How can we use .NET components in COM? Dot Net Interview Questions

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What is CCW (COM callable wrapper) ?, What caution needs to be taken in order that .NET components is compatible with COM ?

.NET components can not be used in straight forward way with COM.You will need to create CCW in order that COM components communicate with .NET assemblies.Following are the different approaches to implement it :-

1.  Explicitly declare interfaces..

Public Interface ICustomer
Property CustomerName() As String
Property CustomerCode() As String
Sub AddCustomer()
End Interface
Public Class Customer
Implements ICustomer
Private PstrCustomerName As String
Private PstrCustomerCode As String
Public Sub AddCustomer() Implements ICustomer.AddCustomer
Try
‘ addin of database code can go here
Catch ex As Exception
Throw ex
End Try

End Sub

Public Property CustomerCode() As String Implements
ICustomer.CustomerCode
Get
Return PstrCustomerCode
End Get
Set(ByVal value As String)
PstrCustomerCode = value
End Set
End Property
Public Property CustomerName() As String Implements
ICustomer.CustomerName
Get
Return PstrCustomerName
End Get
Set(ByVal value As String)
PstrCustomerName = value
End Set
End Property
Public Sub New()
End Sub
End Class

Note :- Source code of this is provided in CD in CODE folder in COMCALLABLEWRAPPER The above customer class is going to be used by COM components so all the properties and methods are declared in interface and implemented in the customer class.Customer Name.Customer Code and AddCustomer are first declared in ICustomer and then implemented in Customer Class.Note also the class must have a default constructor.

2.The second way to create CCW using InteropServices attributes.Here interfaces are created automatically.Following are different type of class attributes :
Note :No class interface is generated for the class.This is default setting when you do not specify
anything.
AutoDispatch :- Interface that supports IDispatch is created for the class. However, no type information is produced.
AutoDual :- A dual interface is created for the class. Typeinfo is produced and made available in the type library.
In below source code we have used the third attribute.

Imports System.Runtime.InteropServices
<ClassInterfaceAttribute(ClassInterfaceType.AutoDual)> _
Public Class ClsCompliant
End Class

Other than class attributes defined up there are other attributes with which you can govern other part of assembly.Example “GuidAttribute” allows you to specify the GUID,”ComVisibleAttribute “ can be used to hide .NET types from COM etc.

Once .NET assembly is created using either interface or using interopservices method we need to create a COM type library using Type library export tool.

Tlbexp (AssemblyName)

The final thing is registering the CCW in registry using regasm tool. regasm AssemblyName [Options]

Finally refer the TLB in your COM IDE Below is figure showing VB6 IDE referencing
the DLL.
Note :- DLL and TLB should be in same directory where the application is executed.

VB6 IDE referencing the CCW