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   ► KBTo/From GuidesDelphiOOP Details  Print This     

Cross Ref > OOP Details

By Mike Prestwood

Delphi versus C#: A side by side comparison between Delphi and C#.

 
OOP Details
 

More object oriented (OO) stuff.

Abstraction

[Other Languages] 

General Info: Abstract Class / Abstract Member

An abstract class member is a member that is specified in a class but not implemented. Classes that inherit from the class will have to implement the abstract member. Abstract members are a technique for ensuring a common interface with descendant classes. An abstract class is a class you cannot instantiate. A pure abstract class is a class with only abstract members.

Languages Focus

Abstraction is supported at various levels with each language. A language could enforce abstraction at the class level (either enforcing a no-instantiation rule or a only abstract members rule), and with class members (member methods and/or properties).

Delphi:   abstract, override

Delphi for Win32 supports abstract class members using the abstract keyword. You can even instantiate instances of a class that contains abstract members. Then you override each abstract member in a descendant class with Override.

Delphi does not support setting an entire class as abstract. You can create an abstract class (a class with one or more abstract methods), but there is no way to tell the compiler to not allow the instantiation of the abstract class.

Delphi does not support abstract member properties directly. To implement an abstract properity, make use of abstract methods. That is, you can read a GetPropertyX abstract function and write to a SetPropertyX abstract procedure. In effect, creating  an abstract property.

Syntax Example:
TCyborg = class(TObject)
public
  procedure Speak(pMessage: String); virtual; abstract;
  procedure Walk; virtual; abstract;
end;
 
TSeries600 = class(TCyborg)
public
  procedure Speak(pMessage: String); override;
  procedure Walk; override;
end;
C#:   abstract, override

C# supports abstract class members and abstract classes using the abstract modifier.

An abstract class is a class with one or more abstract members and you cannot instantiate an abstract class. However, you can have additional implemented methods and properties.

An abstract member is either a method (implicitly virtual), property, indexer, or event in an abstract class. You can add abstract members ONLY to abstract classes using the abstract keyword. Then you override it in a descendant class with Override.

Syntax Example:
abstract public class Cyborg : System.Object
{
  abstract public void Speak(string pMessage);
}
public class Series600 : Cyborg
{
  public override void Speak(string pMessage)  
  {
    MessageBox.Show(pMessage);  
  }
}




Class Helper

[Other Languages] 

A. In Dephi, class helpers allow you to extend a class without using inheritance. With a class helper, you do not have to create and use a new class descending from a class but instead you enhance the class directly and continue using it as you always have (even just with the DCU).

B. In general terms, developers sometimes use the term to refer to any class that helps out another class.

Delphi:  "Class Helpers" class helper for

Delphi allows you to extend an existing class without using inheritance. Buggy in 2005 and not officially supported but stable, usable, and officially supported in 2006 and above.

You declare a class helper similiar to how you declare a class but use the keywords class helper for.

Syntax Example:
TCyborg = class(TObject)
public
  FCyborgName: String;
end;
  
TCyborgHelper = class helper for TCyborg
  procedure ShowClassName;
end;
C#:  "Class Helpers" Not Supported

However, developers sometimes use the term "class helper" to refer to code that helps out a class. Not truly the meaning we are using here, but you should be aware of the term's general usage.





Code Contract

[Other Languages] 

A.k.a. Class Contract and Design by Contracts.

A contract with a method that must be true upon calling (pre) or exiting (post). A pre-condition contract must be true when the method is called. A post-condition contract must be true when exiting. If either are not true, an error is raised. For example, you can use code contracts to check for the validity of input parameters, and results

An invariant is also a code contract which validates the state of the object required by the method.

Delphi:  "Code Contracts" Not Supported

Delphi does not offer built-in Design by Contract features. It does offer an Assert method which tests if an expression is true. If false, an EAssertionFailed exception is raised. Although you can use Assert in a similar manor as you would use a pre-condition contract, the Delphi help clearly says Assert is a debugging tool only and to not use it in production code.

