***Update: C# 8.0 New Feature:*
Beginning with C# 8.0, an interface may define a default implementation for members, including properties. Defining a default implementation for a property in an interface is rare because interfaces may not define instance data fields.**


The advantages of an abstract class are:

- Ability to specify default implementations of methods
- Added invariant checking to functions
- Have slightly more control in how the &quot;interface&quot; methods are called
- Ability to provide behavior related or unrelated to the interface for &quot;free&quot;

Interfaces are merely data passing contracts and do not have these features. However, they are typically more flexible as a type can only be derived from one class, but can implement any number of interfaces.



Abstract classes and interfaces are semantically different, although their usage can overlap.

An abstract class is generally used as a building basis for similar classes. Implementation that is common for the classes can be in the abstract class.

An interface is generally used to specify an ability for classes, where the classes doesn&#39;t have to be very similar.

Another thing to consider is that, since there is no multiple inheritance, if you want a class to be able to implement/inherit from your interface/abstract class, but inherit from another base class, use an interface.

The real question is: whether to use interfaces or base classes. This has been covered before. 

In C#, an abstract class (one marked with the keyword &quot;abstract&quot;) is simply a class from which you cannot instantiate objects. This serves a different purpose than simply making the distinction between base classes and interfaces.



In Visual Studio, there&#39;s the compile flags /MD and /MT which let you choose which kind of C runtime library you want.

I understand the difference in implementation, but I&#39;m still not sure which one to use. What are the pros/cons?

One advantage to /MD that I&#39;ve heard, is that this allows someone to update the runtime, (like maybe patch a security problem) and my app will benefit from this update. Although to me, this almost seems like a non-feature: I don&#39;t want people changing my runtime without allowing me to test against the new version!

Some things I am curious about:

 * How would this affect build times? (presumably /MT is a little slower?)
 * What are the other implications?
 * Which one do most people use?

Should I compile with /MD or /MT?

I need to create an empty map.

    if (fileParameters == null)
        fileParameters = (HashMap&lt;String, String&gt;) Collections.EMPTY_MAP;

The problem is that the above code produces this warning: 
**Type safety: Unchecked cast from Map to HashMap&lt;String,String&gt;**

What is the best way to create this empty map?

Best way to create an empty map in Java

In C#, when should you use interfaces and when should you use abstract classes?  What can be the deciding factor?

Interfaces vs. abstract classes

<p>In C#, when should you use interfaces and when should you use abstract classes?  What can be the deciding factor?</p>


I&#39;m working on a little something and I am trying to figure out whether I can load an XDocument from a string. `XDocument.Load()` seems to take the string passed to it as a path to a physical XML file.

I want to try and bypass the step of first having to create the physical XML file and jump straight to populating the XDocument.

Any ideas?

Populate XDocument from String

If I have some files I want to copy from my project into the `.\bin\debug\` folder on compilation, then it seems I have to put them into the root of the project. Putting them into a subfolder seems to copy them into the `.\bin\debug\` folder in the same structure they&#39;re stored in.

Is there any way to avoid this?

Just to be clear: if I have a `MyFirstConfigFile.txt` and `MySecondConfigFile.txt` in a `ConfigFiles` folder and I set their *Copy to Output* to be *Copy...*, then they appear in the `.\bin\debug\ConfigFiles\` folder.  I want them to appear in the `.\bin\debug\` folder.

Copying files into the application folder at compile time

I read the [C++ version of this question][1] but didn&#39;t really understand it.

Can someone please explain clearly if it can be done in C#, and how?

  [1]: https://stackoverflow.com/questions/321068/returning-multiple-values-from-a-c-function

Return multiple values to a method caller

Is there a lint-like tool for C#?  I&#39;ve got the compiler to flag warnings-as-errors, and I&#39;ve got Stylecop, but these only catch the most egregious errors.  Are there any other must-have tools (especially for newbie C#ers like me) that point out probably-dumb things I&#39;m doing?


Lint for C#

I am just learning about computer vision and C#. It seems like two prominent image processing libraries are [OpenCV][1] and [AForge][2]. What are some of the differences of the two?

I am making a basic image editor in C# and while researching I have come across articles on both. But I don&#39;t really know why I would choose one over the other. I would like to eventually improve the app to include more advanced functions.

Thanks.


  [1]: http://opencv.willowgarage.com/wiki/
  [2]: http://code.google.com/p/aforge/

Differences between AForge and OpenCV

I have a base class that contains the following events:

    public event EventHandler Loading;
    public event EventHandler Finished;

In a class that inherits from this base class I try to raise the event:

    this.Loading(this, new EventHandler()); // All we care about is which object is loading.

I receive the following error:

*The event &#39;BaseClass.Loading&#39; can only appear on the left hand side of += or -= (BaseClass&#39;)*

I am assuming I cannot access these events the same as other inherited members?

C#: Raising an inherited event

ReSharper suggests changing the accessibility of a `public` constructor in an `abstract` class to `protected`, but it does not state the rationale behind this.

Can you shed some light?

Why should constructors on abstract classes be protected, not public?



I have several classes that do not really need any state. From the organizational point of view, I would like to put them into hierarchy.

But it seems I can&#39;t declare inheritance for static classes.

Something like that:

    public static class Base
    {
    }
    
    public static class Inherited : Base
    {
    }

will not work.

Why have the designers of the language closed that possibility?


  [1]: http://stackoverflow.com/questions/556203/how-can-a-static-class-derive-from-an-object

Why can&#39;t I inherit static classes?

