How to Write Simple COM/ATL DLL and Using it with .NET

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Introduction

During discussions in forums, I come across many people who want to write COM/ATL DLL, but don't know how to create properties, methods or raise events from component or strive for fundamental knowledge to create them. For them, I had written this article in VC++ 6.0. Since Visual Studio 6 is now out of support, many beginners requested me for the same article using Visual Studio 2005/2008 as there is much GUI difference between the two.

So, I am presenting the same article using Visual Studio 2005 and as it’s a new article, I have to provide readers with something new, otherwise they will reject this article. .NET application is slowly eating into Visual C++ share of applications. So to follow popular demand, I will show how to use COM component in .NET application instead of Unmanaged Application. As it is a beginner article, I am included a few extra screenshots in the article. Please don't flame for using the same.

Table of Contents

• Creating ATL Component
• Using it in Visual C# Application (.NET 2005)

Creating ATL Component

Let us go step by step to create an ATL component in Visual Studio 2005:

1. Open Visual Studio 2005, and Click on File| New | Project menu, which will show you the following File-Save dialog box, where you can provide a name for your new project. Here I put the name as SimpleATLCom and it is advisable to choose the destination folder, where you want to create your project, otherwise it will clutter your Document Folder. Press OK to confirm your changes.  This will create a blank ATL component.
   
   Figure 1: File Open Dialog

   Now, in the next dialog box (ATL Project wizard), choose default setting and press Finish. If you need MFC Support, check Support MFC checkbox.   

   
   Figure 2: ATL Project Wizard

   Now insert ATL Object in Project which I will show in the next step.

2. Choose Project | Add Class. The following dialog box will appear:
   
   Figure 3: Add Simple ATL Object

   Choose ATL|ATL Simple Object and Press ADD button.

3. Enter new Interface name in ATL Simple Object Wizard. You will see rest of the details are auto filled by Visual Studio development environment when you begin typing the interface name. Also check the Connection Point check box in the Option Tab which provides component ability to fire events.
   
   Figure 4: ATL Simple Object
   
   Figure 5: Select Connection Point Check Box

4. Now if you check the following IDL (Interface Definition Language) will be generated and included into your project:

   

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       1.    // SimpleATLcom.idl : IDL source for SimpleATLcom     2.    //     3.    // This file will be processed by the MIDL tool to     4.    // produce the type library (SimpleATLcom.tlb) and marshalling code.     5.                 6.    import "oaidl.idl";     7.    import "ocidl.idl";     8.             9.    [     10.     object,     11.     uuid(8F4FEC76-F1F6-4D69-A2E8-FFF4F316C101),     12.     dual,     13.     nonextensible,     14.     helpstring("ISimpleCom Interface"),     15.     pointer_default(unique)     16.   ]     17.   interface ISimpleCom : IDispatch{     18.    };     19.    [     20.    uuid(2CA18A17-D157-4D1E-A2C7-3D69EBD87250),     21.    version(1.0),     22.    helpstring("SimpleATLcom 1.0 Type Library")     23.    ]     24.    Library SimpleATLcomLib     25.    {     26.      importlib("stdole2.tlb");     27.      [     28.        uuid(85D35B5B-8DBF-4562-96B0-66A4C4718DD5),     29.         helpstring("_ISimpleComEvents Interface")     30.      ]     31       dispinterface _ISimpleComEvents     32        {     33.         properties:     34.         methods:     35.      };     36.      [     37.        uuid(A7657FC5-D63C-49C3-AEE5-D8799F412C74),     38.        helpstring("SimpleCom Class")     39.      ]     40.       coclass SimpleCom     41.       {     42          [default] interface ISimpleCom;     43.         [default, source] dispinterface _ISimpleComEvents;     44.       };     45.    };

• At Line 11: This is default UUID (Unique ID) generated by Dev environment, this makes the component uniquely identifiable.
• At line 17: That’s our component name and UUID, its contents can be used to uniquely identify a component, you can use this id to create component.
• At Line 31: Our dis-interface i.e. event function.
5. Now Add Property and method using Add Property and Add Method Wizard. This can be found in class view, right click on ISimpleCom interface then ADD->Add Method or Add Property:
   
   Figure 6: Add Method/Add Property

   Add method Calculate with Long as OUT, RETVAL parameter like this:

   
   Figure 7: Add Method

   Add Property ComMark of type Long as shown in the example:

   
   Figure 8: Add Property

   Similarly add property AtlMarks as long and StudentName as BSTR yourself. Sorry I forgot to mention about IN, OUT and RETVAL type in parameter.

