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Can we use com Components in .net?.How ?.can we use .net components in vb?.Explain how ?

22 Sep, 2005

Summary

.NET Classes used :

COM components have different internal architecture from .NET components hence they are not innately compatible. However .NET framework supports invocation of unmanaged code from managed code (and vice-versa) through COM/.NET interoperability. .NET application communicates with a COM component through a managed wrapper of the component called Runtime Callable Wrapper (RCW); it acts as managed proxy to the unmanaged COM component. When a method call is made to COM object, it goes onto RCW and not the object itself. RCW manages the lifetime management of the COM component. Implementation Steps -
Create Runtime Callable Wrapper out of COM component. Reference the metadata assembly Dll in the project and use its methods & properties RCW can be created using Type Library Importer utility or through VS.NET. Using VS.NET, add reference through COM tab to select the desired DLL. VS.NET automatically generates metadata assembly putting the classes provided by that component into a namespace with the same name as COM dll (XYZRCW.dll)
.NET components can be invoked by unmanaged code through COM Callable Wrapper (CCW) in COM/.NET interop. The unmanaged code will talk to this proxy, which translates call to managed environment. We can use COM components in .NET through COM/.NET interoperability. When managed code calls an unmanaged component, behind the scene, .NET creates proxy called COM Callable wrapper (CCW), which accepts commands from a COM client, and forwards it to .NET component. There are two prerequisites to creating .NET component, to be used in unmanaged code:
1. .NET class should be implement its functionality through interface. First define interface in code, then have the class to imlpement it. This way, it prevents breaking of COM client, if/when .NET component changes.
2.Secondly, .NET class, which is to be visible to COM clients must be declared public. The tools that create the CCW only define types based
on public classes. The same rule applies to methods, properties, and events that will be used by COM clients.
Implementation Steps -
1. Generate type library of .NET component, using TLBExporter utility. A type library is the COM equivalent of the metadata contained within
a .NET assembly. Type libraries are generally contained in files with the extension .tlb. A type library contains the necessary information to allow a COM client to determine which classes are located in a particular server, as well as the methods, properties, and events supported by those classes.
2. Secondly, use Assembly Registration tool (regasm) to create the type library and register it.
3. Lastly install .NET assembly in GAC, so it is available as shared assembly.

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