Given the new dynamic support in C# 4, is it possible to write a class in such a way that if a method is invoked on an instance and that method is not present, dispatch is passed to another method? This might look something like:
public class Apple : ... {
// ...
private ... MethodMissing(string name, ...) {
if (name == "TurnIntoOrange") {
// do something
}
}
}
dynamic d = new Apple();
d.TurnIntoOrange(); // Not actually defined on Apple; will pass to MethodMissing.
Other languages would call this "method_missing support", under the more general heading of metaprogramming. I'm not sure what C# calls this specifically. But is it possible?
Absolutely. Either implement IDynamicMetaObjectProvider or derive from DynamicObject for a much simpler route. See the DLR documentation for some good examples.
Here's a quick example of DynamicObject:
using System;
using System.Dynamic;
public class MyDynamic : DynamicObject
{
public override bool TryInvokeMember
(InvokeMemberBinder binder,
object[] args,
out object result)
{
Console.WriteLine("I would have invoked: {0}",
binder.Name);
result = "dummy";
return true;
}
public string NormalMethod()
{
Console.WriteLine("In NormalMethod");
return "normal";
}
}
class Test
{
static void Main()
{
dynamic d = new MyDynamic();
Console.WriteLine(d.HelloWorld());
Console.WriteLine(d.NormalMethod());
}
}
<plug>
I have a bigger example of DynamicObject in the 2nd edition of C# in Depth but I haven't yet implemented IDyamicMetaObjectProvider. I'll do so before the book's release, but the early access edition only has the DynamicObject example at the moment. Btw, if you buy it today it's half price - use the code twtr0711. I'll edit this answer later on to remove that bit :)
</plug>
Related
The main WebServerExtension example from the JUnit5 manual is incomplete and it doesn't fully show how to properly store the configuration (e.g. enableSecurity, server url).
https://github.com/junit-team/junit5/blob/master/documentation/src/main/java/example/registration/WebServerExtension.java
The example ignores or hard codes the values. The manual (section 5.11. Keeping State in Extensions) implies that the "Store" should be used but the ExtensionContext is not yet available yet when the object is constructed -- its not clear how to handle migrating this data to the Store as the ExtensionContext is not yet available in the constructor.
Also its not clear to me that using the Store API for the WebServerExtension programmatic example is even desirable and perhaps it could work just using the internal state (e.g. this.serverUrl, this.enableSecurity, etc.).
Maybe the Store is more applicable to Extensions which don't use this "programmatic" style where multiple instances of the custom extension may exist (appropriately)? In other words its not clear to me from the guide if this a supported paradigm or not?
Other JUnit 5 extension examples online (e.g. org.junit.jupiter.engine.extension.TempDirectory) show how to leverage annotations to handle passing configuration info to the Store but it would be nice if there were a complete programmatic builder type example like WebServerExtension too.
Examples like TempDirectory clearly have access to the ExtensionContext from the beforeXXX() methods whereas the WebServerExtension example does not.
Using the following approach below seems to work fine but I wanted confirmation that this is a supported paradigm (i.e. using fields instead of Stores when using this programmatic approach).
public class WebServerExtension implements BeforeAllCallback {
private final boolean securityEnabled;
private final String serverUrl;
public WebServerExtension(Builder builder) {
this.securityEnabled = builder.enableSecurity;
this.serverUrl = build.serverUrl;
}
#Override
public void beforeAll(ExtensionContext context) {
// is it ok to use this.securityEnabled, this.serverUrl instead of Store API???
