What is the purpose of accessors?

Can somebody help me understand the get & set?

Why are开发者_高级运维 they needed? I can just make a public variable.

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Warning: I am assuming you already know about object-oriented programming.

What are properties?

Properties are language elements that allow you to avoid the repetitive getXYZ() accessors and setXYZ() mutators techniques found in other languages, like Java.

Why do they exist?

They aim to solve the following problems:

1. Saying get and set in the beginning of every access or mutation of a value is annoying and distracting.

   In Java, you often say:

   class person
   {
       private int _age;
       public void setAge(int value) { /*check value first, then set _age*/ }
       public int getAge() { return this._age; }
   }
   

   and then consistently say:

   if (person.getAge() > blah || person.getAge() < 10)
   {
       person.setAge(5);
   }
   

   After a while, the get and set become rather annoying.

2. Providing direct access to the actual variable breaks encapsulation, so that's not an option.

How are they used?

They are used just like variables. You read/write to them just like variables.

How are they created?

They are created as methods. You define a pair of methods that:

1. Return the current value of the property. Oftentimes, this is nothing more than something like the following:

   class Person
   {
       private int _age; //Declare the backing field
   
       public int Age
       {
           get { return this._age; }
           set { ... }
       }
   }
   

2. Set the value of the property:

   class Person
   {
       public int Age
       {
           get { ... }
           set
           {
               if (value < 0) //'value' is what the user provided
               { throw new ArgumentOutOfRangeException(); } //Check validity
               this._age = value;
           }
       }
   }
   

Other notes:

Auto-implemented Properties

C# 3.0 introduced auto-implemented properties:

public int Age { get; set; }

This is equivalent to:

private int _age; //The name is auto-generated
public int Age { get { return this._age; } set { this._age = value; } }

Why does it exist?

It helps you avoiding breaking changes in client executables.

Let's say you're lazy and don't want to type the whole thing, and decide to expose a variable publicly. You then create an executable that reads from or writes to that field. Then you change your mind and decide that you in fact needed a property, so you change it to one.

What happens?

The depending executable breaks, because the code is no longer valid.

Auto-implemented properties help you avoid that, without extra redundancy in your initial code.

Indexers

Indexers extend the property syntax to let you index objects (surprise!), just like arrays.
For C++ users: This is similar to overloading operator [].

Example:

private int[] _elements;

public int this[int index] //Indexed property
{
    get { return this._elements[index]; }
    set
    {
        //Do any checks on the index and value
        this._elements[index] = value;
    }
}

You then use them like obj[5] = 10;, which is equivalent to calling the set method of obj's indexer.
In fact, System.Collections.Generic.List<T> is indexed:

var list = new List<int>();
list.Add(10);
list[0] = 5;  //You're indexing list, as though it were an array!

Isn't that neat? :)

Anything else?

There are many more features to properties, not all of which are available in C#:

• Parametrized properties, of which indexers are a special kind
• Getter/setter access modifiers (in C#)
• Multiple getters or setters (not in C#)
• Et cetera

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They are called Accessors

The accessor of a property contains the executable statements associated with getting (reading or computing) or setting (writing) the property. The accessor declarations can contain a get accessor, a set accessor, or both. The body of the get accessor resembles that of a method. It must return a value of the property type.

http://msdn.microsoft.com/en-us/library/w86s7x04.aspx

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

The set accessor resembles a method whose return type is void. It uses an implicit parameter called value, whose type is the type of the property.

private m_Name;
public string Name {
    get {
        return m_Name;
    }
    set {
        m_Name = value;
    }
}

Then in the incarnation of C# 3, you can do this much easier through auto-properties

public string Name {get; set; } // read and write
public string Name {get; }  // read only
public string Name { get; private set; } //read and parent write

http://msdn.microsoft.com/en-us/library/bb384054.aspx

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Properties act as accessors to the internal state of an object, hiding the implementation of that state.

So, for example, you may have a first name property in a class

public class Example
{
    private string firstName;

    public string FirstName
    {
        get {return this.firstName;}
    }
}

So anyone using the class doesn't need to know how first name is stored, they just know they can get a string representation of it. By adding a set you also add a mutator, something which changes an objects internal state

public class Example
{
    private string firstName;

    public string FirstName
    {
        get {return this.firstName;}
        set {set this.firstName = value;}   
    }
}           

Again you're still isolating how the first name is stored internally (encapsulation), but users can change it by passing in a string.

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Simply put, get and set accessors are the functions called on a Property; that is, when you retrieve the value or when you set it. It forces a type of behavior on the way values are retrieved or set.

For example, you may want to have a mechanism to get/set passwords. Generally speaking, you'll only want to compare the hash of a password instead of storing things plaintext, so you'd have the getter variable retrieve the stored hash, and the setter would take the provided input and hash it for storage.

Here's what I mean:

public class User {
    //Usery properties here, and...
    private string _password;
    public string Password {
        get {
            return _password;
        }
        set {
            _password = SomeHashingFunction(value);
        }
    }
}

value is the variable provided to the setter from what has been given in the variable assignment. e.g.: someuser.Password = "blah";

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Get and set are used in properties. They can each be public, protected, or private. Similar to accessor and mutator methods, they allow some computation when code tries to access/mutate the property. Of course, as long as you define one of get/set, the other is optional.

Example without properties:

private int test;

public int getTest() {
    // some computation on test here, maybe?
    return test;
}

private void setTest(int test) {
    // some error/range checking, maybe?
    this.test = test;
}

With properties:

private int test;
public int Test {
    get {
        // some computation on test here, maybe?
        return test;
    }
    private set {
        // some error/range checking, maybe?
        test = value;   // value is a keyword here
    }
}

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get{} and set{} are accessors that offer up the ability to easily read and write to private fields. Working with a simple example:

public class Foo()
{ 
    //Field
    private int _bar;

    //Property
    public int Bar
    {
        get { return _bar; }
        set { _bar = value; }  
        //value is an implicit parameter to the set acccessor.
        //When you perform an assignment to the property, the value you
        //assign is the value in "value"
    }
}

In this case, Bar is a public property that has a getter and a setter that allows access to the private field _bar that would otherwise be inaccessible beyond class Foo.

Now in a class that has an instace of Foo, you can do this:

public class IHasAFoo()
{
    private Foo _myFoo = new Foo();

    public void SomeMethod()
    {
        _myFoo.Bar = 42;
    }
}

So the public accessor allows you to set the value of the private field back in Foo.

Hope that helps!