//Although not intended as a code contract feature,
//Assert is useful for debugging.
method Cyborg.Walk(pPace);
begin
Assert(pPace > 0);
Assert(pPace < 100);
  
  //Overloaded version with message.
Assert(FEnergyLevel >= 10, 'Energy level too low.');
end;
C#:  "Code Contracts" Not Supported

Although not currently supported, there are plans for the next version of VS.Net and .Net using Requires and Ensures keywords. Look for code contracts in VS.Net 2010 and .Net 4.

If you code with VS.Net 2008 Pro or above and wish to implement contracts now, checkout the following download:

 

Syntax Example:
//Future syntax may look something like:
string SetAge(int pAge) { 
Contract.Requires(pAge>0);
}
  
string GetAge() { 
Contract.Ensures(Contract.Result() != null);
}




Constructor

[Other Languages] 

General Info: Class Constructor

Constructors are called when you instantiate an object from a class. This is where you can initialize variables and put code you wish executed each time the class is created. When you initially set the member fields and properties of an object, you are initializing the state of the object. The state of an object is the values of all it's member fields and properties at a given time.

Languages Focus

What is the syntax? Can you overload constructors? Is a special method name reserved for constructors?

Delphi:  "Constructors" constructor

In Delphi, use the constructor keyword to signify which method or methods are constructors for a class. It is traditional but not required to use a procedure called Create.

In addition to having multiple named constructors, you can overload constructors.

Syntax Example:
//Interface section.
TCyborg = class(TObject)
public
  constructor Create;
end; 

Then implement the class constructor in the Implementation section.

constructor TCyborg.Create;
begin
  inherited;  //Call the parent Create method
end;
C#:  "Constructors" Use class name

In C#, a constructor is called whenever a class or struct is created. A constructor is a method with the same name as the class with no return value and you can overload the constructor.

If you do not create a constructor, C# will create an implicit constructor that initializes all member fields to their default values.

Constructors can execute at two different times. Static constructors are executed by the CLR before any objects are instantiated. Regular constructors are executed when you create an object.

Syntax Example:
public class Cyborg
{
public string CyborgName;
  
  //Constructor.
  public Cyborg(string pName)
{
CyborgName = pName;
}
}




Destructor

[Other Languages] 

General Info: Class Destructor

A special class method called when an object instance of a class is destroyed. With some languages they are called when the object instance goes out of scope, with some languages you specifically have to call the destructor in code to destroy the object, and others use a garbage collector to dispose of object instances at specific times.

Desctructors are commonly used to free the object instance but with languages that have a garbage collector object instances are disposed of when appropriate. Either way, destructors or their equivalent are commonly used to free up resources allocated in the class constructor.

Languages Focus

Are object instances freed with a garbage collector? Or, do you have to destroy object instances.

Delphi:   Free or FreeAndNil

Object Pascal uses a standard virtual destructor named Destroy which is called by the standard Free method. All objects are dynamic, so you need to call MyObject.Free method or the FreeAndNil(MyObject) routine for each object you create.

Syntax Example:  
var
MyObject: TObject;
begin
MyObject := TObject.Create;
 
  //Use it...
  
  MyObject.Free
  //Or use...FreeAndNil(MyObject);
end;
C#:  "Finalizer" ~ClassName

In C# you cannot explicitly destroy a managed object. Instead, the .Net Framework's garbage collector (GC) takes care of destroying all objects. The GC destroys the objects only when necessary. Some situations of necessity are when memory is exhausted or you explicitly call the System.GC.Collect() method. In general, you never need to call System.GC.Collect().

In .Net, a finalizer is used to free non-managed objects such as a file or network resource. In C#, a finalizer is a method with the same name as the class but preceded with a tilde (as in ~ClassName). The finalizer method implicity creates an Object.Finalize method (you cannot directly call nor override the Object.Finalize method). Because you don't know when the garbage collector will call your finalizer, Microsoft recommends you implement the IDisposable interface for non-managed resources and call it's Dispose() method at the appropriate time.

Syntax Example:
class Cyborg {
public:
//Destructor for class Cyborg.
~Cyborg();
  {
  //Free non-managed resources here.
  }
};




Inheritance-Multiple

[Other Languages] 
Delphi:   Not Supported

Delphi does not support multiple implementation inheritance. Each class can have only one parent class (a single inheritance path).

