During any discussions in forums, there are many people who want to write COM/ATL
DLL but have problems creating Properties, Methods or raise Events from
Components.
In this article, I am going to write a Simple, ATL DLL step by step using
VC++ 6.0 and Demonstrate Creating and using of Property, Method and Events.
Convention Used
Symbol --> Means Author Comments
Symbol | Means Menus Operation, i.e.,
FILE |
NEW mean Clicking
New item of File Menu.
Creating the ATL Project:
As
I am writing the article at beginner
level, I have included as many Screen Shots
to explain the creation of COM DLL and explaining it step by step.
1. Open Visual Studio 6.0 and click Menu Item File|NEW
to get this
Dialog Box.
|
Figure 1
|
 |
2. Choose
the ATL COM AppWizard and Give a Project Name
SimpleAtlCom and
click OK to Accept Project Setting, and In next Step this Dialog
Box will be shown.
|
Figure
2
|
 |
3. Click Finish to Accept Project Setting, these above written two steps will
create a blank COM DLL project.
4. Now
,
Click
the Menu Insert|NEW ATL Object... , add a Atl Object to
the Project, you will see this Figure when you click above menu item
|
Figure 3
|
 |
5. Select the Object|Simple Object and click Next, then you will see
this Property page, let take a view in following Figure.
|
Figure 4 & 5
|
 |
 |
6. Here in (Fig 4 ) give the Short Name as SimpleObj and you
will see other Field in this dialog box get self generated.
7. in (Fig 5) let me explain every thing
- First you see the Threading Model in
the wizard; the Compiler default is selected here which is apartment as most of other
application using our component is comfortable with this model.
- Second is Interface, again I have
selected the Wizard Default to Dual (there is one benefit of using dual
interface ,is that you can use it in scripting language also)
- Leave the topics of Aggregation and
Free
Threaded Marshaler ,as they are Too advanced to be used in this article
Now look at ISupportErrorInfo, this is
provided to send rich Textual information back to Client Application which
is using our Interface. As we are not going to send any Error Information
back to Client, I have left this Check Box unchecked.
Last but not the Least, for invoking event
from Component ,you need support of IConnectionPoint Interface, So
check the box for Supporting the Connection Point, click OK to Add the
Object.
8. Now In your Project a New IDL File is get added in your Project (simpleAtlCom.idl)
here take a look at IDL file how
it Looks like.
// SimpleAtlCom.idl : source IDL for our SimpleAtlCom.dll project
9. Now add Property and Method to Our Interface, let make a simple
application based on Class Application. Any way do you know what is property
and method. if not here is brief description about them Method is a name
given to Function in interface and property to variable .but remember one
thing every thing in COM/ATL is based on function, one major difference
between Property and Method you can make Property Read only means you can
only get data from Property but you can
not put them now let get back to our
application.
10. Now put Three Property Both Get and Put and one method in our
interface. Now you going to ask how and where to put them. Right click
on your interface you will get option for putting the both method and
Property .for clarity let take a look on these pictures. Let look at these
figure.
|
Figure 6 and Figure 7
|
 |
 |
11. Figure 6 shows you from where you can add the Property and Method to your
interface ,now as in figure 7 you can add Three property Name ,ATL marks and Com
Marks and One Method calculate and For raising I will tell you at end. After
adding that your interface look like this
interface ISimpleObj : IDispatch
{
[propget, id(1), helpstring("property Name")] HRESULT Name([out, retval] BSTR *pVal);
[propput, id(1), helpstring("property Name")] HRESULT Name([in] BSTR newVal);
[propget, id(2), helpstring("property ATLMarks")] HRESULT ATLMarks([out, retval] short *pVal);
[propput, id(2), helpstring("property ATLMarks")] HRESULT ATLMarks([in] short newVal);
[propget, id(3), helpstring("property COMMarks")] HRESULT COMMarks([out, retval] short *pVal);
[propput, id(3), helpstring("property COMMarks")] HRESULT COMMarks([in] short newVal);
[id(4), helpstring("method Calculate")] HRESULT Calculate();
};
12. Now you will see function for every property and method in you class CSimpleObj
.before going for actual coding, I think you want to know propget, propput
and method in above Interface.
- Propget ??? stand for Property for
getting the value from Component
- PropPut ???Stand
property for Putting property to Component. this can be optional and if you
remove it this can make yor property readonly.
- Method???simple
Function to perform some calculation.
- [in] ---
mean data is going in or your putting some value to Component.
- [out,retval]
--- notation state argument using this will return with data.
