C# – Generate a random string

Generating a random string is useful for unit tests (filling your objects with random garbage). Here is a simple snippet to generate random strings/number/symbols in any combination. Useful for suggesting passwords.

static Random random = new Random(DateTime.Now.Second);
private static string PASSWORD_CHARS_LCASE = "abcdefghijklmnopqrstuvwxyz";
private static string PASSWORD_CHARS_NUMERIC = "0123456789";
private static string PASSWORD_CHARS_SPECIAL = "*$-+?_&=!%{}/";
public static string RandomString(int size, bool allowNumbers, bool allowUpperCase, bool allowLowerCase, bool allowSpecialChars)
{
    StringBuilder builder = new StringBuilder();
    
    List<char> validCharList = new List<char>();
    if (allowNumbers)
        validCharList.AddRange(PASSWORD_CHARS_NUMERIC.ToCharArray());
    if (allowLowerCase)
        validCharList.AddRange(PASSWORD_CHARS_LCASE.ToCharArray());
    if (allowUpperCase)
        validCharList.AddRange(PASSWORD_CHARS_LCASE.ToUpper().ToCharArray());
    if (allowSpecialChars)
        validCharList.AddRange(PASSWORD_CHARS_SPECIAL.ToCharArray());

    while (builder.Length < size)
    {
        builder.Append(validCharList[random.Next(0, validCharList.Count)]);
    }
    
    return builder.ToString();
}