GetClassFile

Returns the CLSID associated with the given filename.

HRESULT GetClassFile(

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

Parameters
szFileName

Points to the filename for which you are requesting the associated CLSID.

pclsid

Points to the location where the the associated CLSID is returned.

Return Values
S_OK

Indicates the CLSID was successfully returned.

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
The CLSID returned by this function is used, for example, by File Monikers to locate the object application that can open the file. For example, when a link to a file-based document is activated, the file moniker uses the returned CLSID to locate the application for the link source.

The following strategies are used to determine an appropriate CLSID:

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

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.
Otherwise, MK_E_INVALIDEXTENSION is returned.