Understanding Properties in C#

Reference from http://www.c-sharpcorner.com/UploadFile/rajeshvs/PropertiesInCS11122005001040AM/PropertiesInCS.aspx

 

In C#, properties are nothing but natural extension of data fields. They are usually known as 'smart fields' in C# community. We know that data encapsulation and hiding are the two fundamental characteristics of any object oriented programming language.In C#, data encapsulation is possible through either classes or structures. By using various access modifiers like private, public, protected, internal etc it is possible to control the accessibility of the class members. 

Usually inside a class, we declare a data field as private and will provide a set of public SET and GET methods to access the data fields. This is a good programming practice, since the data fields are not directly accessible out side the class. We must use the set/get methods to access the data fields. 

An example, which uses a set of set/get methods, is shown below. 

//SET/GET methods
//Author: rajeshvs@msn.com
using System;
class MyClass
{
private int x;
public void SetX(int i)
{
x = i;
}
public int GetX()
{
return x;
}
}
class MyClient
{
public static void Main()
{
MyClass mc = new MyClass();
mc.SetX(10);
int xVal = mc.GetX();
Console.WriteLine(xVal);//Displays 10
}
}

But C# provides a built in mechanism called properties to do the above. In C#, properties are defined using the property declaration syntax. The general form of declaring a property is as follows.          

<acces_modifier> <return_type> <property_name>
{
get
{
}
set
{
}
}

Where <access_modifier> can be private, public, protected or internal. The <return_type> can be any valid C# type. Note that the first part of the syntax looks quite similar to a field declaration and second part consists of a get accessor and a set accessor. 

For example the above program can be modifies with a property X as follows.      

class MyClass
{
private int x;
public int X
{
get
{
return x;
}
set
{
x = value;
}
}
}

The object of the class MyClass can access the property X as follows.

MyClass mc = new MyClass();

mc.X = 10; // calls set accessor of the property X, and pass 10 as value of the standard field 'value'.
This is used for setting value for the data member x.
Console.WriteLine(mc.X);// displays 10. Calls the get accessor of the property X. 

The complete program is shown below. 

//C#: Property
//Author: rajeshvs@msn.com
using System;
class MyClass
{
private int x;
public int X
{
get
{
return x;
}
set
{
x = value;
}
}
}
class MyClient
{
public static void Main()
{
MyClass mc = new MyClass();
mc.X = 10;
int xVal = mc.X;
Console.WriteLine(xVal);//Displays 10
}
} 
 
Remember that a property should have at least one accessor, either set or get. The set accessor has a free variable available in it called value, which gets created automatically by the compiler. We can't declare any variable with the name value inside the set accessor.

We can do very complicated calculations inside the set or get accessor. Even they can throw exceptions. 

Since normal data fields and properties are stored in the same memory space, in C#, it is not possible to declare a field and property with the same name. 

Static Properties

C# also supports static properties, which belongs to the class rather than to the objects of the class. All the rules applicable to a static member are applicable to static properties also. 

The following program shows a class with a static property. 

//C# : static Property
//Author: rajeshvs@msn.com
using System;
class MyClass
{
private static int x;
public static int X
{
get
{
return x;
}
set
{
x = value;
}
}
}
class MyClient
{
public static void Main()
{
MyClass.X = 10;
int xVal = MyClass.X;
Console.WriteLine(xVal);//Displays 10
}
}  

Remember that set/get accessor of static property can access only other static members of the class. Also static properties are invoking by using the class name. 

Properties & Inheritance 

The properties of a Base class can be inherited to a Derived class. 

//C# : Property : Inheritance
//Author: rajeshvs@msn.com
using System;
class Base
{
public int X
{
get
{
Console.Write("Base GET");
return 10;
}
set
{
Console.Write("Base SET");
}
}
}
class Derived : Base
{
}
class MyClient
{
public static void Main()
{
Derived d1 = new Derived();
d1.X = 10;
Console.WriteLine(d1.X);//Displays 'Base SET Base GET 10'
}
}

The above program is very straightforward. The inheritance of properties is just like inheritance any other member. 

Properties & Polymorphism 

A Base class property can be polymorphicaly overridden in a Derived class. But remember that the modifiers like virtual, override etc are using at property level, not at accessor level. 

//C# : Property : Polymorphism
//Author: rajeshvs@msn.com
using System;
class Base
{
public virtual int X
{
get
{
Console.Write("Base GET");
return 10;
}
set
{
Console.Write("Base SET");
}
}
}
class Derived : Base
{
public override int X
{
get
{
Console.Write("Derived GET");
return 10;
}
set
{
Console.Write("Derived SET");
}
}
}
class MyClient
{
public static void Main()
{
Base b1 = new Derived();
b1.X = 10;
Console.WriteLine(b1.X);//Displays 'Derived SET Derived GET 10'
}
}

Abstract Properties 

A property inside a class can be declared as abstract by using the keyword abstract. Remember that an abstract property in a class carries no code at all. The get/set accessors are simply represented with a semicolon. In the derived class we must implement both set and get assessors. 

If the abstract class contains only set accessor, we can implement only set in the derived class. 

The following program shows an abstract property in action. 

//C# : Property : Abstract
//Author: rajeshvs@msn.com
using System;
abstract class Abstract
{
public abstract int X
{
get;
set;
}
}
class Concrete : Abstract
{
public override int X
{
get
{
Console.Write(" GET");
return 10;
}
set
{
Console.Write(" SET");
}
}
}
class MyClient
{
public static void Main()
{
Concrete c1 = new Concrete();
c1.X = 10;
Console.WriteLine(c1.X);//Displays 'SET GET 10'
}
} 

The properties are an important features added in language level inside C#. They are very useful in GUI programming. Remember that the compiler actually generates the appropriate getter and setter methods when it parses the C# property syntax.

转载于:https://www.cnblogs.com/sandy_liao/archive/2010/12/02/1894275.html