Instanciating an interface + Excel.Application

Question

Hi everyone I am going to manipulate an Excel workbook via C# programming. I looked to some information to do so. A C# statement disturbed me while reading. Here is the statement : Excel.Application oXl = new Excel.Application; Excel is an alias for a namespace. The statement disturbed me because Application is an interface and it does not contain any code by definition. The fact that the object library allows the programmer to control Excel may justify the instanciation. Could someone explain me clearly why the instanciation is possible ? Thanks in advance.

John Penn · Answer

Although the Object Browser displays Application interfaces for the Microsoft Word and Microsoft Excel PIAs it does not directly expose Application classes for Word and Excel. However when you use the Application interface to instantiate an Application object the common language runtime internally uses the Application class. http://msdn.microsoft.com/en-us/library/ms247302%28v=office.11%29.aspx

Vulpes · Answer

No problem Sam. I think it's a good question because most developers faced with Office interop just copy boilerplate code without worrying too much how it works. The problem now is remembering what the answer was if it comes up again :)

Sam Hobbs · Answer

I apologize. I did not understand the question. Probably my biggest mistake was assuming that the question is a beginner question; it is not. I deleted my message that was my second reply.

Vulpes · Answer

I meant that the line which the OP gave won't work without the namespace alias which he said he'd put in. It would of course work if he used fully qualified names throughout.

Vulpes · Answer

The OP said that Excel was an alias for a namespace so that line is certain to exist in his code and won't work without it. I also used it in my sample program for decompiling with .NET Reflector.

Sam Hobbs · Answer

Are you sure there is not a line such as: using Excel = Microsoft.Office.Interop.Excel; Somewhere in the soruce code? If there is such a line try commenting it out to see what happens.

David Rougier · Answer

Thank you for providing me information. The mechanism is clearer to me now.

John Penn · Answer

So that's how it does it. Thanks Vulpes!

Vulpes · Answer

That's a useful link which John's dug out. The only place I've seen this previously was in an obscure book I have on Office interop. The Application class is well and truly hidden and the Object Browser is unable to show it probably because it can't cope with both an interface and a class which are within the same namespace having the same name! However if you look at this line: Excel.Application oXl = new Excel.Application(); with Redgate Reflector then it decompiles back to this line: Application application1 = (Application) Activator.CreateInstance(Type.GetTypeFromCLSID(new Guid(00024500-0000-0000-C000-000000000046))); proving that the Application object is created behind the scenes using its CLSID and then cast back to the Application interface. So evidently the C# compiler is somehow able to sort out what's needed.