jstedfast/MailKit · error · ArgumentNullException

Value cannot be null. (Parameter 'rights')

Error message

Value cannot be null. (Parameter 'rights')

What it means

AccessRights.AddRange(string rights) parses a string of single-character access rights (e.g. "lrs") and adds each as an AccessRight. A null rights string throws ArgumentNullException(nameof(rights)) at MailKit/AccessRights.cs:160. The guard exists because there is no meaningful way to interpret a null rights specification.

Solutions

1. Pass a valid rights string (e.g. "lrs") to AddRange.
2. Ensure the code producing the rights string cannot return null; default to string.Empty when absent.
3. Check the argument for null before calling AddRange and skip or substitute a default set.

Example fix

// before
string aclRights = GetRightsFromConfig(); // may be null
accessRights.AddRange(aclRights);

// after
string aclRights = GetRightsFromConfig() ?? string.Empty;
if (aclRights.Length > 0)
    accessRights.AddRange(aclRights);

if (rights != null)
    accessRights.AddRange(rights);

bool HasRights(string rights) => !string.IsNullOrEmpty(rights);

try
{
    accessRights.AddRange(rights);
}
catch (ArgumentNullException ex) when (ex.ParamName == "rights")
{
    logger.LogWarning("No rights string available; using empty rights");
}

When it happens

Trigger: Calling rights.AddRange(null) with the string overload — e.g. a null string from a parsed server response, config value, or an uninitialized variable.

Common situations: Building IMAP ACL rights from a decoded GETACL response where the rights token was missing; passing a settings value that was never configured.

Related errors

• Value cannot be null. (Parameter 'name')
• Value cannot be null. (Parameter 'array')
• Value cannot be null. (Parameter 'message')
• Value cannot be null. (Parameter 'entry')
• Value cannot be null. (Parameter 'specifier')

AI-assisted analysis of jstedfast/MailKit@9d3859a785 (2026-09-15). Data as JSON: /api/errors/0126e2054f38119a. Report an issue: GitHub.

Appendix: source

Thrown at MailKit/AccessRights.cs:160

		public bool Add (char right)
		{
			return Add (new AccessRight (right));
		}

		/// <summary>
		/// Add the rights specified by the characters in the given string.
		/// </summary>
		/// <remarks>
		/// Adds the rights specified by the characters in the given string.
		/// </remarks>
		/// <param name="rights">The rights.</param>
		/// <exception cref="System.ArgumentNullException">
		/// <paramref name="rights"/> is <see langword="null" />.
		/// </exception>
		public void AddRange (string rights)
		{
			if (rights == null)
				throw new ArgumentNullException (nameof (rights));

			for (int i = 0; i < rights.Length; i++)
				Add (new AccessRight (rights[i]));
		}

		/// <summary>
		/// Add the range of specified rights.
		/// </summary>
		/// <remarks>
		/// Adds the range of specified rights.
		/// </remarks>
		/// <param name="rights">The rights.</param>
		/// <exception cref="System.ArgumentNullException">
		/// <paramref name="rights"/> is <see langword="null" />.
		/// </exception>
		public void AddRange (IEnumerable<AccessRight> rights)
		{
			if (rights == null)

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