dotnet/wpf · error · ApplicationException

Flat enums can't be flag type

Error message

Flat enums can't be flag type: {0}

What it means

FormatNativeEnum in the mcg codegen tool's EnumHelper throws this ApplicationException when the model contains a flat (non-nested) enum that is marked as a flags enum (e.Flags). The generator only supports flag enums in the nested form, so a flat enum declared with flags is a modeling error in the input XML and code generation aborts.

Solutions

  1. Remove the flags marker from the flat enum in the input XML, or
  2. Convert the enum to the nested form the generator supports for flag types.
  3. Regenerate and confirm the model validates before re-running the generator.

Example fix

// before (model XML)
<enum name='MyEnum' flags='true'> ... flat values ... </enum>
// after
<enum name='MyEnum'> ... flat values ... </enum>
Defensive patterns

Strategy: validation

Validate before calling

// before running the generator, check the model:
// every enum with flags='true' must use the nested form
foreach (var e in model.Enums)
    if (e.Flags && e.IsFlat) throw new Exception($"{e.UnmanagedName}: flat enums cannot be flags");

Try / catch

try { generator.Run(args); }
catch (ApplicationException ex) when (ex.Message.StartsWith("Flat enums can't be flag type")) { /* fix the enum declaration in the model XML */ }

Prevention

When it happens

Trigger: Running the mcg generator over an XML definition where an enum is declared flat but flagged as a flags type (Flags=true), causing FormatNativeEnum to hit its invariant check while emitting native enum comments/values.

Common situations: Hand-editing the mcg input XML and adding a flags attribute to a plain enum; migrating enum definitions between generator versions with different flag-enum rules; copy-pasting an enum declaration from another model that used the nested flag form.

Understand the failure class

Background: Invalid enum value errors: "Unknown type", "Invalid scope", "must be one of" — when a string is not on the library's allowed list — this error's family across 23 libraries.

Related errors

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/c73e429566a2b6b1. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/WpfGfx/codegen/mcg/helpers/EnumHelper.cs:78

                        [[/inline]]
                        ;
                }
                else
                {
                    return comment +
                        [[inline]]
                            BEGIN_MILENUM( [[e.UnmanagedName]] )
                                [[GetEnumMembers(e)]]
                            END_MILENUM
                        [[/inline]]
                        ;
                }
            }
            else
            {
                if (e.Flags)
                {
                    throw new ApplicationException(String.Format(
                        "Flat enums can't be flag type: {0}",
                        e.UnmanagedName));
                }

                return comment +
                    [[inline]]
                        typedef enum
                        {
                            [[GetEnumMembers(e)]]

                            [[e.UnmanagedName]]_[[GetNativeEnumForceSize(e)]]
                        } [[e.UnmanagedName]];
                    [[/inline]]
                    ;
            }
        }

        /// <summary>

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