When creating a simple object hierarchy in Python, I&#39;d like to be able to invoke methods of the parent class from a derived class.  In Perl and Java, there is a keyword for this ([`super`][1]).  In Perl, I might do this:


    package Foo;
    
    sub frotz {
        return &quot;Bamf&quot;;
    }
    
    package Bar;
    @ISA = qw(Foo);
    
    sub frotz {
       my $str = SUPER::frotz();
       return uc($str);
    }

In Python, it appears that I have to name the parent class explicitly from the child.
In the example above, I&#39;d have to do something like `Foo::frotz()`.  

This doesn&#39;t seem right since this behavior makes it hard to make deep hierarchies.  If children need to know what class defined an inherited method, then all sorts of information pain is created.  

Is this an actual limitation in python, a gap in my understanding or both?


  [1]: https://docs.oracle.com/javase/tutorial/java/IandI/super.html

How do I call a parent class&#39;s method from a child class in Python?

In JavaScript, every object is at the same time an instance and a class. To do inheritance, you can use any object instance as a prototype.

In Python, C++, etc.. there are classes, and instances, as separate concepts. In order to do inheritance, you have to use the base class to create a new class, which can then be used to produce derived instances.

Why did JavaScript go in this direction (prototype-based object orientation)? what are the advantages (and disadvantages) of prototype-based OO with respect to traditional, class-based OO?

prototype based vs. class based inheritance

I imagine that we all (when we can be bothered!) comment our interfaces.  e.g.

    /// &lt;summary&gt;
    /// Foo Interface
    /// &lt;/summary&gt;
    public interface Foo
    {
        /// &lt;summary&gt;
        /// Will &#39;bar&#39;
        /// &lt;/summary&gt;
        /// &lt;param name=&quot;wibble&quot;&gt;Wibble factor&lt;/param&gt;
        void Bar(string wibble);
    }


Do you also comment the implementation (which may also be provided to clients, e.g. as part of a a library)?  If so how do you manage keeping the two in sync?  Or do you just add a &#39;See interface for documentation&#39; comment?

Thanks

Comment the interface, implementation or both?

I have had recently two telephone interviews where I&#39;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&#39;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.

**Interface:**

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 &quot;What if you had an Abstract class with only abstract methods? How would that be different from an interface?&quot; I didn&#39;t know the answer but I think it&#39;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&#39;t have a public variable inside an interface. I didn&#39;t know what he wanted to hear but he wasn&#39;t satisfied either.

**See Also**: 

 - [When to use an interface instead of
   an abstract class and vice versa][1]
   
 - [Interfaces vs. Abstract Classes][2]

 - [How do you decide between using an Abstract Class and an Interface?][3]
 
 - [What is the difference between an interface and abstract class?][4]

  [1]: https://stackoverflow.com/questions/479142/when-to-use-an-interface-instead-of-an-abstract-class-and-vice-versa
  [2]: https://stackoverflow.com/questions/747517/interfaces-vs-abstract-classes
  [3]: https://stackoverflow.com/questions/558164/how-do-you-decide-between-using-an-abstract-class-and-an-interface-closed
  [4]: https://stackoverflow.com/questions/1913098/what-is-the-difference-between-an-interface-and-abstract-class?rq=1


Interface vs Abstract Class (general OO)

Could someone please explain to me the differences between **abstract classes**, **interfaces**, and **mixins**? I&#39;ve used each before in my code but I don&#39;t know the technical differences.

Abstract classes vs. interfaces vs. mixins

I haven&#39;t touched Java since using JBuilder in the late 90&#39;s while at University, so I&#39;m a little out of touch - at any rate I&#39;ve been working on a small Java project this week, and using Intellij IDEA as my IDE, for a change of pace from my regular .Net development.

I notice it has support for adding interfaces and @interfaces, what is an @interface, and how does it differ from a normal interface?

    public interface Test {
    }


vs.

    public @interface Test {
    }

I&#39;ve done a bit of searching, but couldn&#39;t find a great deal of useful info referring to @interface.


What&#39;s the difference between interface and @interface in java?

I have an existing project that uses `@Override` on methods that override *interface* methods, rather than superclass methods. I cannot alter this in code, but I would like Eclpse to stop complaining about the annotation, as I can still build with Maven.

How would I go about disabling this error?

**Note: Due to project requirements, I need to compile for Java 1.5.**

Why does Eclipse complain about @Override on interface methods?

I was wondering what would make a programmer to choose either Pimpl idiom or pure virtual class and inheritance.

I understand that pimpl idiom comes with one explicit extra indirection for each public method and the object creation overhead.

The Pure virtual class in the other hand comes with implicit indirection(vtable) for the inheriting implementation and I understand that no object creation overhead.  
**EDIT**: But you&#39;d need a factory if you create the object from the outside

What makes the pure virtual class less desirable than the pimpl idiom?

Pimpl idiom vs Pure virtual class interface

In java, if a class implements Serializable but is abstract, should it have a serialVersionUID long declared, or do the subclasses only require that?

In this case it is indeed the intention that all the sub classes deal with serialization as the purpose of the type is to be used in RMI calls.

Should an abstract class have a serialVersionUID

I&#39;m originally a Java programmer who now works with Objective-C. I&#39;d like to create an abstract class, but that doesn&#39;t appear to be possible in Objective-C. Is this possible?

If not, how close to an abstract class can I get in Objective-C?


Content Type	Original Author	Original Content on Stackoverflow
Question	tush1r	View Question on Stackoverflow
Solution 1 - C#	JaredPar	View Answer on Stackoverflow
Solution 2 - C#	Guffa	View Answer on Stackoverflow
Solution 3 - C#	Zifre	View Answer on Stackoverflow
Solution 4 - C#	Dave Van den Eynde	View Answer on Stackoverflow

Interfaces vs. abstract classes

C# Problem Overview

C# Solutions

Solution 1 - C#

Solution 2 - C#

Solution 3 - C#

Solution 4 - C#

Best way to create an empty map in Java

Should I compile with /MD or /MT?

Attributions