Smallest but fastest GUID generator function (using API)

				







An API-based CreateGUID function requires at the very least a single API function

call. The CoCreateGuid function seems to be the most popular API used for this

task. However, as the remarks in its documentation points out, it in fact calls

another API to do the real job. The RPC function UuidCreate generates the actual

Universally Unique Identifiers (UUID), which Microsoft has implemented as the GUID.

Manually converting the GUID/UUID structure to a String can be quite cumbersome

to do in VB. Fortunately, two APIs already exists for this purpose: the

UuidToString function and the StringFromGUID2 function. One drawback when using

UuidToString is that the function RpcStringFree needs to be called to release the

memory allocated by the RPC run-time library. StringFromGUID2, on the other hand,

just needs to be provided a buffer to which it will write the results.

StringFromGUID2 does have a limitation though, in that it only works with Unicode

strings. But that is easily resolved by declaring the lpsz parameter As Long and

by using StrPtr.

When allocating buffers, the intrinsic VB functions Space$ and String$ are often

used. It has already been proven that Space$ is the faster of the two. But if

utmost speed is a major factor, then SysAllocStringLen/SysReAllocStringLen are

the functions to call. While writing a Declare statement for SysAllocStringLen

that returns a "proper" VB String looks impossible (it is best called from a

Type Library), SysReAllocStringLen's syntax, on the other hand, is much easier

to convert.

Here are the necessary API declarations:
Private Type UUID
    Data1    As Long
    Data2    As Integer
    Data3    As Integer
    Data4(7) As Byte
End Type

Private Declare Function UuidCreate Lib "rpcrt4.dll" ( _
    ByRef lpUuid As Any _
) As Long
Private Declare Function StringFromGUID2 Lib "ole32.dll" ( _
    ByRef rguid As Any, _
    ByVal lpsz As Long, _
    ByVal cchMax As Long _
) As Long
Private Declare Function SysReAllocStringLen Lib "oleaut32.dll" ( _
    ByVal pBSTR As Long, _
    Optional ByVal pszStrPtr As Long, _
    Optional ByVal Length As Long _
) As Long

UuidCreate's lpUuid parameter is typed As Any to allow either GUID or UUID

structures (which are identical) or even Long Arrays. The same thing is done to

StringFromGUID2's rguid argument.

SysReAllocStringLen's first parameter should be passed a pointer to a BSTR.

Because VB Strings are also known as BSTRs, a String variable's address is

therefore retrieved with VarPtr. The second parameter, on the other hand, should

be passed a pointer to some other string's contents, i.e., with StrPtr. If this

parameter is skipped, SysReAllocStringLen will not initialize the data; thus the

resulting string will contain garbage. The final optional parameter specifies how

large the new string will be. If not given, a zero-length string ("") will be

returned.

And now, for the CreateGUID function:
Public Function CreateGUID() As String
    Const RPC_S_OK = 0&: Dim udtUUID As UUID

    If UuidCreate(udtUUID) = RPC_S_OK Then _
    SysReAllocStringLen VarPtr(CreateGUID), , 38&: _
    StringFromGUID2 udtUUID, StrPtr(CreateGUID), 39&
End Function

If UuidCreate returns successfully, then the implicit function variable is used

as a buffer and allocated with the exact size required (SysReAllocStringLen

appends a Null character before returning the string). By not specifying a string

to copy from, SysReAllocStringLen leaves the output string uninitialized. This

method makes it faster than using the Space$ function. Finally, StringFromGUID2

writes the converted value into the supplied buffer, which now becomes

CreateGUID's return value.





The included benchmark module compares the performance of this function against

several other GUID functions from this site, as well as from two other.