}
public String getServerUrl() {
return this.serverUrl;
}
public boolean isSecurityEnabled() {
return this.securityEnabled;
}
public static Builder builder() {
return new Builder();
}
public static class Builder {
private boolean enableSecurity;
private String serverUrl;
public Builder enableSecurity(boolean b) {
this.enableSecurity = b;
return this;
}
public Builder serverUrl(String url) {
this.serverUrl = url;
return this;
}
public WebServerExtension build() {
return new WebServerExtension(this);
}
}
}
Thanks!
newbie here...thanks for your patience. I'm interested in writing a test class for the following controller, but not sure where to begin:
public class savecontroller
{
private final Emp__c emps;
public savecontroller(ApexPages.StandardController controller)
{
this.emps= (Emp__c)controller.getRecord();
}
public void autosave()
{
upsert emps;
}
}
Thank you
Your code is a little bit strange... From this part:
public savecontroller(ApexPages.StandardController controller)
it looks like your controller is not really a "controller", but more like an extension of a standard controller for Emp__c object. I know, it doesn't affect anythingin your post (except maybe semantics), BUT (!) it does have an affect on how you write your test class. Since this is an extension, test class would look something like this:
#isTest
public class saveconttroller_test {
public static Emp__c test_emp; // declaration
static {
test_emp = new Emp__c();
insert test_emp; //since you have upsert you can leave this out
}
static testMethod void testsavecotroller() {
Test.startTest();
//in the next two lines we contruct standard controller and the extension
ApexPages.StandardController sc = new ApexPages.StandardController(test_emp);
savecontroller ext = new savecontroller(sc);
ext.autosave();
Test.stopTest();
}
}
Now, let me point out a few things... first, as I'm sure you know, test should cover as much code as possible. SF requires 75%, but the closer you get to 100% the better. But (!), you should always include something to assert if your method is doing what it is suppose to be doing. For example in your case, i would change method autosave() like this:
public PageReference autosave()
{
try {
upsert emps;
return new ApexPages.StandardController(test_emp).view();
} catch(Exception e) {
return null;
}
}
By doing so, you can include System.assertEquals(ref1, ref2); in your test class, wher ref1 is reference you would expect (if the upsertion was successful this would be test_emp page reference) and ref2 would be the reference you actually get from the test.
Second thing is using static method in the test. Whatever you write in this method will always execute on Test.startTest(); call.
Hope this helps you! :)
Cheers, G.
I saw this post and I want to know if this is possible in VB.
So like extension method, do extension properties exists in VB.Net?
Here I've read they do, but cannot find any examples.
I believe that person is incorrect. From MSDN
You cannot define an extension property, field, or event.
This is almost possible. Learned this neat trick from Daniel Cazzulino.
You return a type from an extension method which exposes the properties. This is C#, but should be understandable.
public static class ListExtensions
{
// this extension method returns the type with properties
public static ListExtender<T> Extend<T>(this List<T> target)
{
//null check skipped
return new ListExtender<T>(target);
}
}
public sealed class ListExtender<T>
{
private List<T> _target;
// this is a pseudo extension property
public T First { get { return _target[0]; } }
public ListExtender(List<T> target)
{
_target = target;
}
}
Other than that, the answer is no.
According to the MSDN(draft) documentation for Visual Studio 11, extension properties are not available in VS 11 (i.e., .NET 4.5) either.
It's odd though searching does throw up a few instances where bloggers, etc., think it to be possible, including Ayende in an article on his blog here.
Referring to the below link:
http://www.javaworld.com/javaworld/jw-11-1998/jw-11-techniques.html?page=2
The composition approach to code reuse provides stronger encapsulation
than inheritance, because a change to a back-end class needn't break
any code that relies only on the front-end class. For example,
changing the return type of Fruit's peel() method from the previous
example doesn't force a change in Apple's interface and therefore
needn't break Example2's code.
