Interface vs Abstract Class (general OO)


I have had recently two telephone interviews where I’ve been asked about the differences between an Interface and an Abstract class. I have explained every aspect of them I could think of, but it seems they are waiting for me to mention something specific, and I don’t know what it is.

From my experience I think the following is true. If I am missing a major point please let me know.


Every single Method declared in an Interface will have to be implemented in the subclass. Only Events, Delegates, Properties (C#) and Methods can exist in a Interface. A class can implement multiple Interfaces.

Abstract Class:

Only Abstract methods have to be implemented by the subclass. An Abstract class can have normal methods with implementations. Abstract class can also have class variables beside Events, Delegates, Properties and Methods. A class can only implement one abstract class only due non-existence of Multi-inheritance in C#.

  1. After all that, the interviewer came up with the question “What if you had an Abstract class with only abstract methods? How would that be different from an interface?” I didn’t know the answer but I think it’s the inheritance as mentioned above right?
  2. An another interviewer asked me what if you had a Public variable inside the interface, how would that be different than in Abstract Class? I insisted you can’t have a public variable inside an interface. I didn’t know what he wanted to hear but he wasn’t satisfied either.

See Also:


While your question indicates it’s for “general OO”, it really seems to be focusing on .NET use of these terms.

In .NET (similar for Java):

  • interfaces can have no state or implementation
  • a class that implements an interface must provide an implementation of all the methods of that interface
  • abstract classes may contain state (data members) and/or implementation (methods)
  • abstract classes can be inherited without implementing the abstract methods (though such a derived class is abstract itself)
  • interfaces may be multiple-inherited, abstract classes may not (this is probably the key concrete reason for interfaces to exist separately from abtract classes – they permit an implementation of multiple inheritance that removes many of the problems of general MI).

As general OO terms, the differences are not necessarily well-defined. For example, there are C++ programmers who may hold similar rigid definitions (interfaces are a strict subset of abstract classes that cannot contain implementation), while some may say that an abstract class with some default implementations is still an interface or that a non-abstract class can still define an interface.

Indeed, there is a C++ idiom called the Non-Virtual Interface (NVI) where the public methods are non-virtual methods that ‘thunk’ to private virtual methods:


Comparison : interface methods vs virtual methods vs abstract methods


What are the advantages and disadvantages of each of these?

  • interface methods
  • virtual methods
  • abstract methods

When one should choose what? What are the points one should keep in mind when making this decision?


Virtual and abstract are almost the same. A virtual method has an implementation in the base class that can optionally be overridden, while an abstract method hasn’t and must be overridden in a child class. Otherwise they are the same. Choosing between them depends on the situation. If you got a base implementation, you use virtual. If you don’t, and you need every descendant to implement it for itself, you choose abstract.

Interface methods are implementations of a method that is declared in an interface that the class implements. This is quite unrelated to the other two. I think a method can be both virtual and interface. The advantage of interfaces is that you declare one interface (duh) that can be implemented by two totally different classes. That way, you can run the same code on two different classes, as long as the methods you’d like to call are declared in an interface they share.


Unable to lock the administration directory (/var/lib/dpkg/) is another process using it?


Remove your /var/lib/dpkg/lock file and force package reconfiguration.

sudo rm /var/lib/dpkg/lock
sudo dpkg --configure -a

It should work after this.

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When to use Common table expression (CTE)


One example, if you need to reference/join the same data set multiple times you can do so by defining a CTE. Therefore, it can be a form of code re-use.

An example of self referencing is recursion: Recursive Queries Using CTE

For exciting Microsoft definitions Taken from Books Online:

A CTE can be used to:

  • Create a recursive query. For more information, see Recursive Queries Using Common Table Expressions.
  • Substitute for a view when the general use of a view is not required; that is, you do not have to store the definition in metadata.
  • Enable grouping by a column that is derived from a scalar subselect, or a function that is either not deterministic or has external access.
  • Reference the resulting table multiple times in the same statement.

The SQL OVER() clause – when and why is it useful?


The OVER clause is powerful in that you can have aggregates over different ranges (“windowing”), whether you use a GROUP BY or not

Example: get count per SalesOrderID and count of all

    SalesOrderID, ProductID, OrderQty
    ,COUNT(OrderQty) AS 'Count'
    ,COUNT(*) OVER () AS 'CountAll'
FROM Sales.SalesOrderDetail 
     SalesOrderID IN(43659,43664)
     SalesOrderID, ProductID, OrderQty
Get different COUNTs, no GROUP BY

    SalesOrderID, ProductID, OrderQty
    ,COUNT(OrderQty) OVER(PARTITION BY SalesOrderID) AS 'CountQtyPerOrder'
    ,COUNT(OrderQty) OVER(PARTITION BY ProductID) AS 'CountQtyPerProduct',
    ,COUNT(*) OVER () AS 'CountAllAgain'
FROM Sales.SalesOrderDetail 
     SalesOrderID IN(43659,43664)

How to clear NuGet package cache using command line?


First, download the nuget command line tool from here.

Next, open a command prompt and cd to the directory to which nuget.exe was downloaded.

You can list the local caches with this command:

nuget locals all -list

You can clear all caches with this command:

nuget locals all -clear