This command allows the user to select the desired union field. In the presence of unions, the decompiler can not always detect the correct union field.
The decompiler tries to reuse the union selection information from the disassembly listing. If there is no information in the disassembly listing, the decompiler uses an heuristic rule to choose the most probable union field based on the field types. However, it may easily fail in the presence of multiple union fields with the same type or when there is no information how the union field is used.
If both the above methods of selecting the union field fail, then this command can be used to specify the desired field. It is especially useful for analyzing device drivers (I/O request packets are represented with a long union), or COM+ code that uses VARIANT data types.
See also: interactive operation