Surely if you change the return type of peel() (see code below) this means getPeelCount() wouldn't be able to return an int any more? Wouldn't you have to change the interface, or get a compiler error otherwise?
class Fruit {
// Return int number of pieces of peel that
// resulted from the peeling activity.
public int peel() {
System.out.println("Peeling is appealing.");
return 1;
}
}
class Apple {
private Fruit fruit = new Fruit();
public int peel() {
return fruit.peel();
}
}
class Example2 {
public static void main(String[] args) {
Apple apple = new Apple();
int pieces = apple.peel();
}
}
With a composition, changing the class Fruit doesn't necessary require you to change Apple, for example, let's change peel to return a double instead :
class Fruit {
// Return String number of pieces of peel that
// resulted from the peeling activity.
public double peel() {
System.out.println("Peeling is appealing.");
return 1.0;
}
}
Now, the class Apple will warn about a lost of precision, but your Example2 class will be just fine, because a composition is more "loose" and a change in a composed element does not break the composing class API. In our case example, just change Apple like so :
class Apple {
private Fruit fruit = new Fruit();
public int peel() {
return (int) fruit.peel();
}
}
Whereas if Apple inherited from Fruit (class Apple extends Fruit), you would not only get an error about an incompatible return type method, but you'd also get a compilation error in Example2.
** Edit **
Lets start this over and give a "real world" example of composition vs inheritance. Note that a composition is not limited to this example and there are more use case where you can use the pattern.
Example 1 : inheritance
An application draw shapes into a canvas. The application does not need to know which shapes it has to draw and the implementation lies in the concrete class inheriting the abstract class or interface. However, the application knows what and how many different concrete shapes it can create, thus adding or removing concrete shapes requires some refactoring in the application.
interface Shape {
public void draw(Graphics g);
}
class Box implement Shape {
...
public void draw(Graphics g) { ... }
}
class Ellipse implements Shape {
...
public void draw(Graphics g) { ... }
}
class ShapeCanvas extends JPanel {
private List<Shape> shapes;
...
protected void paintComponent(Graphics g) {
for (Shape s : shapes) { s.draw(g); }
}
}
Example 2 : Composition
An application is using a native library to process some data. The actual library implementation may or may not be known, and may or may not change in the future. A public interface is thus created and the actual implementation is determined at run-time. For example :
interface DataProcessorAdapter {
...
public Result process(Data data);
}
class DataProcessor {
private DataProcessorAdapter adapter;
public DataProcessor() {
try {
adapter = DataProcessorManager.createAdapter();
} catch (Exception e) {
throw new RuntimeException("Could not load processor adapter");
}
}
public Object process(Object data) {
return adapter.process(data);
}
}
static class DataProcessorManager {
static public DataProcessorAdapter createAdapter() throws ClassNotFoundException, InstantiationException, IllegalAccessException {
String adapterClassName = /* load class name from resource bundle */;
Class<?> adapterClass = Class.forName(adapterClassName);
DataProcessorAdapter adapter = (DataProcessorAdapter) adapterClass.newInstance();
//...
return adapter;
}
}
So, as you can see, the composition may offer some advantage over inheritance in the sense that it allows more flexibility in the code. It allows the application to have a solid API while the underlaying implementation may still change during it's life cycle. Composition can significantly reduce the cost of maintenance if properly used.
For example, when implementing test cases with JUnit for Exemple 2, you may want to use a dummy processor and would setup the DataProcessorManager to return such adapter, while using a "real" adapter (perhaps OS dependent) in production without changing the application source code. Using inheritance, you would most likely hack something up, or perhaps write a lot more initialization test code.
As you can see, compisition and inheritance differ in many aspects and are not preferred over another; each depend on the problem at hand. You could even mix inheritance and composition, for example :
static interface IShape {
public void draw(Graphics g);
}
static class Shape implements IShape {
private IShape shape;
public Shape(Class<? extends IShape> shape) throws InstantiationException, IllegalAccessException {
this.shape = (IShape) shape.newInstance();
}
public void draw(Graphics g) {
System.out.print("Drawing shape : ");
shape.draw(g);
}
}
static class Box implements IShape {
#Override
public void draw(Graphics g) {
System.out.println("Box");
}
}
static class Ellipse implements IShape {
#Override
public void draw(Graphics g) {
System.out.println("Ellipse");
}
}
static public void main(String...args) throws InstantiationException, IllegalAccessException {
IShape box = new Shape(Box.class);
IShape ellipse = new Shape(Ellipse.class);
box.draw(null);
ellipse.draw(null);
}
Granted, this last example is not clean (meaning, avoid it), but it shows how composition can be used.
