Unity-Technologies/UnityCsReference · error · NotSupportedException

Unsupported enum base type for {0}

Error message

Unsupported enum base type for {0}

What it means

MultiselectDataSource checks EnumDataUtility.GetCachedEnumData(...).serializable before building flag/mask data, and throws NotSupportedException when false. The serialization layer only supports enums whose underlying type is int; enums backed by byte, sbyte, short, ushort, uint, long, or ulong are considered non-serializable and cannot be used with the multiselect/mask dropdown.

Source

Thrown at Editor/Mono/Inspector/Core/AdvancedDropdown/DataSources/MultiselectDataSource.cs:31

        private int[] m_OptionMaskValues;
        private string[] m_OptionNames;
        private int[] m_SelectedOptions;

        string[] m_DisplayNames;
        int[] m_FlagValues;

        public Enum enumFlags => m_EnumFlag;
        public int mask => m_Mask;

        public MultiselectDataSource(Enum enumValue)
        {
            m_EnumFlag = enumValue;
            var enumType = enumFlags.GetType();

            var enumData = EnumDataUtility.GetCachedEnumData(enumType);
            if (!enumData.serializable)
                // this is the same message used in SerializedPropertyEnumHelper.cpp
                throw new NotSupportedException(string.Format("Unsupported enum base type for {0}", enumType.Name));

            m_Mask = EnumDataUtility.EnumFlagsToInt(enumData, enumFlags);
            m_DisplayNames = enumData.displayNames;
            m_FlagValues = enumData.flagValues;

            MaskFieldGUI.GetMenuOptions(m_Mask, m_DisplayNames, m_FlagValues, out var buttonText, out var buttonTextMixed, out m_OptionNames, out m_OptionMaskValues, out m_SelectedOptions);
        }

        public MultiselectDataSource(int mask, string[] displayedOptions, int[] flagValues)
        {
            m_DisplayNames = displayedOptions;
            m_FlagValues = flagValues;
            MaskFieldGUI.GetMenuOptions(mask, displayedOptions, flagValues, out var buttonText, out var buttonTextMixed, out m_OptionNames, out m_OptionMaskValues, out m_SelectedOptions);
        }

        protected override AdvancedDropdownItem FetchData()
        {
            var rootGroup = new AdvancedDropdownItem(string.Empty);

View on GitHub (pinned to 225b0fbdb5)

Solutions

1. Change the enum's underlying type to int (remove `: byte`/`: long` etc.).
2. If you cannot change the enum, map its values to an int-backed enum before passing to MultiselectDataSource.
3. Use the int-mask constructor overload MultiselectDataSource(int mask, string[], int[]) instead of the Enum-based one.
4. Verify with Enum.GetUnderlyingType(typeof(MyEnum)) == typeof(int) before constructing.

Example fix

// before
[Flags] public enum MyFlags : byte { A = 1, B = 2 } // -> NotSupportedException

// after
[Flags] public enum MyFlags { A = 1, B = 2 } // int-backed, serializable

var underlying = Enum.GetUnderlyingType(enumValue.GetType());
if (underlying != typeof(int))
    throw new InvalidOperationException("MultiselectDataSource requires an int-backed enum.");

static bool IsSerializableEnum(Type t) =>
    t.IsEnum && Enum.GetUnderlyingType(t) == typeof(int) &&
    EnumDataUtility.GetCachedEnumData(t).serializable;

When it happens

Trigger: Constructing MultiselectDataSource with an Enum whose underlying type is not int (e.g. `enum Flags : byte`, `enum Flags : long`). The shared comment notes this matches SerializedPropertyEnumHelper.cpp behavior, so the same enums fail in serialized property enum fields.

Common situations: Using a [Flags] enum declared with a non-int base type for a mask field. Interop/data-compact enums that specify `: byte` for size, then fed into an editor mask dropdown. Third-party enums with custom base types.

Related errors

• Parameter enumValue must be of type System.Enum
• Unsupported enum base type for {0}
• Parameter asset is null
• Cannot serialize json value of unknown type
• Command ({id}, {command.hint}) does not match the hinting {h

AI-assisted analysis of Unity-Technologies/UnityCsReference@225b0fbdb5 (2026-08-13). Data as JSON: /api/errors/56ded3cbf76acac2. Report an issue: GitHub.