User validateUser = new User();

validateUser.firstName = model.firstName.Trim();
validateUser.lastName = model.lastName.Trim();
validateUser.email = model.Email.Trim();

//Create Email and Mobile Verification Codes
string randomEmailCode = "";
randomEmailCode = _notifyService.GenerateEmailCode();
validateUser.emailVerificationCode = randomEmailCode;

string randomMobileCode = "";
randomMobileCode = _notifyService.GenerateMobileCode();
validateUser.mobileVerificationCode = randomMobileCode;

public string GenerateEmailCode()
{
    StringBuilder builder = new StringBuilder();

    builder.Append(RandomString(4, true));
    builder.Append(RandomNumber(1000, 9999));
    builder.Append(RandomString(2, false));
    return builder.ToString();
}

public string GenerateMobileCode()
{
    StringBuilder builder = new StringBuilder();

    builder.Append(RandomString(3, true));
    builder.Append(RandomNumber(1000, 9999));
    builder.Append(RandomString(2, false));
    return builder.ToString();
}

private int RandomNumber(int min, int max)
{
    Random random = new Random();
    return random.Next(min, max);
}

private string RandomString(int size, bool lowerCase)
{
    StringBuilder builder = new StringBuilder();
    Random random = new Random();
    char ch;
    for (int i = 0; i < size; i++)
        {
            ch = Convert.ToChar(Convert.ToInt32(Math.Floor(26 * random.NextDouble() + 65)));
            builder.Append(ch);
        }
        if (lowerCase)
            return builder.ToString().ToLower();
        return builder.ToString();
    }

_accountService.AddUser(validateUser);
_accountService.Save();

randomEmailCode = _notifyService.GenerateEmailCode();

validateUser.emailVerificationCode = "emailCode";
validateUser.mobileVerificationCode = "mobileCode";

static Random r = new Random();

public static string generateCode()
        {
            string chrs = "abcdefghijklmnopqrstuvwxyz";
            char[] arr = chrs.ToCharArray();
            string code = "";
            for (int i = 0; i < 5; i++)
            {
                code += arr[r.Next(arr.Count())];
            }
            return code;
        }

var firstCode  = generateCode();
 var secondCode = generateCode();