Method Enumeration in ATL COM Components

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Introduction

Usually when we are working with COM objects/controls in Visual Basic or ASP, we see method enumeration, like ListView.ColumnHeaders.Add (some arguments). In order to develop such COM components in VC++ using the ATL COM wizard, which allows the user to enumerate the methods of one COM object inside the method of another COM object, this article helps in getting the method enumeration when using a ATL based COM components in the Visual Basic and ASP Environments. It is also possible to access them in the VC++. Here are set of steps to be followed in building such Components

Steps in Developing the Component

1. Using the ATL COM wizard, create a DLL project called CoolCodeComponent.
2. Insert a 'Simple' object called "coolcode", with class name Ccoolcode and Interface name Icoolcode
3. Add a Method called EnumMethod to the Icoolcode interface as follows

   HRESULT EnumMethod([out,retval] pEnumComponent *Res)

   Note : Here the pEnumComponent is the pointer to the interface whose methods are to be enumerated. It will be declared as a typedef in the .IDL in step 8 in this article.

4. Insert another 'Simple' COM Object into the workspace called 'EnumComponent' with class name CEnumComponent and Interface name IEnumComponent.
5. Add two methods to this interface:

   HRESULT Add(int x, int y,[out,retval] int *Sum) 

   Note : this method is used to add two numbers and return their sum

   HRESULT Multiply(int x, int y, [out,retval] int *Mul)

   Note : this method is used to multiply two numbers and return their product

6. Provide the implementation for the above two methods as follows inside the EnumComponent.cpp

   STDMETHODIMP CEnumComponent::Add(int x, int y, int *Sum)
   {
   	*Sum =  x + y; // Add this line
   
   	return S_OK;
   }

   STDMETHODIMP CEnumComponent::Multiply(int x, int y, int *Mul)
   {
   	*Mul = x * y; // Add this line
   
   	return S_OK;
   }

7. In coolcode.h file make the following changes:
   • add the header file of EnumComponent

     #include "EnumComponent.h"

   • declare a member variable to CEnumComponent like

      public :
           CComPtr<IEnumComponent> pEnumComp;
     

   • In the constructor of coolcode create the EnumComponent

     Ccoolcode()
     {
         pEnumComp.CoCreateInstance(CLSID_EnumComponent); // Add this line
     
     }

   • In the destructor of coolcode, release the interface to IEnumComponent

     ~Ccoolcode()
     {
         if(pEnumComp != NULL)
     	pEnumComp.Release();
     }

8. Changes to the .IDL file:
   • Declare a variable of type IEnumComponent before the import "oaidl.idl";

     typedef IEnumComponent *pEnumComponent;

9. In coolcode.cpp, add the following code to EnumMethod

   STDMETHODIMP Ccoolcode::EnumMethod(pEnumComponent *Res)
   {
         *Res = pEnumComp; // Add this line
   
         return S_OK;
   }

Now build the component in Win32 Release MinDependency mode

How to Use

Check the component in Visual Basic:

1. Create a reference to

   CoolCodeComponent 1.0 Type Library

2. Add a command button to the form and write the following code in button click

       Private Sub Command1_Click()
   	   Dim a As New COOLCODECOMPONENTLib.coolcode
               MsgBox a.EnumMethod.Add(10, 12)
               MsgBox a.EnumMethod.Multiply(10, 20)
       End Sub

3. When you run the application it will display the result of the two method calls in message box.