Both encapsulated and nonencapsulated unions share a common union_arm_selector<> format:

arm1_case_value<4> offset_to_arm_description<2>
armN_case_value<4> offset_to_arm_description<2>

The union_arms<2> field consists of two parts. If the union is a MIDL 1.0 style union, the upper 4 bits contain the alignment of the union arm (alignment of greatest aligned arm). Otherwise the upper 4 bits are zero. The lower 12 bits contain the number of arms in the union. In other words:

alignment<highest nibble> arm_counter<three lower nibbles>

The offset_to_arm_description<2> fields contain a relative signed offset to the arm's type description. However, the field is overloaded with optimization for simple types. For these, the upper byte of this offset field is FC_MAGIC_UNION_BYTE (0x80) and the lower byte of the short is the actual format character type of the arm. As such, there are two ranges for the offset values: "80 xx" means that xx is a type format string; and everything else within range (80 FF .. 7f FF) means an actual offset. This makes offsets from range <80 00 .. 80 FF > unavailable as offsets. The compiler checks that as of MIDL version 5.1.164.

The default_arm_description<2> field indicates the type of union arm for the default arm, if any. If there is no default arm specified for the union then the default_arm_description<2> field is 0xFFFF and an exception is raised if the switch_is value does not match any of the arm case values. If the default arm is specified but empty, then the default_arm_description<2> field is zero. Otherwise the default_arm_description<2> field has the same semantics as the offset_to_arm_description<2> fields.

The following is a summary:

  • 0 - empty default
  • FFFF - no default
  • 80xx - simple type
  • other - relative offset

Encapsulated Unions

An encapsulated union comes from a special union syntax in IDL. Effectively, an encapsulated union is a bundle structure with a discriminant field at the beginning of the structure and the union as the only other member.


An encapsulated union's switch_type<1> field has two parts. The lower nibble provides the actual switch type, and the upper nibble provides the memory increment to step over that is an amount that the memory pointer must be incremented to skip past the switch_is field, which includes any padding between the switch_is() field of the stub-constructed structure and the actual union field.

The memory_size<2> field is the size of the union only, and is identical to nonencapsulated unions. To obtain the total size of the structure that contains the union, add memory_size<2> to memory increment to step over, that is to the upper nibble of the switch_type<1> field, then align by the alignment corresponding to the increment.

Nonencapsulated Unions

A nonencapsulated union is a typical situation where a union is one argument or field and the switch is another argument or field, respectively.



The switch_type<1> field is a format character for the discriminant.

The switch_is_descriptor<> field is a correlation descriptor and has 4 or 6 bytes depending on whether /robust is used. However, for the switch_is_description<>, if the union is embedded in a structure, the offset field of the switch_is_description<> is the offset to the switch_is field from the union's position in the structure (not from the beginning of the structure).

The offset_to_size_and_arm_description<2> field gives the offset to the union's size and arm description, which is identical to that for encapsulated unions and is shared by all nonencapsulated unions of the same type :