static string GetRandomString(int count, Options opt)
{
    var num = Enumerable.Range(48, 10); // 0 ~ 9
    var upp = Enumerable.Range(65, 26); // A ~ Z
    var low = Enumerable.Range(97, 26); // a ~ z
    
    char GetRandomChar()
    {
        var tmp = Enumerable.Empty<int>();
        
        if (opt.HasFlag(Options.Upper))
            tmp = tmp.Concat(upp);
        if (opt.HasFlag(Options.Lower))
            tmp = tmp.Concat(low);
        if (opt.HasFlag(Options.Number))
            tmp = tmp.Concat(num);

        if (!tmp.Any())
            throw new System.InvalidOperationException();

        return (char)tmp.ElementAt(new Random().Next(tmp.Count()));
    }

    string result = string.Empty;

    for (int i = 0; i < count; i++)
        result += GetRandomChar();

    return result;
}