Component-Based Software Engineering Component Technologies

1. Component-Based Software EngineeringComponent Technologies COM Overview of COM Component technology

2. Overview of COM • What is COM • History and Background • Fundamental Concepts • Software Components as Building Blocks • The Component Object Model Defined • Resources • Existent COM Technologies • Useful COM Tools • C++ vs. COM • COM Advantages Component technology

3. What is COM • Component software model Component def. “…binary, self-contained, reusable, language neutral piece of software” • COM is a Microsoft solution • Based on • The OO paradigm • Binary reuse • Interfaces • Language independent • Clients bind to objects at runtime • Provides • Integrated services, easy-to-use tools, available applications • Market for reusable, off-the-shelf, client and server components Component technology

4. History & Background • Microsoft’s Component Object Model (COM) • The first component model. • 1988 - OO infrastructures that provides reuse based on components • OLE2 project • OLE - Object Linking & Embedding 4 high-level requirement for a new component architecture • Component-based • Based on the OO paradigm • Language independent • Interprocess communication Component technology

5. History & Background, cont. • OLE = COM • but COM was an essential piece of the OLE2 project • COM specification • Specific memory layout • One or more COM interfaces • Communication only through interfaces • 1995 DCOM • Allows creating and accessing COM objects on remote location • 1999 • Over 150 million systems worldwide Component technology

6. Fundamental Concepts • COM Interface • a collection of abstract operations one can perform on an object • COM Object • a collection of vptrs in memory that follow the COM identity laws • has methods and state • COM Class (or coclass) • is a named body of code that can be used to produce COM objects • Class object or Class factory • initializes the creation of an object. • SCM (Service Control Manager)or class loader • on demand, load all coclasses • COM library • infrastructure used to support COM on a given platform Component technology

7. A Simple COM Object • A COM object has methods and state • Language independent • All Access is through the interface • Supports multiple interfaces • Each interface has a GUID Component technology

8. IUnkown • All COM objects must implement IUnknown • AddRef • Release • QueryInterface IUnknown Component technology

9. 4. Invoke methods 3. Return pointer to interface 2. Create Objectand get interface pointer 1. IClassFactory::CreateInstance Creating a local object using a class factory Client Application Object ClassFactory Server Component technology

10. The Component Model Defined • COM interfaces and COM classes • Uniquely identified by GUIDs • A class may implement several interfaces • Several classes may implement the same interface • COM servers • Are either in-process (.dll) or out-of-process (.exe) • A server may implement several classes • The COM library • Used by clients to create COM objects Component technology

11. Resources • Essential COM, Don Box • Programming Distributed Applications with COM and Ms VB 6.0, Ted Pattison • Fundamentals of COM(+), (Part 1 + 2), ppt presentation by, Don Box • Links • http://www.develop.com/dbox • http://www.microsoft.com/com/ • http://msdn.microsoft.com/library/default.asp?URL=/library/en-us/dnasp/html/comtutorial.asp • http://www.eki.mdh.se/kurshemsidor/informatik/ei0230/com/ht00/index.htm • http://www.codeproject.com/com/ • http://www.microsoft.com/com/news/drgui.asp Component technology

12. Existent COM Technologies • DCOM Distributed COM • COM + Extension of COM • MSMQ Message Queuing • MTS Microsoft Transaction Server • ActiveX Controls • Distribution of components over high-latency networks • Provide integration of controls into Web browsers Component technology

13. Useful COM Tools • GUIDGEN.EXE Generates GUID’s • DCOMCNFG.EXE Configuration tool • OLEVIEW.EXE Configuration tool • MIDL.EXE Compiler • REGSVR32.EXE Install & Uninstall in-process server • COM library • OLE32.DLL • core class/interface functionality • OLEAUT32.DLL • Visual Basic®-centric type infrastructure • Environments • FrontPage, Visual Studio, Adobe PageMaker, Borland Intrabuilder etc. Component technology

14. C++ vs COM • COM as a better C++ • Improved flexibility, extensibility, maintainability. • Binary reuse • C++ • Reuse of user-defined types • Include libraries • CBSE with C++ • Deploy the class as a DLL • Use the notations of interfaces and implementations • Add dynamically selection with e.g LoadLibrary() • Add dynamically interrogation Component technology

15. C++ vs COM, Cont. • Problems - CBSE with C++ • Clients statically link to import library • Client imports resolved at load time • C++ compilers non-standard for DLLs • DLL and clients must be built using same compiler/linker • Adding new public methods • OK when statically linked • Dangerous to a DLL-based class • New client expects method to be there • Old DLLs have never heard of this method • Change definition of a data type => massive rebuild/redeployment of client/object • Clients must not program in terms of classes • Does not provide a reasonable binary component architecture for encapsulation Component technology

16. C++ vs COM, Cont. • Interface-based programming • Splits the world into two to solve the replaceable component problem • Define interfaces • The types the client programs against can never change • Since classes need to change, these better not be classes • Encapsulation of implementation • Prevents clients changing the underlying class hierarchy • In C++ • For bridging interfaces and classes • use abstract base (classes as interfaces) Component technology

17. COM advantages Taking advantage of COM allows you to: • Package your objects in a uniform way as executables or dynamic-link libraries. • Break your application into really reusable components, ones that you can redistribute easily to others. • Be prepared to distribute your components on multiple machines, through a simple change in configuration. As COM is being ported to other platforms, you can even take advantage of real, standardized, efficient interoperability between different platforms. Markus Horstmann, Microsoft. From C++ to COM • COM provides the ability to work with CBSE (Component-Based Software Engineering) which is hard if not impossible in C++ Component technology