COM

Interop Services?

The common language runtime provides two mechanisms for interoperating with unmanaged code:
• Platform invoke, which enables managed code to call functions exported from an unmanaged library.
• COM interop, which enables managed code to interact with COM objects through interfaces.
Both platform invoke and COM interop use interop marshaling to accurately move method arguments between caller and callee and back, if required.

How does u handle this COM components developed in other programming languages in .NET?

What is RCW (Runtime Callable Wrappers)?

The common language runtime exposes COM objects through a proxy called the runtime callable wrapper (RCW). Although the RCW appears to be an ordinary object to .NET clients, its primary function is to marshal calls between a .NET client and a COM object.

What is CCW (COM Callable Wrapper)

A proxy object generated by the common language runtime so that existing COM applications can use managed classes, including .NET Framework classes, transparently.

How CCW and RCW is working?

Calling COM components from .NET Client

Generally COM components will expose interfaces to communicate with other objects. A .NET client cannot directly communicate with a COM component because the interfaces exposed by a COM component may not be read by the .NET application. So, to communicate with a COM component, the COM component should be wrapped in such a way that the.NET client application can understand the COM component. This wrapper is known as Runtime Callable Wrapper (RCW).

The .NET SDK provides Runtime Callable Wrapper (RCW) which wraps the COM components and exposes it into to the .NET client application.




To communicate with a COM component, there should be Runtime Callable Wrapper (RCW). RCW can be generated by using VS.NET or by the use of TlbImp.exe utility. Both the ways will read the type library and uses System.Runtime.InteropServices.TypeLibConverter class to generate the RCW. This class reads the type library and converts those descriptions into a wrapper (RCW). After generating the RCW, the .NET client should import its namespace. Now the client application can call the RCW object as native calls.

When a client calls a function, the call is transferred to the RCW. The RCW internally calls the native COM function coCreateInstance there by creating the COM object that it wraps. The RCW converts each call to the COM calling convention. Once the object has been created successfully, the .NET client application can access the COM objects as like native object calls.

Calling .NET components from COM Client

When a COM client requests a server, first it searches in the registry entry and then the communication starts. Calling a .NET component from a COM component is not a trivial exercise. The .NET objects communicate through Objects. But the Object based communication may not be recognized by the COM clients. So, to communicate with the .NET component from the COM component, the .NET component should be wrapped in such a way that the COM client can identify this .NET component. This wrapper is known as COM Callable Wrapper (CCW). The COM Callable Wrapper (CCW) will be used to wrap the .NET components and used to interact with the COM clients.

Using the Type Library Exporter (Tlbexp.exe) the classes and interfaces contained in an assembly are converted into to a COM type library. Once the TypeLib is created COM clients can create an instance of the .NET class and call the methods of that object, just as if it were a COM object.

CCW will be created by the .NET utility RegAsm.exe. This reads metadata of the .NET component and generates the CCW. This tool will make a registry entry for the .NET components.




Generally COM client instantiates objects through its native method coCreateInstance. While interacting with .NET objects, the COM client creates .NET objects by coCreateInstance through CCW.

Internally, when coCreateInstance is called, the call will redirect to the registry entry and the registry will redirect the call to the registered server, mscoree.dll. This mscoree.dll will inspect the requested CLSID and reads the registry to find the .NET class and the assembly that contains the class and rolls a CCW on that .NET class.

When a client makes a call to the .NET object, first the call will go to CCW. The CCW

How will you register com+ services?

The .NET Framework SDK provides the .NET Framework Services Installation Tool (Regsvcs.exe - a command-line tool) to manually register an assembly containing serviced components. You can also access these registration features programmatically with the System.EnterpriseServicesRegistrationHelper class by creating an instance of class RegistrationHelper and using the method InstallAssembly

What is use of ContextUtil class?

Obtains information about the COM+ object context. This class cannot be inherited.

What is the new three features of COM+ services, which are not there in COM (MTS)?

Is the COM architecture same as .Net architecture? What is the difference between them?

Can we copy a COM dll to GAC folder?

What is Pinvoke?

Platform invoke is a service that enables managed code to call unmanaged functions implemented in dynamic-link libraries (DLLs), such as those in the Win32 API. It locates and invokes an exported function and marshals its arguments (integers, strings, arrays, structures, and so on) across the interoperation boundary as needed.

Is it true that COM objects no longer need to be registered on the server?

Answer: Yes and No. Legacy COM objects still need to be registered on the server before they can be used. COM developed using the new .NET Framework will not need to be registered. Developers will be able to auto-register these objects just by placing them in the 'bin' folder of the application.

Can .NET Framework components use the features of Component Services?

Answer: Yes, you can use the features and functions of Component Services from a .NET Framework component. http://msdn.microsoft.com/library/techart/Pahlcompserv.htm