How can I use a User Defined Type (UDT) in a COM server?

Question

I have a COM server with a method currently returning an integer:

[
    object,
    uuid("..."),
    dual,
    helpstring("IMyCOMServer Interface"),
    pointer_default(unique)
]
__interface IMyCOMServer : IDispatch
{
    [id(1), helpstring("method MyQuery")]
    HRESULT MyQuery([in] BSTR instr, [out,retval] int* outint);
};

This compiles fine, but I'd rather return an enum: (this code is actually above the interface definition)

typedef
[
    uuid("..."),
    v1_enum,
    helpstring("Enum")
]
enum {
    value_a,
    value_b,
    value_c
} MyEnum;

Which again compile fine of its own right, but as soon as I change the int* to MyEnum* in the interface and implementation, I get linker errors:

[id(1), helpstring("method MyQuery")]
HRESULT MyQuery([in] BSTR instr, [out,retval] MyEnum* outint);

error MIDL2025 : syntax error : expecting a type specification near "MyEnum"

Whichever way I do it, I can't get it to compile.

---

Thanks to Euro Micelli it turns out that the real problem is that my User Defined Type (the enum) wasn't making it into the generated .IDL file. Judging by forum queries online, this seems to be a common problem.

A blog article Star Tech: UDT (User Defined Types) and COM guided me down the right path. It seems that a workaround is needed when using attributed ATL.

In summary, I made the following changes:

Created udt.idl:

import "oaidl.idl";
import "ocidl.idl";

[
    uuid("..."),
    v1_enum,
    helpstring("Enum")
]
typedef enum MyEnum {
    value_a,
    value_b,
    value_c
} MyEnum_t;

[
    version(1.0),
    uuid(...),
    helpstring(...)
]
library MyLibrary
{
    enum MyEnum;
}

Added the following line prior to the module attribute in the main .cpp file so that the above IDL gets imported into the generated file:

[importidl("udt.idl")];

Answer 1

This is very compiler-dependent, since enums don't have a fixed storage size.

There's also the compatibility angle - how would you represent that enum in, say, Visual Basic, or C#? The underlying storage is something like an integer, so that's what COM allows.

Answer 2

You've almost got it, but the idl compiler has a little stricter syntax than the cl.exe. You need to have the initial enum name before the enum like this.

typedef
[uuid("..."), v1_enum, helpstring("Enum")]
enum tagMyEnum
{
    value_a,
    value_b,
    value_c
} MyEnum;

If you build and register your tlb then scripting languages should be able to access your enum in scripts and .NET.

Answer 3

(This is adapted from an actual IDL, so I know it works)

[uuid(XXXXXXXX-XXXX-XXXX-XXXX-XXXXXXXXXXXX), v1_enum, helpstring("Enum")]
enum MyEnum {
    value_a,
    value_b,
    value_c
} ;

Plus, in your Library section you must include the enum as well, or the enum won't be exported to the type library:

library MyLib
{
    enum MyEnum;
...
};