• Propget – stands for property for getting the value from Component
• PropPut – stands for property for putting property to Component. This can be optional and if you remove it, this can make your property readonly
• Method — simple function to perform some calculation
• [in] - means data is going in or you are putting some value to Component
• [out,retval] - Notation states that argument using this will return with data
• HRESULT - Standard Error reporting variable
6. Now add Event almost in the same way, we add method in interface. Now right click on _ISimpleComEvents and Add | Add method.
   
   Figure 9: Add Event

7. Once everything is done, modified IDL File would look like this:

   

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       1.  interface ISimpleCom : IDispatch     2.  {     3.   [id(1), helpstring("method Calculate")] HRESULT Calculate([out,retval]    4.           LONG* a_lTotalMarks);     5.   [propget, id(2), helpstring("property ComMarks")] HRESULT ComMarks([out,    6.           retval] LONG* pVal);     7.   [propput, id(2), helpstring("property ComMarks")] HRESULT ComMarks([in]    8.            LONG newVal);     9.   [propget, id(3), helpstring("property AtlMarks")] HRESULT AtlMarks([out,    10.            retval] LONG* pVal);     11.  [propput, id(3), helpstring("property AtlMarks")] HRESULT AtlMarks([in]    12.            LONG newVal);     13.  [propget, id(4), helpstring("property StudentName")] 					HRESULT StudentName([out,    14.            retval] BSTR* pVal);     15.  [propput, id(4), helpstring("property StudentName")] 					HRESULT StudentName([in]    16.            BSTR newVal);     17.  };     18.            19.  dispinterface _ISimpleComEvents     20.  {     21.    properties:     22.    methods:     23.    [id(1), helpstring("method TotalMarks")] HRESULT TotalMarks([in] LONG    24.            a_lTotalMark);     25.  }; 

8. Now add connection points into the main interface. For adding it, right click on CSimpleCom and click ADD | Add Connection Point. The following screenshot will appear:
   
   Figure 10: Chose Add Connection Point

   Choose > button to move _ISimpleComEvents source interface to implement connection point and click on finish. A new function Fire_TotalMarks is added to CSimpleCom class which can be used to raise event. 

   
   Figure 11: Add Connection Wizard

9. Now code all properties and functions present in CSimpleCom class, i.e. Add Class Variable for ComMarks, ATLMarks and StudentName for storing information coming from the outer world. After coding, our class would look like this:

   

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           		1.   //// Declare following private variable in Header File		2.	long m_lComMarks,m_lAtlMarks;		3.		CComBSTR m_bstName;		4.		5.	////  Source File		6. 	STDMETHODIMP CSimpleCom::Calculate(LONG* a_lTotalMarks) 		7. 	{ 		8. 	  long lTotalMarks = m_lAtlMarks + m_lComMarks; 		9. 	  * a_lTotalMarks = lTotalMarks; 		10. 	  Fire_TotalMarks(lTotalMarks); //Fire Event 		11. 	  return S_OK; 		12.	} 		13. /// Return Com marks		14. STDMETHODIMP CSimpleCom::get_ComMarks(LONG* pVal) 		15.	{ 		16. 	* pVal = m_lComMarks; 		17. 	return S_OK; 		18. 	} 		19. // Store Com Marks in Local Storage		20. STDMETHODIMP CSimpleCom::put_ComMarks(LONG newVal) 		21.	{ 		22. 	m_lComMarks = newVal; 		23. 	return S_OK; 		24.	} 		25. // return ATL Marks		26.	STDMETHODIMP CSimpleCom::get_AtlMarks(LONG* pVal) 		27.	{ 		28.  * pVal = m_lAtlMarks; 		29.	 return S_OK; 		30.	 } 		31. //Store ATL marks in local storage		32. STDMETHODIMP CSimpleCom::put_AtlMarks(LONG newVal) 		33. { 		34.  m_lAtlMarks = newVal; 		35.  return S_OK; 		36. }		37.  //return Student Name		38. STDMETHODIMP CSimpleCom::get_StudentName(BSTR* pVal) 		39. { 		40.  *pVal = m_bstName.Copy(); 		41.  return S_OK; 		42. } 		43. //Store Student name in local Storage.		44. STDMETHODIMP CSimpleCom::put_StudentName(BSTR newVal) 		45. { 		46.  m_bstName = newVal; 		47.  return S_OK; 		48. }    