Bottom line is that both examples, DataProcessor and Shape are "solid" classes, and their API should not change. However, the adapter classes may change and if they do, these changes should only affect their composing container, thus limit the maintenance to only these classes and not the entire application, as opposed to Example 1 where any change require more changes throughout the application. It all depends how flexible your application needs to be.
If you would change Fruit.peel()'s return type, you would have to modify Apple.peel() as well. But you don't have to change Apple's interface.
Remember: The interface are only the method names and their signatures, NOT the implementation.
Say you'd change Fruit.peel() to return a boolean instead of a int. Then, you could still let Apple.peel() return an int. So: The interface of Apple stays the same but Fruit's changed.
If you would have use inheritance, that would not be possible: Since Fruit.peel() now returns a boolean, Apple.peel() has to return an boolean, too. So: All code that uses Apple.peel() has to be changed, too. In the composition example, ONLY Apple.peel()'s code has to be changed.
The key word in the sentence is "interface".
You'll almost always need to change the Apple class in some way to accomodate the new return type of Fruit.peel, but you don't need to change its public interface if you use composition rather than inheritance.
If Apple is a Fruit (ie, inheritance) then any change to the public interface of Fruit necessitates a change to the public interface of Apple too. If Apple has a Fruit (ie, composition) then you get to decide how to accomodate any changes to the Fruit class; you're not forced to change your public interface if you don't want to.
Return type of Fruit.peel() is being changed from int to Peel. This doesn't meant that the return type of Apple.peel() is being forced to change to Peel as well. In case of inheritance, it is forced and any client using Apple has to be changed. In case of composition, Apple.peel() still returns an integer, by calling the Peel.getPeelCount() getter and hence the client need not be changed and hence Apple's interface is not changed ( or being forced to be changed)
Well, in the composition case, Apple.peel()'s implementation needs to be updated, but its method signature can stay the same. And that means the client code (which uses Apple) does not have to be modified, retested, and redeployed.
This is in contrast to inheritance, where a change in Fruit.peel()'s method signature would require changes all way into the client code.
I am new to OOP. Though I understand what polymorphism is, but I can't get the real use of it. I can have functions with different name. Why should I try to implement polymorphism in my application.
Classic answer: Imagine a base class Shape. It exposes a GetArea method. Imagine a Square class and a Rectangle class, and a Circle class. Instead of creating separate GetSquareArea, GetRectangleArea and GetCircleArea methods, you get to implement just one method in each of the derived classes. You don't have to know which exact subclass of Shape you use, you just call GetArea and you get your result, independent of which concrete type is it.
Have a look at this code:
#include <iostream>
using namespace std;
class Shape
{
public:
virtual float GetArea() = 0;
};
class Rectangle : public Shape
{
public:
Rectangle(float a) { this->a = a; }
float GetArea() { return a * a; }
private:
float a;
};
class Circle : public Shape
{
public:
Circle(float r) { this->r = r; }
float GetArea() { return 3.14f * r * r; }
private:
float r;
};
int main()
{
Shape *a = new Circle(1.0f);
Shape *b = new Rectangle(1.0f);
cout << a->GetArea() << endl;
cout << b->GetArea() << endl;
}
An important thing to notice here is - you don't have to know the exact type of the class you're using, just the base type, and you will get the right result. This is very useful in more complex systems as well.
Have fun learning!
Have you ever added two integers with +, and then later added an integer to a floating-point number with +?
Have you ever logged x.toString() to help you debug something?
I think you probably already appreciate polymorphism, just without knowing the name.
