TechnitiumSoftware/DnsServer · error · ArgumentException

Username cannot be null or empty.

Error message

Username cannot be null or empty.

What it means

Thrown by User.IsUsernameValid(username, throwException: true) when username is null, empty, or whitespace. It is an ArgumentException (parameter Username). IsUsernameValid is the shared validator used by username-setting paths; with throwException it reports the problem instead of returning false.

Source

Thrown at DnsServerCore/Auth/User.cs:171

        public static User CreateSsoUser(string displayName, string username, string ssoIdentifier)
        {
            User user = new User();

            user.SetUsername(username);
            user.DisplayName = displayName;
            user._isSsoUser = true;
            user._ssoIdentifier = ssoIdentifier;

            return user;
        }

        public static bool IsUsernameValid(string username, bool throwException = false)
        {
            if (string.IsNullOrWhiteSpace(username))
            {
                if (throwException)
                    throw new ArgumentException("Username cannot be null or empty.", nameof(Username));

                return false;
            }

            if (username.Length > 255)
            {
                if (throwException)
                    throw new ArgumentException("Username length cannot exceed 255 characters.", nameof(Username));

                return false;
            }

            foreach (char c in username)
            {
                if ((c >= 97) && (c <= 122)) //[a-z]
                    continue;

                if ((c >= 65) && (c <= 90)) //[A-Z]

View on GitHub (pinned to d0484b6c1e)

Solutions

1. Supply a non-empty, non-whitespace username.
2. Validate presence at the request boundary before calling IsUsernameValid.
3. Use IsUsernameValid(username) without throwException to branch cleanly on invalid input.

Example fix

// before
User.IsUsernameValid(username, throwException: true);

// after
if (!User.IsUsernameValid(username))
    return BadRequest("Username cannot be null or empty.");

// Use the non-throwing overload to branch cleanly.
if (!User.IsUsernameValid(username))
    return BadRequest("Username cannot be null or empty.");

try { User.IsUsernameValid(username, throwException: true); }
catch (ArgumentException ex) when (ex.ParamName == "Username" && ex.Message.Contains("null or empty"))
{ return BadRequest(ex.Message); }

When it happens

Trigger: Calling IsUsernameValid(blank, true) directly, or SetUsername/Create paths that delegate to it with a blank username when throwException is enabled.

Common situations: A create-user form submitted without a username; an import row with a blank username column; an SSO provisioning path that passes a null identifier through to the validator.

Related errors

• Username length cannot exceed 255 characters.
• Username can contain only alpha numeric, '@', '-', '_', or '
• User account is disabled. Please contact your administrator.
• Cannot create more than 255 users.
• User already exists: {username}

AI-assisted analysis of TechnitiumSoftware/DnsServer@d0484b6c1e (2026-08-13). Data as JSON: /api/errors/9d293fac6ce3ff29. Report an issue: GitHub.