Using Properties (C# Programming Guide)

The body of the get accessor is similar to that of a method. It must return a value of the property type. The execution of the get accessor is equivalent to reading the value of the field. For example, when you are returning the private variable from the get accessor and optimizations are enabled, the call to the get accessor method is inlined by the compiler so there is no method-call overhead. However, a virtual get accessor method can not be inlined since the compiler does not know at compile-time which method may actually be called at run time. The following is a get accessor that returns the value of a private field name:

class Person
{
    private string name;  // the name field
    public string Name    // the Name property
    {
        get
        {
            return name;
        }
    }
}

When you reference the property, except as the target of an assignment, the get accessor is invoked to read the value of the property. For example:

Person p1 = new Person();
//...

System.Console.Write(p1.Name);  // the get accessor is invoked here

The get accessor must terminate in a return or throw statement, and control cannot flow off the accessor body.

It is a bad programming style to change the state of the object by using the get accessor. For example, the following accessor produces the side effect of changing the state of the object each time the number field is accessed.

private int number;
public int Number
{
    get
    {
        return number++;   // Don't do this
    }
}

The get accessor can either be used to return the field value or to compute it and return it. For example:

class Employee
{
    private string name;
    public string Name
    {
        get
        {
            return name != null ? name : "NA";
        }
    }
}

In the preceding code segment, if you do not assign a value to the Name property, it will return the value NA.

The set accessor is similar to a method whose return type is void. It uses an implicit parameter called value, whose type is the type of the property. In the following example, a set accessor is added to the Name property:

class Person
{
    private string name;  // the name field
    public string Name    // the Name property
    {
        get
        {
            return name;
        }
        set
        {
            name = value;
        }
    }
}

When you assign a value to the property, the set accessor is invoked with an argument that provides the new value. For example:

Person p1 = new Person();
p1.Name = "Joe";  // the set accessor is invoked here                

System.Console.Write(p1.Name);  // the get accessor is invoked here

It is an error to use the implicit parameter name, value, for a local variable declaration in a set accessor.

Properties can be marked as public, private, protected, internal, or protected internal. These access modifiers define how users of the class can access the property. The get and set accessors for the same property may have different access modifiers. For example, the get may be public to allow read-only access from outside the type, and the set may be private or protected. For more information, see Access Modifiers (C# Programming Guide).

A property may be declared as a static property using the static keyword. This makes the property available to callers at any time, even if no instance of the class exists. For more information, see Static Classes and Static Class Members (C# Programming Guide).

A property may be marked as a virtual property using the virtual keyword. This allows derived classes to override the property behavior using the override keyword. For more information about these options, see Inheritance (C# Programming Guide).

A property overriding a virtual property can also be sealed, specifying that for derived classes it is no longer virtual. Lastly, a property can be declared abstract, meaning there is no implementation in the class, and derived classes must write their own implementation. For more information about these options, see Abstract and Sealed Classes and Class Members (C# Programming Guide).

Note
It is an error to use a virtual (C# Reference), abstract (C# Reference), or override (C# Reference) modifier on an accessor of a static property.

Employee number: 101

Employee name: Claude Vige

Name in the derived class is: John, Manager

Name in the base class is: Mary

The following are important points in the preceding example:

• The property Name in the derived class hides the property Name in the base class. In such a case, the new modifier is used in the declaration of the property in the derived class:

  C#

  Copy

  public new string Name
  
  

• The cast (Employee) is used to access the hidden property in the base class:

  C#

  Copy

  ((Employee)m1).Name = "Mary";
  
  

  For more information on hiding members, see the new Modifier (C# Reference).

abstract class Shape
{
    public abstract double Area
    {
        get;
        set;
    }
}

class Square : Shape
{
    public double side;

    public Square(double s)  //constructor
    {
        side = s;
    }

    public override double Area
    {
        get
        {
            return side * side;
        }
        set
        {
            side = System.Math.Sqrt(value);
        }
    }
}

class Cube : Shape
{
    public double side;

    public Cube(double s)
    {
        side = s;
    }

    public override double Area
    {
        get
        {
            return 6 * side * side;
        }
        set
        {
            side = System.Math.Sqrt(value / 6);
        }
    }
}

class TestShapes
{
    static void Main()
    {
        // Input the side:
        System.Console.Write("Enter the side: ");
        double side = double.Parse(System.Console.ReadLine());

        // Compute the areas:
        Square s = new Square(side);
        Cube c = new Cube(side);

        // Display the results:
        System.Console.WriteLine("Area of the square = {0:F2}", s.Area);
        System.Console.WriteLine("Area of the cube = {0:F2}", c.Area);
        System.Console.WriteLine();

        // Input the area:
        System.Console.Write("Enter the area: ");
        double area = double.Parse(System.Console.ReadLine());

        // Compute the sides:
        s.Area = area;
        c.Area = area;

        // Display the results:
        System.Console.WriteLine("Side of the square = {0:F2}", s.side);
        System.Console.WriteLine("Side of the cube = {0:F2}", c.side);
    }
}

Input

4
24

Enter the side: 4

Area of the square = 16.00

Area of the cube = 96.00

Enter the area: 24

Side of the square = 4.90

Side of the cube = 2.00

Tasks

Reference

Concepts