In Delphi, you can use multiple interface usage to design in a multiple class way horizontally in a class hierarchy.

More Info / Comment
C#:   Not Supported

C# does not support multiple implementation inheritance. Each class can have only one parent class (a single inheritance path). In C#, you can use multiple interface usage to design in a multiple class way horizontally in a class hierarchy.

More Info / Comment




Interface

[Other Languages] 

An element of coding where you define a common set of properties and methods for use with the design of two or more classes.

Both interfaces and abstract classes are types of abstraction. With interfaces, like abstract classes, you cannot provide any implementation. However, unlike abstract classes, interfaces are not based on inheritance. You can apply an Interface to any class in your class tree. In a real sense, interfaces are a technique for designing horizontally in a class hierarchy (as opposed to inheritance where you design vertically). Using interfaces in your class design allows your system to evolve without breaking existing code.

Delphi:  "Interfaces" IInterface, TInterfacedObject

In Delphi, you use interfaces for both com objects and language interfaces and make use of IUnknown, IInterface, and/or TInterfacedObject.

For a pure language interface, add your specified proprieties, procedures, and functions to an interface that descends from IInterface (the base interface) as an interface, no implementation. Then have your implementing class inherit from TInterfacedObject and implement the interface.

For extending the VCL, you descend from the class you wish to extend, then implement an interface from IInterface and add the required functions QueryInterface, _AddRef, and _Release methods (refer to TInterfacedObject for an example).

For a com object, you descend from IUnknown. Descending from IUnknown instead of IInterface informs the Delphi compiler that the interface must be compatible with COM objects -- a Windows feature).

When defining an interface, define it in the type block just like you do for a class but you use the interface keyword instead of the class keyword and in the interfaces section only. Since interfaces, by definition, do not have any implementation details, all you do is specify it in the type block. Then implement in all classes that support the interface.

Syntax Example:
//Language interface:
//Interface section of unit.
IHuman = Interface(IInterface)
  //Specify interface methods and properties here.

end;
  
TCyborg = class(TInterfacedObject)
end;
  
TCyborgHuman = class(TCyborg, IHuman)
//Specify each here and implement in
//implementation section.
end;
C#:  "Interfaces" interface

Classes and structs can inherit from interfaces in a manner similar to how classes can inherit a base class or struct, but a class or struct can inherit more than one interface and it inherits only the method names and signatures, because the interface itself contains no implementations.

class MyClass: IMyInterface
{  
  public object Clone()
{
return null;
}

// IMyInterface implemented here...
}
Syntax Example:
interface IMyInterface
{
  bool IsValid();
}




Overriding

[Other Languages] 

General Info: Method Overriding

Where you define or implement a virtual method in a parent class and then replace it in a descendant class.

When you decide to declare a method as virtual, you are giving permission to derived classes to extend and override the method with their own implementation. You can have the extended method call the parent method's code too.

In most OO languages you can also choose to hide a parent method. When you introduce a new implementation of the same named method with the same signature without overriding, you are hiding the parent method.

Delphi:   virtual, override

In Delphi, you specify a virtual method with the virtual keyword in a parent class and extend (or replace) it in a descendant class using the override keyword. Call Inherited in the descendant method to execute the code in the parent method.

Syntax Example:
TRobot = class(TObject)
public
procedure Speak; virtual;
end;
  
TCyborg = class(TRobot)
procedure Speak; Override;
end;
C#:   virtual, override

In C#, you specify a virtual method with the virtual keyword in a parent class and extend (or replace) it in a descendant class using the override keyword.

Use the base keyword in the descendant method to execute the code in the parent method, i.e. base.SomeMethod().

Syntax Example:
class Robot
{
  public virtual void Speak()
  {
  }
}

class Cyborg:Robot
{
  public override void Speak()
  {
  }
}




Partial Class

[Other Languages] 

A partial class, or partial type, is a class that can be split into two or more source code files and/or two or more locations within the same source file. Each partial class is known as a class part or just a part. Logically, partial classes do not make any difference to the compiler. The compiler puts the class together at compile time and treats the final class or type as a single entity exactly the same as if all the source code was in a single location.