- HRESULT ---
Standard Error reporting variable
13. Now add a some useful variable in Class that will take care of above
properties add char Name[100] ,short AtlMarks and short COMMarks in your CSimpleObj
Class and I have coded rest of simple class for you. Let see and explain each
function, I have included one more property viz Total which will return the
Total till we do not implement EVENTS
Here is our SimpleObj Class code
STDMETHODIMP CSimpleObj::get_Name(BSTR *pVal)
{
//return Name of Student
CComBSTR bstStr(this->Name);
*pVal=bstStr.Detach();
return S_OK;
}
STDMETHODIMP CSimpleObj::put_Name(BSTR newVal)
{
//put Name of Student
::wcstombs(this->Name,newVal,99);
return S_OK;
}
STDMETHODIMP CSimpleObj::get_ATLMarks(short *pVal)
{
//return ATL marks
*pVal=this->ATLMarks;
return S_OK;
}
STDMETHODIMP CSimpleObj::put_ATLMarks(short newVal)
{
return Put of marks of atl
this->ATLMarks=newVal;
return S_OK;
}
STDMETHODIMP CSimpleObj::get_COMMarks(short *pVal)
{
// get the marks for COM
*pVal=this->COMMarks;
return S_OK;
}
STDMETHODIMP CSimpleObj::put_COMMarks(short newVal)
{
//put marks for COM
this->COMMarks=newVal;
return S_OK;
}
STDMETHODIMP CSimpleObj::get_Total(short *pVal)
{
//return total number of marks
*pVal=this->m_iTotalMarks;
return S_OK;
}
STDMETHODIMP CSimpleObj::Calculate()
{
//Calculate total number of marks and store it total number variable
this->m_iTotalMarks=this->ATLMarks+this->COMMarks;
return S_OK;
}
14. Now compile and build the SimpleAtlCom dll using BUILD|BUILD
SimpleATLCom.dll, and I think , you successfully get yourself a
SimpleATLCom.dll.
15. Now Develop a simple Visual Basic Project for it, I have created a sample
UI for above component ,let take a look on it.
|
Figure 8
|
 |
16. now let go for coding Side, first add reference of our com dll to project
,you can found option for it in PROJECT|REFRENCES , after clicking that
you will get a dialog box like this ,just find our SimpleAtlCom library in it
and check the box against it and press OK here is Dialog applicationlook like
|
Figure 9
|
 |
17. now let look at backend coding of VB application .
//Our Component Object
Private Obj As SIMPLEATLCOMLib.SimpleObj
Private Sub cmdPutValue_Click()
'give memory to Com object
Set Obj = New SIMPLEATLCOMLib.SimpleObj
'put atl marks in component
Obj.ATLMarks = txtPutATL
'put com marks
Obj.COMMarks = Me.txtPutCom
'put Name
Obj.Name = Me.txtPutName
End Sub
Private Sub cmdGetValue_Click()
'calculate the marks
Obj.Calculate
'put atl marks in component
Me.txtGetAtl = Obj.ATLMarks
'put com marks
Me.txtGetCom = Obj.COMMarks
'put Name
Me.txtGetName = Obj.Name
'get total marks and display it on the Component
Me.txtTotalMarks = Obj.Total
End Sub
18. Before going for Events , let see test running of our application.,
here it is -->
|
Figure 10
|
 |
19. I think, till now nobody have face problem I n getting above. Now take a
look at events and how to create them and raise them. You can see a Interface
names _ISimpleObjEvents this is disinterface created by APP Wizard ehich
provide support for Events. You can see a prefix '???_??? (underscore) ,this
underscore notify MIDL (Microsoft IDL compiler) that this interface is
Disinteface and don???t make it part of TLB files. Now right click on this
interface and add a method say
Void TOTAL([in]short marks);
Like shown in this fiqure
|
Figure 11
|
 |
20. Now right Click on your CSimpleObj class and Click on
Implement Connection Point Option. You will see this figure, check on _ISimpleObjEvents
Check Box and Click OK .voila the CProxy_ISimpleObjEvents class
is get added to your project. This is proxy class that will fire Event for you.
you can see it contain this function
VOID Fire_TotalMarks(SHORT TotalMarks)
Which is proxy function ,which raise events . ohh I forget ,take a look at
picture for Connection Point implementation
|
Figure 12
|
 |
21. Now event is added to your class, now let modify the
CSimpleObj::Calculate()
Now new CSimpleObj::Calculate() look like this
STDMETHODIMP CSimpleObj::Calculate()
{
this->m_iTotalMarks=this->ATLMarks + this->COMMarks;
//when you use this pointer ,Fire_Totalmarks support is added to your class
//after you implement the Connection points
->This event will fire the Total though event to Client
this->Fire_TotalMarks(this->m_iTotalMarks);
return S_OK;
}
22. Now let modify our Visual Basic application to handle events
Private Obj As SIMPLEATLCOMLib.SimpleObj
Change it to
Private withevents Obj As SIMPLEATLCOMLib.SimpleObj
This help us to implement Events, look this figure for more Clarity
|
Figure 14
|
 |
23. Now Look at code for Implementation of events, as Total marks come
from Event ,just add a line to show total marks. so our Event code look like
Private Sub Obj_TotalMarks(ByVal TotalMarks As Integer)
MsgBox "total marks " & TotalMarks
End Sub
24. Now test and run the application here it is the message box showing
event.
|
Figure 15
|
 |
Download Codes Includes
Source Code Include
#1 SimpleAtlCom.DLL (with source code)
#2 Test Project in Visual Basic.
Download the Source
code by clicking the link.
Test Project Include
#1 Compiled Test Application
#2 Compiled SimpleAtlCom.DLL
Download the Project
files by clicking the link.
Using of Demo Application
If you directly use the Com dll and Test application ,don???t forget to
register ComDLL ie SimpleAtlCom.dll to your computer.
You can use this command line to register the Component
Drive:> %sys%regsvr32 SimpleAtlCom.dll to register
application.
Author Comment
I tried my level best to tell each and every simple aspect of com Dll,if it
is missing something feel free to contact me
Special Thanks :
To My Mother and Father