   Now Build the solution, your DLL would be registered automatically, when the build process is over.

Using it in Visual C# Application

Now again move forward step by step to create a project in C#.NET and include support of COM component in project.

1. Add a new C# Window application Project (SimpleATLcomTest) in the same solution and click to accept default configuration to create a test project.
   
   Figure 12: Add a New C# Project

2. Design the interface as shown in the screenshot given below; add three Textboxes for putting values into the Component and four boxes for retrieving all values including the calculated value.

   
   Figure 13: Test Application Design

3. Now, add a reference for SimpleATLCom.dll into your project, by right clicking on Project name and clicking on Add Reference menu item.

   
   Figure 14: Add Reference Menu

   Click on COM Tab item and select SimpleATLCom.dll to add reference of Component into the project.

   
   Figure 15: Add Reference

4. Now, write the following code to use the component into your project.

   

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           1.  // Include SimpleAtlComLib          2.  using SimpleATLcomLib;         3.  namespace SimpleAtlComTest          4.  {         5.  public partial class SimpleATLCom : Form         6.  {         7.  // Create Object of Main Component Interface         8.  ISimpleCom objSimpleCom = new SimpleComClass();         9.  // Also declare Event variable         10. _ISimpleComEvents_Event event1 = null;         11.	public SimpleATLCom()         12.	{         13.	InitializeComponent();         14.	// Initialize event1 variable with object of simplecom         15.	event1 = (_ISimpleComEvents_Event)objSimpleCom;         16.        17.	// Add Event to listen event raised by Component         18.	event1.TotalMarks += new _ISimpleComEvents_TotalMarksEventHandler			(ISimpleCom_TotalMarksEvent);        19.	        20.	}         21.	// this function will put the values back to Component         22.	private void btnPutValues_Click(object sender, EventArgs e)         23.	{         24.	objSimpleCom.AtlMarks = int.Parse(txtPATLMarks.Text);         25.	objSimpleCom.ComMarks = int.Parse(txtPComMarks.Text);         26.	objSimpleCom.StudentName = txtPName.Text;         27.	}         28.	// this function will bring values stored into Component         29.	// and populate into TextBox present on Form         30.	private void btnGetDetails_Click(object sender, EventArgs e)         31.	{         32.	txtGAtlMarks.Text = objSimpleCom.AtlMarks.ToString();         33.	txtGComMarks.Text = objSimpleCom.ComMarks.ToString();         34.	txtGName.Text = objSimpleCom.StudentName;         35.	objSimpleCom.Calculate();         36.	}         37.	// this Event will called, when invoke method Calculate 		// of simpleATLCom.         38.	void ISimpleCom_TotalMarksEvent(int lTotalMarks)         39.	{         40.	txtTotalMarks.Text = lTotalMarks.ToString();         41.	MessageBox.Show("Total Marks is " + txtTotalMarks.Text);         42.	}         43.	}         44.	}     

5. Run the Test application, and you will see the result:
   
   Figure 16: Running Application

About the Download Code

Source Code Includes

• SimpleAtlCom.dll (with source code)
• Test Project in Visual C#

Using of Demo Application

If you like, use the Com DLL and Test application first. Don't forget to register Com DLL, i.e., SimpleAtlCom.dll, to your computer. You can use this command line to register the Component.

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Drive:> %sys%regsvr32 path_to_dll\SimpleAtlCom.dll

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 or leave your precious comments in the discussion forum below.