Languages Focus

For languages that have implemented partial classes, you need to know usage details and restrictions. Can you split a class into two or more files? Can you split a class within a source code file into two or more locations? What are the details of inheritance? Does it apply to interfaces as well?

Delphi:  "Partial Classes" Not Supported

As of Delphi 2009, partial classes are not supported. The main reason given in the thread below was that the Delphi compiler is a single pass compiler. Here is a link to a discussion thread on the subject:

C#:  "Partial Classes" partial

C# uses the keyword partial to specify a partial class. All parts must be in the same namespace.

Syntax Example:
class partial Cyborg: System.Object
{
}




Polymorphism

[Other Languages] 

A coding technique where the same named function, operator, or object behaves differently depending on outside input or influences. Usually implemented as parameter overloading where the same named function is overloaded with other versions that are called either with a different type or number of parameters. Polymorphism is a general coding technique and other specific implementations are common such as inheritance, operator overloading, and interfaces.

Languages Focus

Many languages support built-in polymorphism such as a "+" operator that can add both integers and decimals. The following documents the ability to implement developer defined polymorphism.

Delphi: 

Delphi supports the following types of polymorphism:

More Info / Comment
C#: 

C# supports the following types of polymorphism:

More Info / Comment




Prevent Derivation

[Other Languages] 

Languages Focus

How do you prevent another class from inheriting and/or prevent a class from overriding a member.

Delphi:  "Sealed class" sealed, final

With Delphi, use the sealed keyword to prevent a class from being inherited from and use the final keyword to prevent a method from being overridden.

Syntax Example:
type
  Robot = class sealed(TObject)
public
  procedure Speak(pSentence: String); virtual; final;
end;
C#:   sealed

With C#, use the sealed keyword to prevent a class from being inherited from and to prevent a method from being overridden.

A method marked sealed must override an ancestor method. If you mark a class sealed, all members are implicitly not overridable so the sealed keyword on members is not legal.

Syntax Example:
public class Machine : System.Object
{
public virtual void Speak(String pSentence)
{
MessageBox.Show(pSentence);
}
}

public class Robot : Machine
{
public sealed override void Speak(String pSentence)
{
MessageBox.Show(pSentence);
}
}
  
public sealed class Cyborg : Robot
{
}




Static Member

[Other Languages] 

General Info: Static Class / Static Member

A static member is a member you can have access to without instantiating the class into an object. For example, you can read and write static properties and call static methods without ever creating the class. Static members are also called class members (class methods, class properties, etc.) since they belong to the class and not to a specific object. A static class is a class that contains only static members. In the UML, these classes are described as utility classes.

Languages Focus

Languages that support static members usually at least support static member fields (the data). Some languages also support static methods, properties, etc. in which case the class member is held in memory at one location and shared with all objects. Finally, some languages support static classes which usually means the compiler will make sure a static class contains only static members.

Delphi:  "Class Members" Class

Object Pascal supports static methods, but not static member fields. For static member fields, use traditional Pascal-like global variables.

Since Object Pascal is a hybrid language, you can use global functions and data so the need for class methods is diminished but still useful. For example, since Object Pascal does not have automatic reference counting, you could use a class method to keep track of the number of object instances.

Delphi 1-7: All classes in a unit are friendly (see eachother's private members), some developers like to put each class in it's own unit and reserve putting multiple classes in the same unit until they wish to implement friendly classes.

Delphi 2005+: New strict keyword allows you to indicate friendly.

Delphi Prism: The Strict keyword was introduced from the beginning (Delphi.Net preview that shipped with D7).

Syntax Example:
type
TMyUtils = class(TObject)
public
class function MyStaticMethod: Integer;
end;

In implimentation:

class function TMyUtils.MyStaticMethod: Integer;
C#:  "Static Members" static

C# supports both static members and static classes using the static keyword. You can add a static method, field, property, or event to an existing class. Also, you can designate a class as static and the compiler will ensure all members in that class are static. You can add a constructor to a static class to initialize values.

The CLR automatically loads static classes with the program or namespace.

Syntax Example:
//Static Class Example
public static class MyStaticClass
{
  //Static Method Example
  public static void MyStaticMethod()
{
// static method code
}
}





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