Component Naming and GUIDs by Gopalan Suresh Raj

Food for Thought...

import "unknwn.idl";

[uuid(6B8DEE50-E8C5-11d3-987F-006097A7D34F), dual]
interface IChecking : IDispatch {
HRESULT create ([in] BSTR key, [in] BSTR name, [in] double startingBalance);
HRESULT credit ([in] double amount, [in] BSTR key);
HRESULT debit ([in] double amount, [in] BSTR key);
HRESULT getBalance ([in] BSTR key, [out, retval] double* balance);
HRESULT getCustomerName ([in] BSTR key, [out, retval] BSTR* name);
};

[uuid (6B8DEE51-E8C5-11d3-987F-006097A7D34F), version (1.0)]
library Bank {
importlib ("stdole32.tlb");

[uuid(6B8DEE52-E8C5-11d3-987F-006097A7D34F)]
coclass Checking {
interface IChecking;
};
}
- Yours Truly

Dim checkingAccount As IChecking                                      'declare interface-based reference
Set checkingAccount = New Checking                                  'use CoCreateInstance to load component
'invoke IChecking.create method
bool found = checkingAccount.create ("10001-101","Athul", 1000000.10)
checkingAccount.credit (450001.23, "10001-101")                 'invoke IChecking.credit method
Dim balance = checkingAccount.getBalance ("10001-101") 'invoke IChecking.getBalance method
Set checkingAccount = Nothing                                              'release reference to object
                                                                                                               - Yours Truly

In C++, Global Namespace Corruption happens when we place names in global scope and a collision occurs with the same names that are in use elsewhere within the system being compiled and linked as a single executable module.

In COM+, the global namespace is the aggregate of all possible computing devices in the world. Because your COM+ component may be remotely activated on any system in the world, it is important that the name given to the component implementation and interfaces are really unique across all possible execution environments.

Globally Unique Identifiers (GUID)
COM+ uses Globally Unique Identifiers (GUIDs) to name Interfaces and their Implementations. The concept of a GUID was originally conceived by the Open Software Foundation (OSF) for their Distributed Computing Environment (DCE). The GUID is a unique value in both space and time. It is generated on your development machine by a standard OSF DCE algorithm. It is this DCE algorithm that allows rapid development of COM+ objects without requiring use of namespace partitions. COM+ provides a tool called GUIDGEN that generates a unique GUID based on spacial and temporal information.

GUIDs are 16-byte extremely long integers. They are defined as a struct in C/C++. The internal representation of a GUID is shown below:

Since this structure is large for an identifier, a datatype is defined in the standard headers that is used to pass GUIDs by value. These datatypes are defined as follows:

GUIDs are used to eliminate name collisions across libraries. Each interface has a GUID called an Interface ID (IID). Each COM+ implementation has a unique GUID called the Class ID (CLSID). Both IID and CLSID are typedefs of GUID and are therefore type compatible with one another.

GUIDs are generated by a tool called GUIDGEN.EXE which is available in the Windows Platform SDK and is also bundled for convenience with development tools like Microsoft Visual Studio. Internally, GUIDGEN calls the COM+ API CoCreateGUID() to generate a unique GUID. You can use the COM API to dynamically create GUIDs. It is defined as follows:

Similarly, if you ever need to define a GUID in a C/C++ header file, use the DEFINE_GUID macro as follows:

The algorithm used for generating the GUID is decentralized which means you can generate unique GUIDs from your local development machine.

The following code snippet shows how GUIDs can be generated programmatically using Java code in Visual J++.

The following code snippet shows how GUIDs can be generated programmatically using Visual Basic.

Option Explicit
Private Type GUID
Data1 As Long
Data2 As Integer
Data3 As Integer
Data4(7) As Byte
End Type
Private Declare Function CoCreateGuid Lib "OLE32.DLL" (pGuid As GUID) As Long
Private Const S_OK = 0 ' return value from CoCreateGuid

Function GetGUID() As String
Dim lResult As Long
Dim lguid As GUID
Dim MyguidString As String
Dim MyGuidString1 As String
Dim MyGuidString2 As String
Dim MyGuidString3 As String
Dim DataLen As Integer
Dim StringLen As Integer
Dim i%

On Error GoTo error_olemsg

lResult = CoCreateGuid(lguid)

If lResult = S_OK Then

MyGuidString1 = Hex$(lguid.Data1)
StringLen = Len(MyGuidString1)
DataLen = Len(lguid.Data1)
'First 4 bytes (8 hex digits)
MyGuidString1 = LeadingZeros(2 * DataLen, StringLen) & MyGuidString1

MyGuidString2 = Hex$(lguid.Data2)
StringLen = Len(MyGuidString2)
DataLen = Len(lguid.Data2)
'Next 2 bytes (4 hex digits)
MyGuidString2 = LeadingZeros(2 * DataLen, StringLen) & Trim$(MyGuidString2)

MyGuidString3 = Hex$(lguid.Data3)
StringLen = Len(MyGuidString3)
DataLen = Len(lguid.Data3)
'Next 2 bytes (4 hex digits)
MyGuidString3 = LeadingZeros(2 * DataLen, StringLen) & Trim$(MyGuidString3)

GetGUID = MyGuidString1 & MyGuidString2 & MyGuidString3

For i% = 0 To 7
MyguidString = MyguidString & Format$(Hex$(lguid.Data4(i%)), "00")
Next i%

'MyGuidString contains last 8 bytes of Guid (16 hex digits)
GetGUID = GetGUID & MyguidString

Else
GetGUID = "00000000" ' return zeros if function unsuccessful
End If

Exit Function

error_olemsg:
MsgBox "Error " & Str(Err) & ": " & Error$(Err)
GetGUID = "00000000"
Exit Function

End Function

Function LeadingZeros(ExpectedLen As Integer, ActualLen As Integer) As String
LeadingZeros = String$(ExpectedLen - ActualLen, "0")
End Function

Private Sub Command1_Click()
MsgBox GetGUID
End Sub

About the Author...

Gopalan Suresh Raj is a Software Architect, Developer and an active Author. He is contributing author to a couple of books "Enterprise Java Computing-Applications and Architecture" and "The Awesome Power of JavaBeans". His expertise spans enterprise component architectures and distributed object computing. Visit him at his Web Cornucopia^© site (http://www.execpc.com/~gopalan) or mail him at gopalan@execpc.com.

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