COM component object model

a component which can attached with any component and create a new component (aggregation) each component must have a 128 bit id that must be register on ther registry of windows

Dcom distributed component model

a component which can be used on network

COM is a Specification defined by microsoft in order enhance the productibility of one application in to other.COM is a FrameWork which when implemented both the application can talk each other.COM is set of rules and it is language independent. COM talk about Language interability. and reusablility.

3/ DCOM had to solve the problems of

- serializing and deserializing the arguments and return values of method calls over the wire .

- ensuring that references held by clients of interfaces are released when for example the client process crashed or the network connection was lost.

1. Language Independent (by implementing Dual interfaces IDispatch for Nonpointer based languages and VTable for Pointer based languages)
2. Location transparency ( By guid and clsids stored in windows registry we can have location transparency which cannot be done using dlls.
3. Immutable:Old clients are not affected if we change the old component by new one this cannot be done in case of dlls which is also known as dll hell.