devlooped/moq · error · ArgumentException

Value cannot be an empty string.

Error message

Value cannot be an empty string.

What it means

Guard.NotNullOrEmpty validates that a string argument is non-null and non-empty before Moq proceeds. After the null check throws ArgumentNullException, an empty string (Length == 0) throws ArgumentException with "Value cannot be an empty string.". It protects APIs that require a meaningful string identifier (e.g. member or event names).

Solutions

1. Pass a non-empty string value (trim user/config input before calling).
2. Default the value to a valid fallback when it would be empty.
3. If null vs empty semantics differ, ensure the caller passes null (which throws ArgumentNullException instead) only when the API allows it.
4. Validate inputs at your API boundary before invoking Moq.

Example fix

// before
string name = config.MemberName ?? ""; // may be ""
moqHelper.Invoke(name); // throws
// after
string name = config.MemberName;
if (string.IsNullOrEmpty(name)) throw new InvalidOperationException("MemberName not configured");
moqHelper.Invoke(name);

if (string.IsNullOrEmpty(name)) throw new InvalidOperationException("Name must be non-empty before calling the API");

static string RequireNonEmpty(string? s, [CallerArgumentExpression(nameof(s))] string? p = null) => string.IsNullOrEmpty(s) ? throw new ArgumentException("Value cannot be empty", p) : s;

try { api.Invoke(name); }
catch (ArgumentException ex) when (ex.Message == "Value cannot be an empty string.") { /* supply a valid name */ throw; }

When it happens

Trigger: Passing string.Empty to any Moq API routed through this guard, e.g. empty property/member names in weak-event or reflection-based helpers, empty event names in Raise/Event setups.

Common situations: Building names dynamically from config or reflection and getting an empty result; default-initialized string fields; copy-paste leaving a placeholder empty.

Understand the failure class

Background: "must not be empty", "cannot be empty" — required-field validation errors across open-source libraries — this error's family across 41 libraries.

Related errors

• Unsupported expression
• Could not determine the correct positions for all argument…
• The return type of the last member shown above is not…
• Value cannot be null. (Parameter 'value')
• Unsupported expression

AI-assisted analysis of devlooped/moq@89a5be629c (2026-09-16). Data as JSON: /api/errors/511bcc1679601aba. Report an issue: GitHub.

Appendix: source

Thrown at src/Moq/Guard.cs:195

                throw new ArgumentNullException(paramName);
            }
        }

        /// <summary>
        /// Ensures the given string <paramref name="value"/> is not null or empty.
        /// Throws <see cref="ArgumentNullException"/> in the first case, or 
        /// <see cref="ArgumentException"/> in the latter.
        /// </summary>
        public static void NotNullOrEmpty(string value, string paramName)
        {
            if (value == null)
            {
                throw new ArgumentNullException(paramName);
            }

            if (value.Length == 0)
            {
                throw new ArgumentException(Resources.ArgumentCannotBeEmpty, paramName);
            }
        }

        public static void NotField(MemberExpression memberAccess)
        {
            if (memberAccess.Member is FieldInfo)
                throw new NotSupportedException(
                    string.Format(
                        Resources.FieldsNotSupported,
                        memberAccess.ToStringFixed()));
        }

        public static void IsMockable(Type type)
        {
            if (!type.IsMockable())
            {
                throw new NotSupportedException(
                    string.Format(

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