A Runtime Callable Wrapper (RCW) is a proxy object 
generated by the .NET Common Language Runtime (CLR) in 
order to allow a Component Object Model (COM) object to be 
accessed from managed code. 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.

For example, a managed application written in C# might make 
use of an existing COM library written in C++ or Visual 
Basic 6, via RCWs.

The runtime creates exactly one RCW for each COM object, 
regardless of the number of references that exist on that 
object. The runtime maintains a single RCW per process for 
each object. If you create an RCW in one application domain 
or apartment, and then pass a reference to another 
application domain or apartment, a proxy to the first 
object will be used.

Runtime Callable wrapper is created by CLR. It enables com 
dot net components to access com components.

It Creates Bridge between dot net component and com 
component.

It filters Data Type between both component.