JamesNK/Newtonsoft.Json · error · FormatException

Integer string '{0}' is not allowed.

Error message

Integer string '{0}' is not allowed.

What it means

ParseEnum (EnumUtils.cs:296-323), after determining the value parses as a number (char.IsDigit or sign at the start and Convert.ChangeType succeeds), checks the disallowNumber flag at EnumUtils.cs:317-320. If disallowNumber is true, it throws FormatException reporting the integer string. disallowNumber is set by callers that want to reject raw integer enum values (notably StringEnumConverter with AllowIntegerValues=false).

Source

Thrown at Src/Newtonsoft.Json/Utilities/EnumUtils.cs:319

                value = value.Trim();
                object? temp = null;

                try
                {
                    temp = Convert.ChangeType(value, underlyingType, CultureInfo.InvariantCulture);
                }
                catch (FormatException)
                {
                    // We need to Parse this as a String instead. There are cases
                    // when you tlbimp enums that can have values of the form "3D".
                    // Don't fix this code.
                }

                if (temp != null)
                {
                    if (disallowNumber)
                    {
                        throw new FormatException("Integer string '{0}' is not allowed.".FormatWith(CultureInfo.InvariantCulture, value));
                    }

                    return Enum.ToObject(enumType, temp);
                }
            }

            ulong result = 0;

            int valueIndex = firstNonWhitespaceIndex;
            while (valueIndex <= value.Length) // '=' is to handle invalid case of an ending comma
            {
                // Find the next separator, if there is one, otherwise the end of the string.
                int endIndex = value.IndexOf(EnumSeparatorChar, valueIndex);
                if (endIndex == -1)
                {
                    endIndex = value.Length;
                }

View on GitHub (pinned to 4f73e74372)

Solutions

1. Send enum values as their string member names in the JSON (e.g. "Active" rather than 3).
2. If integer values are acceptable, set the StringEnumConverter's AllowIntegerValues=true (and AllowNullValues if needed).
3. Pre-process the payload to map numbers to names before deserialization.
4. Use a custom JsonConverter that maps disallowed integers to a default member.

Example fix

// before: StringEnumConverter { AllowIntegerValues = false }, JSON = 3 -> throws
// after (option A): emit "Active" in JSON
// after (option B): new StringEnumConverter { AllowIntegerValues = true }

// If integers are disallowed, ensure the value is a name, not a number.
if (disallowNumber && value.Trim().TrimStart('-','+').All(char.IsDigit) && value.Trim().Length > 0)
    throw new FormatException($"Integer string '{value}' is not allowed.");
// or: set StringEnumConverter.AllowIntegerValues = true if integers are acceptable.

static bool LooksLikeInteger(string s) { s = s.Trim(); return s.Length > 0 && (char.IsDigit(s[0]) || s[0]=='-' || s[0]=='+') && s.TrimStart('-','+').All(char.IsDigit); }

try { return EnumUtils.ParseEnum(enumType, null, value, disallowNumber: true); } catch (FormatException ex) when (ex.Message.Contains("not allowed")) { /* map to a name or accept integers */ }

When it happens

Trigger: An enum field receives a numeric value (e.g. JSON `"status": 3` or the string "3") while the configured converter has AllowIntegerValues=false, so integer representations are forbidden.

Common situations: StringEnumConverter configured with AllowIntegerValues=false but the source emits integers (common with System.Text.Json interop or older producers); schema enforcing string-only enum values; serializers migrated from one that emitted numbers.

Related errors

• Enum name '{0}' already exists on enum '{1}'.
• Enum type {0} is not a set of flags.
• Unexpected merge array handling when merging JSON.
• value
• value

AI-assisted analysis of JamesNK/Newtonsoft.Json@4f73e74372 (2026-08-07). Data as JSON: /api/errors/53859cd9dba8ada8. Report an issue: GitHub.