Cannot create a service reference with namespace XXX becuase the name is already in use by an existing service reference, folder or file

If you get the above error when trying to add a WCF service reference that you have previously deleted then try:

• deleting the Service References.DeviceService.Reference.cs.dll file you'll find under obj\Debug\TempPE. 
• deleting the service folder you'll still find under Service References on disk, even though you deleted the reference in Visual Studio.
• commenting out all references to the service's namespace in your code
• rebuilding

Could not find default endpoint element that references contract...

I created a WCF service today after a long time and I hosted it using a Windows service. While doing so I created the wcf proxy client and its config settings were generated. Then I need to use this client in my web application to call the service methods to do some work. When I did that I got the error "Could not find default endpoint element that references contract....". 

Solution: To get this error resolved one has to add the section with all the contents from proxy's app.config file to the web.config file of the web application.

Abstract Class v/s Interface

While designing software systems we often come across the situation where we need to decide on having interfaces or abstract class. To help on this following are few points which can be think upon to arrive at an informed decision:

1. Interfaces are the contracts which contain the signature/declaration of the functions which are forced to be implemented by the deriving classes. Abstract class needs atleast one function that remains abstract and all other functions can be implemented in the abstract class itself.

2. Interfaces are strongly coupled design and abstract class provides more flexibility in terms of implementation. Abstract classes may also provide members that have already been implemented. Therefore, you can ensure a certain amount of identical functionality with an abstract class, but cannot with an interface.

3. You should use abstract class when there is certain implementation uncertain to its subclass and other part of the behavior is same for all of the subclass.

4. Abstract classes should be used primarily for objects that are closely related, whereas interfaces are best suited for providing common functionality to unrelated classes.

5. Abstract classes are helpful while designing large functional units and interfaces shall be used while designing small, concise bits of functionality.

6. In case the requirement is to provide common, implemented functionality across all implementations of your component, use abstract classes. Abstract classes allow you to partially implement your class, whereas interfaces contain no implementation for any members.