How could HRESULT appear in an MIDL file?

Question

I am developing some COM interfaces with IDL files. Some interface methods return HRESULT, but I have checked the MIDL language reference on MSDN, there's not a clue of HRESULT. So where could I find the official definition of this data type?

Update

Thanks to Shog9, I found it in wtypes.idl. I paste it here for other's view:

    ...
    cpp_quote("#ifndef _HRESULT_DEFINED")
    cpp_quote("#define _HRESULT_DEFINED")
    #if defined(_STRICT_HRESULT)
    typedef struct _HRESULT_STRUCT {
            DWORD Data1;
    } HRESULT_STRUCT, *PHRESULT_STRUCT;
    typedef PHRESULT_STRUCT HRESULT;
    #else // defined(_STRICT_HRESULT)
    cpp_quote("#ifdef __midl")
    typedef LONG HRESULT;
    ...

However, when I use the DWORD or LONG explicitly in my IDL files, the MIDL compiler will report an error saying:

"error MIDL2269: procedures in an object interface must return an HRESULT"

Kind of ridiculous...

score 4 · Accepted Answer · answered Aug 10 '10 at 15:55

4

Any practical .idl file should start with

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

Which declares essential types. Like HRESULT and VARIANT. Etcetera.

answered Aug 10 '10 at 15:55

Hans Passant

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I have checked these 2 files. Not found the definition of HRESULT. Do I miss something? – smwikipedia Aug 10 '10 at 16:22
You probably missed all the include directives in those files. Do you have a *real* problem? – Hans Passant Aug 10 '10 at 16:33

score 4 · Answer 2 · edited May 23 '17 at 12:01

It's actually just an alias for DWORD (which is an alias for unsigned long). It's #defined for MIDL in wtypes.idl, which as Hans notes is brought in by the standard import mechanism.

The purpose of HRESULT is to represent return codes in a consistently recognizable manner - success and failure can be identified regardless of the source or other information encoded in the value, and certain system-defined values are given special meaning when coupled with standard interfaces. See MSDN for details.

How could HRESULT appear in an MIDL file?

Update

2 Answers2