IWMHeaderInfo3::AddAttribute method

The AddAttribute method adds a metadata attribute. To change the value of an existing attribute, use the IWMHeaderInfo3::ModifyAttribute method.


HRESULT AddAttribute(
  [in]        WORD              wStreamNum,
  [in]        LPCWSTR           pszName,
  [out]       WORD              *pwIndex,
  [in]        WMT_ATTR_DATATYPE Type,
  [in]        WORD              wLangIndex,
  [in]  const BYTE              *pValue,
  [in]        DWORD             dwLength


wStreamNum [in]

WORD containing the stream number of the stream to which the attribute applies. Setting this value to zero indicates an attribute that applies to the entire file.

pszName [in]

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

pwIndex [out]

Pointer to a WORD. On successful completion of the method, this value is set to the index assigned to the new attribute.

Type [in]

Type of data used for the new attribute. For more information about the types of data supported, see WMT_ATTR_DATATYPE.

wLangIndex [in]

WORD containing the language index of the language to be associated with the new attribute. This is the index of the language in the language list for the file. Setting this value to zero indicates that the default language will be used. A default language is created and set according to the regional settings on the computer running your application.

pValue [in]

Pointer to an array of bytes containing the attribute value.

dwLength [in]

DWORD containing the length of the attribute value, in bytes.

Return value

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

Return codeDescription

The method succeeded.


An illegal parameter combination, data type, or attribute name was used.


The method is not implemented on a reader object.


A pointer is not valid.


The size specified by dwLength is too small.


wStreamNum is not a valid stream number.



This method appends a null character to the end of the string passed in pwszName if one is not present. In this case, the buffer needed to retrieve the attribute name will be two bytes larger than the input buffer.

When setting attributes for MP3 files, the metadata editor automatically inserts a byte-order mark in accordance with the Unicode specification. If you manually insert a byte-order mark, this method will not fail, but the value will then have two marks, which can cause problems when reading the attribute.

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.

Note  Be careful to use the correct type representations of values. For example, when setting WM/MediaClassPrimaryID or WM/MediaClassSecondaryID attributes the values need to be represented as GUIDs converted to a byte array instead of strings converted to a byte array.


Minimum supported client

Windows 2000 Professional [desktop apps only]

Minimum supported server

Windows 2000 Server [desktop apps only]


Windows Media Format 9 Series SDK, or later versions of the SDK


Wmsdkidl.h (include Wmsdk.h)


WMStubDRM.lib (if you use DRM)

See also

IWMHeaderInfo3 Interface