# GetClassFile

Supplies the CLSID associated with the given filename.

WINOLEAPI GetClassFile(

LPCWSTR szFileName, | //Pointer to filename for which you are requesting a CLSID
CLSID * pclsid | //Pointer to location for returning the CLSID
); |

Parameters

szFileName
  [in] Points to the filename for which you are requesting the associated CLSID.
pclsid
  [out] Points to the location where the associated CLSID is written on return.

Return Values

S_OK
  Indicates the CLSID was successfully supplied.
MK_E_CANTOPENFILE
  Indicates unable to open the specified filename.
MK_E_INVALIDEXTENSION
  Indicates the specified extension in the registry is invalid.

Note This function can also return any file system errors.

Comments

When given a filename, the GetClassFile function finds the CLSID associated with that file. Examples of its use are in OleCreateFromFile, which is passed a file name and requires an associated CLSID, and in the OLE implementation of IMoniker::BindToObject, which, when a link to a file-based document is activated, calls GetClassFile to locate the object application that can open the file.

GetClassFile uses the following strategies to determine an appropriate CLSID:

1. If the file contains a storage object, as determined by a call to the StgIsStorageFile function, GetClassFile returns the CLSID that was written with the IStorage::SetClass method.

2. If the file is not a storage object, the GetClassFile function attempts to match various bits in the file against a pattern in the registry. A pattern in the registry can contain a series of entries of the form:

    regdb key = offset, cb, mask, value

The value of the offset item is an offset from the beginning or end of the file and the cb item is a length in bytes. These two values represent a particular byte range in the file. (A negative value for the offset item is interpreted from the end of the file). The mask value is a bit mask that is used to perform a logical AND operation with the byte range specified by offset and cb. The result of the logical AND operation is compared with the value item. If the mask is omitted, it is assumed to be all ones.

Each pattern in the registry is compared to the file in the order of the patterns in the database. The first pattern where each of the value items matches the result of the AND operation determines the CLSID of the file. For example, the pattern contained in the following entries of the registry requires that the first four bytes be AB CD 12 34 and that the last four bytes be FE FE FE FE:

    HKEY_CLASSES_ROOT
    	FileType
    	  {12345678-0000-0001-C000-000000000095}
    	    0 = 0, 4, FFFFFFFF, ABCD1234
    	    1 = -4, 4, , FEFEFEFE

If a file contains such a pattern, the CLSID {12345678-0000-0001-C000-000000000095} will be associated with this file.

- 3. If the above strategies fail, the GetClassFile function searches for the File Extension key in the registry that corresponds to the .ext portion of the filename. If the database entry contains a valid CLSID, this function returns that CLSID.

- 4. If all strategies fail, the function returns MK_E_INVALIDEXTENSION.

See Also

WriteClassStg