C# Java-like inline extension of classes?

Question

I would look this up on Google/MSDN, but I have no idea what it's called so I'm asking here.

In Java, I seem to remember you can do this really cool thing like:

Class MyClass
{
  int number;

  MyClass() { }

  void setNumber(int number)
  {
    this.number = number;
  }
}

and then do something like:

MyClass myClass = new MyClass()
  {
    override void setNumber(int Number)
    {
      this.number = 2 * number;
    }
  };

...or something. Forgive any mistakes I made above - I haven't actually touched Java in about 6 years.

The point is, I remember you could pseudo-extend a class inline.

Right now, I need to extend a C# WinForms control, but I only need to use it once, and the modifications are very minor. All I need to do is to override the CreateParams property and OnPaint() handler.

My solution is already getting huge with classes all over the place, it seems like a shame to include yet another class which is basically identical to a standard .Net control, just with very slightly different behaviour.

Is it possible to do this inline-extension in C# like you could in Java? If so, how? (and what is it called so I can look it up on MSDN?)

score 5 · Accepted Answer · 2011-03-23 01:12:00Z

5

This (explicit nominative anonymous types) is not possible in C#3/4.

The types must be created explicitly and then constructed. Tasks are sometimes "inverted" in C# with the use of Events and Delegates (class invokes Event which supplies implementation such as "NeedDataSource") -- arguably because of this, although it just makes sense in many cases.

If is possible to create explicit non-nominative types: var x = new { P = 1, }; but only in a local scope. Implicit methods include delegates/lambdas/anonymous functions and do not apply here.

Happy coding.

edited Mar 23, 2011 at 1:12

answered Mar 23, 2011 at 1:05

user166390

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1 Comment

Ozzah Over a year ago

Thanks. That answers my question, but it's not the answer I was hoping for.

Ondra Meca · Accepted Answer · 2012-04-29 14:07:52Z

1

I understand that you said this cannot be done in C#, but its possible to rewrite this to C# but with not too much complicated method?

new EffectClause("Sleep Clause", SleepEffect.class)
{
    public String getClauseDescription()
    {
        return "Bla bla bla";
    }

    public boolean isEnabledByDefault()
    {
        return true;
    }
};

answered Apr 29, 2012 at 14:07

Ondra Meca

111 bronze badge

1 Comment

MakePeaceGreatAgain Over a year ago

There is no class-property in C# but in JAVA. Furthermore: is this a question or an answer? And last: how is this concerned to the question?

Cody · Accepted Answer · 2011-03-23 01:12:39Z

0

The feature you're looking for in C# is called Extension Methods. They are similar, but instead you make code such as:

public static void setNumber(int Number, MyClass target)
{
    target.number = 2 * number;
}

You usually include this in your own extensions namespace, then bring it into scope when necessary by using MyExtensions; C# is able to figure out that you mean to call this method when you call whatever.setNumber(x);

I'm not 100% sure if this will let you override a method that already exists in a class however. Your other option is to inherit from the target class with a class specifically for the purpose of overriding the target method.

answered Mar 23, 2011 at 1:12

Cody

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4 Comments

user166390 Over a year ago

Extension methods are different and do not provide the same functionality. In the above, imagine: T one = new T { public void () { IMPL1(); } }; T two = new T { public void () { IMPL2(); } }; -- that is, Extension methods can decorate a statically-known type but can't allow different anonymous subtypes to be created.

Jeff Mercado Over a year ago

Perhaps his use of "extension" confused you. He's not talking about C# extension methods but extending (in the Java sense) a class. Or in other words, inheriting from a class inline.

Brent Stewart Over a year ago

Not what the op was asking for. Extension methods do not allow you to override existing methods or properties.

Ozzah Over a year ago

Yes, I see what you are doing in the snippet you provided but I cannot see how it would enable me to override something like OnPaint() in a foreign class. Thanks anyway :)

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