IWMHeaderInfo::GetAttributeByName method (wmsdkidl.h)

Статья
4/26/2023

[The feature associated with this page, Windows Media Format 11 SDK, is a legacy feature. It has been superseded by Source Reader and Sink Writer. Source Reader and Sink Writer have been optimized for Windows 10 and Windows 11. Microsoft strongly recommends that new code use Source Reader and Sink Writer instead of Windows Media Format 11 SDK, when possible. Microsoft suggests that existing code that uses the legacy APIs be rewritten to use the new APIs if possible.]

The GetAttributeByName method returns a descriptive attribute that is stored in the header section of the ASF file. Now that attribute names can be duplicated in a file, this method is obsolete. To find attributes of a particular name, use IWMHeaderInfo3::GetAttributeIndices.

Syntax

HRESULT GetAttributeByName(
  [in, out] WORD              *pwStreamNum,
  [in]      LPCWSTR           pszName,
  [out]     WMT_ATTR_DATATYPE *pType,
  [out]     BYTE              *pValue,
  [in, out] WORD              *pcbLength
);

Parameters

[in, out] pwStreamNum

Pointer to a WORD containing the stream number, or zero to indicate any stream. Although this parameter is a pointer, the method does not change the value.

[in] pszName

Pointer to a null-terminated string containing the name of the attribute. Attribute names are limited to 1024 wide characters.

[out] pType

Pointer to a variable that receives a value from the WMT_ATTR_DATATYPE enumeration type. The returned value specifies the data type of the attribute.

[out] pValue

Pointer to a byte array that receives the value of the attribute. The caller must allocate the array. To determine the required array size, set this parameter to NULL and check the value returned in the pcbLength parameter.

[in, out] pcbLength

On input, the length of the pValue array, in bytes. On output, if the method succeeds, the actual number of bytes that were written to pValue.

Return value

The method returns an HRESULT. Possible values include, but are not limited to, those in the following table.

Return code	Description
S_OK	The method succeeded.
ASF_E_NOTFOUND	The specified attribute is not defined in this file.
E_INVALIDARG	pwStreamNum is not a valid stream number, pszName does not point to a wide-character string, or another parameter does not contain a valid value.
E_POINTER	A parameter is not a valid pointer.
E_UNEXPECTED	The method failed for an unspecified reason.
NS_E_INVALID_STATE	The object is not in a configurable state, or no profile has been set.

Remarks

Typically, an application should call this method twice for each attribute that it retrieves. On the first call, set the pValue parameter to NULL. The pcbLength parameter receives the buffer size needed to hold the attribute value. Then, allocate a sufficient byte array and call the method again, passing the address of the array in the pType parameter. The method fills the buffer with the value of the attribute. Coerce the buffer to the data type indicated by the value returned in pType.

If the file does not contain the specified attribute, the method might return ASF_E_NOTFOUND. The method can also succeed but return the value zero for pcbLength.

The objects of the Windows Media Format SDK perform type checking on some supported metadata attributes, but not all of them. You should ensure that any attributes you use are set using the data type specified in the Attributes section of this documentation. Likewise, you cannot assume that an attribute set by another application will use the correct data type.

Attributes in MP3 files cannot be specific to a particular stream. For MP3 files, always set the stream number to zero.

For a list of all the predefined attributes, see Attributes.

Examples

The following example code shows how to retrieve the "Title" attribute.


HRESULT hr;
IWMHeaderInfo *pInfo;
WORD wStreamNum = 0;
WMT_ATTR_DATATYPE Type;
WORD cbLength;
//
// First, retrieve the length of the string, and allocate memory.
//
hr = pInfo->GetAttributeByName( &wStreamNum, L"Title", 
                                &Type, NULL, &cbLength );
if( FAILED( hr ) )
{
    return( hr );
}
WCHAR *pwszTitle = (WCHAR *) new BYTE[ cbLength ];
if( !pwszTitle )
{
    return( E_OUTOFMEMORY );
}
//
// Now, retrieve the string itself.
//
hr = pInfo->GetAttributeByName( &wStreamNum, L"Title", &Type, 
                                (BYTE *) pwszTitle, &cbLength );
if( FAILED( hr ) )
{
    return( hr );
}

Requirements

Requirement	Value
Minimum supported client	Windows 2000 Professional [desktop apps only],Windows Media Format 7 SDK, or later versions of the SDK
Minimum supported server	Windows 2000 Server [desktop apps only]
Target Platform	Windows
Header	wmsdkidl.h (include Wmsdk.h)
Library	Wmvcore.lib; WMStubDRM.lib (if you use DRM)

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