In a strictly typed language, polymorphism is important in order to have a list/collection/array of objects of different types. This is because lists/arrays are themselves typed to contain only objects of the correct type.
Imagine for example we have the following:
// the following is pseudocode M'kay:
class apple;
class banana;
class kitchenKnife;
apple foo;
banana bar;
kitchenKnife bat;
apple *shoppingList = [foo, bar, bat]; // this is illegal because bar and bat is
// not of type apple.
To solve this:
class groceries;
class apple inherits groceries;
class banana inherits groceries;
class kitchenKnife inherits groceries;
apple foo;
banana bar;
kitchenKnife bat;
groceries *shoppingList = [foo, bar, bat]; // this is OK
Also it makes processing the list of items more straightforward. Say for example all groceries implements the method price(), processing this is easy:
int total = 0;
foreach (item in shoppingList) {
total += item.price();
}
These two features are the core of what polymorphism does.
Advantage of polymorphism is client code doesn't need to care about the actual implementation of a method.
Take look at the following example.
Here CarBuilder doesn't know anything about ProduceCar().Once it is given a list of cars (CarsToProduceList) it will produce all the necessary cars accordingly.
class CarBase
{
public virtual void ProduceCar()
{
Console.WriteLine("don't know how to produce");
}
}
class CarToyota : CarBase
{
public override void ProduceCar()
{
Console.WriteLine("Producing Toyota Car ");
}
}
class CarBmw : CarBase
{
public override void ProduceCar()
{
Console.WriteLine("Producing Bmw Car");
}
}
class CarUnknown : CarBase { }
class CarBuilder
{
public List<CarBase> CarsToProduceList { get; set; }
public void ProduceCars()
{
if (null != CarsToProduceList)
{
foreach (CarBase car in CarsToProduceList)
{
car.ProduceCar();// doesn't know how to produce
}
}
}
}
class Program
{
static void Main(string[] args)
{
CarBuilder carbuilder = new CarBuilder();
carbuilder.CarsToProduceList = new List<CarBase>() { new CarBmw(), new CarToyota(), new CarUnknown() };
carbuilder.ProduceCars();
}
}
Polymorphism is the foundation of Object Oriented Programming. It means that one object can be have as another project. So how does on object can become other, its possible through following
Inheritance
Overriding/Implementing parent Class behavior
Runtime Object binding
One of the main advantage of it is switch implementations. Lets say you are coding an application which needs to talk to a database. And you happen to define a class which does this database operation for you and its expected to do certain operations such as Add, Delete, Modify. You know that database can be implemented in many ways, it could be talking to file system or a RDBM server such as MySQL etc. So you as programmer, would define an interface that you could use, such as...
public interface DBOperation {
public void addEmployee(Employee newEmployee);
public void modifyEmployee(int id, Employee newInfo);
public void deleteEmployee(int id);
}
Now you may have multiple implementations, lets say we have one for RDBMS and other for direct file-system
public class DBOperation_RDBMS implements DBOperation
// implements DBOperation above stating that you intend to implement all
// methods in DBOperation
public void addEmployee(Employee newEmployee) {
// here I would get JDBC (Java's Interface to RDBMS) handle
// add an entry into database table.
}
public void modifyEmployee(int id, Employee newInfo) {
// here I use JDBC handle to modify employee, and id to index to employee
}
public void deleteEmployee(int id) {
// here I would use JDBC handle to delete an entry
}
}
Lets have File System database implementation
public class DBOperation_FileSystem implements DBOperation
public void addEmployee(Employee newEmployee) {
// here I would Create a file and add a Employee record in to it
}
public void modifyEmployee(int id, Employee newInfo) {
// here I would open file, search for record and change values
}
public void deleteEmployee(int id) {
// here I search entry by id, and delete the record
}
}
Lets see how main can switch between the two
public class Main {
public static void main(String[] args) throws Exception {
Employee emp = new Employee();
... set employee information
DBOperation dboper = null;
// declare your db operation object, not there is no instance
// associated with it
if(args[0].equals("use_rdbms")) {
dboper = new DBOperation_RDBMS();
// here conditionally, i.e when first argument to program is
// use_rdbms, we instantiate RDBM implementation and associate
// with variable dboper, which delcared as DBOperation.
