.NET assembly file format: flags

How flags are shown

Some fields hold one of several values in a group of bits (visibility, layout); every other bit is one flag. The explorer prints the number in hex and the names that are set; a bit with no name is printed as hex so nothing is hidden. The values below are the ones the explorer names.

TypeAttributes (TypeDef.Flags, 4 bytes)

Mask	Values
0x7 visibility	0 NotPublic, 1 Public, 2 NestedPublic, 3 NestedPrivate, 4 NestedFamily, 5 NestedAssembly, 6 NestedFamANDAssem, 7 NestedFamORAssem
0x18 layout	0 AutoLayout, 0x8 SequentialLayout, 0x10 ExplicitLayout
0x20	0 Class, 0x20 Interface
0x30000 string format	0 AnsiClass, 0x10000 UnicodeClass, 0x20000 AutoClass, 0x30000 CustomFormatClass
single bits	0x80 Abstract, 0x100 Sealed, 0x400 SpecialName, 0x800 RTSpecialName, 0x1000 Import, 0x2000 Serializable, 0x4000 WindowsRuntime, 0x40000 BeforeFieldInit, 0x200000 HasSecurity, 0x40000000 Forwarder

FieldAttributes (Field.Flags, 2 bytes)

Mask	Values
0x7 access	0 CompilerControlled, 1 Private, 2 FamANDAssem, 3 Assembly, 4 Family, 5 FamORAssem, 6 Public
single bits	0x10 Static, 0x20 InitOnly, 0x40 Literal, 0x80 NotSerialized, 0x100 HasFieldRVA, 0x200 SpecialName, 0x400 RTSpecialName, 0x1000 HasFieldMarshal, 0x2000 PinvokeImpl, 0x8000 HasDefault

MethodAttributes (MethodDef.Flags, 2 bytes)

Mask	Values
0x7 access	0 CompilerControlled, 1 Private, 2 FamANDAssem, 3 Assem, 4 Family, 5 FamORAssem, 6 Public
0x100	0 ReuseSlot, 0x100 NewSlot
single bits	0x10 Static, 0x20 Final, 0x40 Virtual, 0x80 HideBySig, 0x200 CheckAccessOnOverride, 0x400 Abstract, 0x800 SpecialName, 0x1000 RTSpecialName, 0x2000 PinvokeImpl, 0x4000 UnmanagedExport, 0x8000 HasSecurity

MethodImplAttributes (MethodDef.ImplFlags, 2 bytes)

Mask	Values
0x3 code type	0 IL, 1 Native, 2 OPTIL, 3 Runtime
0x4	0 Unmanaged, 0x4 Managed
single bits	0x8 ForwardRef, 0x10 PreserveSig, 0x20 Synchronized, 0x40 NoInlining, 0x80 NoOptimization, 0x100 AggressiveInlining, 0x200 AggressiveOptimization, 0x1000 InternalCall

ParamAttributes (Param.Flags, 2 bytes)

0x1 In, 0x2 Out, 0x4 Lcid, 0x8 Retval, 0x10 Optional, 0x1000 HasDefault, 0x2000 HasFieldMarshal.

EventAttributes and PropertyAttributes

Event.EventFlags: 0x200 SpecialName, 0x400 RTSpecialName. Property.Flags: 0x200 SpecialName, 0x400 RTSpecialName, 0x1000 HasDefault.

Method semantics (MethodSemantics.Semantics, 2 bytes)

0x1 Setter, 0x2 Getter, 0x4 Other, 0x8 AddOn, 0x10 RemoveOn, 0x20 Fire.

GenericParamAttributes (GenericParam.Flags, 2 bytes)

Mask 0x3 variance: 0 None, 1 Covariant, 2 Contravariant. Bits: 0x4 ReferenceTypeConstraint (class), 0x8 NotNullableValueTypeConstraint (struct), 0x10 DefaultConstructorConstraint (new()), 0x20 AllowByRefLike (allows ref struct).

PInvokeAttributes (ImplMap.MappingFlags, 2 bytes)

Mask	Values
0x6 character set	0 CharSetNotSpec, 0x2 CharSetAnsi, 0x4 CharSetUnicode, 0x6 CharSetAuto
0x700 calling convention	0x100 Platformapi, 0x200 Cdecl, 0x300 Stdcall, 0x400 Thiscall, 0x500 Fastcall
0x3000 best fit	0x1000 BestFitEnabled, 0x2000 BestFitDisabled
single bits	0x1 NoMangle, 0x40 SupportsLastError

AssemblyFlags (Assembly.Flags, AssemblyRef.Flags, 4 bytes)

0x1 PublicKey (the key column holds the full key, not a token), 0x100 Retargetable, 0x4000 DisableJITcompileOptimizer, 0x8000 EnableJITcompileTracking.

CLR header flags

0x1 ILONLY, 0x2 32BITREQUIRED, 0x4 IL_LIBRARY, 0x8 STRONGNAMESIGNED, 0x10 NATIVE_ENTRYPOINT, 0x10000 TRACKDEBUGDATA, 0x20000 32BITPREFERRED. An image that is not ILONLY holds native code as well: a mixed-mode (C++/CLI) image, or a precompiled one.

Not described

Known, not yet covered

Reference: the meaning of each flag in the runtime (what BeforeFieldInit changes about type initialisation, how HideBySig affects overload resolution) is not explained; this page lists the values.
Viewer: the ExportedType flags, ManifestResource visibility and File flags are named as in the specification; flag combinations the specification forbids are not reported.

.NET Assembly File Format – Flags