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IDataObject::GetCanonicalFormatEtc

  • Article

This method provides a standard FORMATETC structure that is logically equivalent to one that is more complex. You use this method to determine whether two different FORMATETC structures would return the same data, removing the need for duplicate rendering.

HRESULT GetCanonicalFormatEtc(
  FORMATETC* pFormatetcIn,
  FORMATETC* pFormatetcOut
);

Parameters

  • pFormatetcIn
    [in] Pointer to the FORMATETC structure that defines the format, medium, and target device that the caller wants to use to retrieve data in a subsequent call such as to the IDataObject::GetData method. The TYMED member is not significant in this case and should be ignored.
  • pFormatetcOut
    [out] Pointer to a FORMATETC structure that contains the most general information possible for a specific rendering, making it canonically equivalent to pFormatetcIn. The caller must allocate this structure and the GetCanonicalFormatEtc method must fill in the data. To retrieve data in a subsequent call such as IDataObject::GetData, the caller uses the supplied value of pFormatetcOut, unless the value supplied is NULL. This value is NULL if the method returns DATA_S_SAMEFORMATETC. The TYMED member is not significant in this case and should be ignored.

Return Values

This method supports the standard return values E_INVALIDARG, E_UNEXPECTED, and E_OUTOFMEMORY. The following table shows the additional return values for this method.

Value Description
S_OK The returned FORMATETC structure is different from the one that was passed.
DATA_S_SAMEFORMATETC The FORMATETC structures are the same and NULL is returned in pFormatetcOut.
DV_E_LINDEX Invalid value for lindex; currently, only -1 is supported.
DV_E_FORMATETC Invalid value for pFormatetc.
OLE_E_NOTRUNNING The object application is not running.

Remarks

If a data object can supply exactly the same data for more than one requested FORMATETC structure, IDataObject::GetCanonicalFormatEtc can supply a canonical, or standard FORMATETC that gives the same rendering as a set of more complicated FORMATETC structures. For example, it is common for the data returned to be insensitive to the target device specified in any one of a set of otherwise similar FORMATETC structures.

Notes to Callers

A call to this method can determine whether two calls to IDataObject::GetData on a data object, specifying two different FORMATETC structures, would actually produce the same renderings, thus eliminating the need for the second call and improving performance. If the call to GetCanonicalFormatEtc results in a canonical format being written to the pFormatetcOut parameter, the caller then uses that structure in a subsequent call to IDataObject::GetData.

To determine whether the platform supports this interface, see Determining Supported COM APIs.

Notes to Implementers

Conceptually, it is possible to think of FORMATETC structures in groups defined by a canonical FORMATETC structure that provides the same results as each of the group members. In constructing the canonical FORMATETC structure, you should make sure it contains the most general information possible that still produces a specific rendering.

For data objects that never provide device-specific renderings, the simplest implementation of this method is to copy the input FORMATETC structure to the output FORMATETC structure, store a NULL in the ptd field of the output FORMATETC structure, and return DATA_S_SAMEFORMATETC.

Requirements

OS Versions: Windows CE 3.0 and later.
Header: Objidl.h, Objidl.idl.
Link Library: Ole32.lib, Uuid.lib.

See Also

IDataObject::GetData | FORMATETC

Last updated on Wednesday, April 13, 2005

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