// this is where runtime binding of polymorphism kicks in
// JVM is allowing this assignment because DBOperation_RDBMS
// has a "is a" relationship with DBOperation.
} else if(args[0].equals("use_fs")) {
dboper = new DBOperation_FileSystem();
// similarly here conditionally we assign a different instance.
} else {
throw new RuntimeException("Dont know which implemnation to use");
}
dboper.addEmployee(emp);
// now dboper is refering to one of the implementation
// based on the if conditions above
// by this point JVM knows dboper variable is associated with
// 'a' implemenation, and it will call appropriate method
}
}
You can use polymorphism concept in many places, one praticle example would be: lets you are writing image decorer, and you need to support the whole bunch of images such as jpg, tif, png etc. So your application will define an interface and work on it directly. And you would have some runtime binding of various implementations for each of jpg, tif, pgn etc.
One other important use is, if you are using java, most of the time you would work on List interface, so that you can use ArrayList today or some other interface as your application grows or its needs change.
Polymorphism allows you to write code that uses objects. You can then later create new classes that your existing code can use with no modification.
For example, suppose you have a function Lib2Groc(vehicle) that directs a vehicle from the library to the grocery store. It needs to tell vehicles to turn left, so it can call TurnLeft() on the vehicle object among other things. Then if someone later invents a new vehicle, like a hovercraft, it can be used by Lib2Groc with no modification.
I guess sometimes objects are dynamically called. You are not sure whether the object would be a triangle, square etc in a classic shape poly. example.
So, to leave all such things behind, we just call the function of derived class and assume the one of the dynamic class will be called.
You wouldn't care if its a sqaure, triangle or rectangle. You just care about the area. Hence the getArea method will be called depending upon the dynamic object passed.
One of the most significant benefit that you get from polymorphic operations is ability to expand.
You can use same operations and not changing existing interfaces and implementations only because you faced necessity for some new stuff.
All that we want from polymorphism - is simplify our design decision and make our design more extensible and elegant.
You should also draw attention to Open-Closed Principle (http://en.wikipedia.org/wiki/Open/closed_principle) and for SOLID (http://en.wikipedia.org/wiki/Solid_%28Object_Oriented_Design%29) that can help you to understand key OO principles.
P.S. I think you are talking about "Dynamic polymorphism" (http://en.wikipedia.org/wiki/Dynamic_polymorphism), because there are such thing like "Static polymorphism" (http://en.wikipedia.org/wiki/Template_metaprogramming#Static_polymorphism).
You don't need polymorphism.
Until you do.
Then its friggen awesome.
Simple answer that you'll deal with lots of times:
Somebody needs to go through a collection of stuff. Let's say they ask for a collection of type MySpecializedCollectionOfAwesome. But you've been dealing with your instances of Awesome as List. So, now, you're going to have to create an instance of MSCOA and fill it with every instance of Awesome you have in your List<T>. Big pain in the butt, right?
Well, if they asked for an IEnumerable<Awesome>, you could hand them one of MANY collections of Awesome. You could hand them an array (Awesome[]) or a List (List<Awesome>) or an observable collection of Awesome or ANYTHING ELSE you keep your Awesome in that implements IEnumerable<T>.
The power of polymorphism lets you be type safe, yet be flexible enough that you can use an instance many many different ways without creating tons of code that specifically handles this type or that type.
Tabbed Applications
A good application to me is generic buttons (for all tabs) within a tabbed-application - even the browser we are using it is implementing Polymorphism as it doesn't know the tab we are using at the compile-time (within the code in other words). Its always determined at the Run-time (right now! when we are using the browser.)