Introduction

Attributes are implemented with a standard “template” of code
Remember, “attribute” is the abstract concept of some data stored in an object; “instance variable” is the way that data is actually stored
First, declare an instance variable for the attribute
Then write a “getter” method for the instance variable
Then write a “setter” method for the instance variable
With this combination of instance variable and methods, the object has an attribute that can be read (with the getter) and written (with the setter)

For example, this code implements a “width” attribute for the class Rectangle:

class Rectangle
{
    private int width;
    public void SetWidth(int value)
    {
        width = value;
    }
    public int GetWidth()
    {
        return width;
    }
}

Note that there is a lot of repetitive or “obvious” code here:
- The name of the attribute is intended to be “width,” so you must name the instance variable width, and the methods GetWidth and SetWidth, repeating the name three times.
- The attribute is intended to be type int, so you must ensure that the instance variable is type int, the getter has a return type of int, and the setter has a parameter type of int. Similarly, this repeats the data type three times.
- You need to come up with a name for the setter’s parameter, even though it also represents the width (i.e. the new value you want to assign to the width attribute). We usually end up naming it “widthParameter” or “widthParam” or “newWidth” or “newValue.”
Properties are a “shorthand” way of writing this code: They implement an attribute with less repetition.
Note that properties are not present in every object-oriented programming language: for example, Java does not have properties.

Writing properties

Declare an instance variable for the attribute, like before
A property declaration has 3 parts:
- Header, which gives the property a name and type (very similar to variable declaration)
- get accessor, which declares the “getter” method for the property
- set accessor, which declares the “setter” method for the property

Example code, implementing the “width” attribute for Rectangle (this replaces the code in the previous example):

class Rectangle
{
    private int width;
    public int Width
    {
        get
        {
            return width;
        }
        set
        {
            width = value;
        }
    }
}

Header syntax: [public/private] [type] [name]
Convention (not rule) is to give the property the same name as the instance variable, but capitalized – C# is case sensitive
get accessor: Starts with the keyword get, then a method body inside a code block (between braces)
- get is like a method header that always has the same name, and its other features are implied by the property’s header
- Access modifier: Same as the property header’s, i.e. public in this example
- Return type: Same as the property header’s type, i.e. int in this example (so imagine it says public int get())
- Body of get section is exactly the same as body of a “getter”: return the instance variable
set accessor: Starts with the keyword set, then a method body inside a code block
- Also a method header with a fixed name, access modifier, return type, and parameter
- Access modifier: Same as the property header’s, i.e. public in this example
- Return type: Always void (like a setter)
- Parameter: Same type as the property header’s type, name is always “value”. In this case that means the parameter is int value; imagine the method header says public void set(int value)
- Body of set section looks just like the body of a setter: Assign the parameter to the instance variable (and the parameter is always named “value”). In this case, that means width = value

Using properties

Properties are members of an object, just like instance variables and methods
Access them with the “member access” operator, aka the dot operator
- For example, myRect.Width will access the property we wrote, assuming myRect is a Rectangle

A complete example (available as a complete solution), where the “length” and “width” attributes are implemented with properties:

class Rectangle
{
  private int width;
  public int Width
  {
    get { return width; }
    set { width = value; }
  }
  private int length;
  public int Length
  {
    get { return length; }
    set { length = value; }
  }
}

Properties “act like” variables: you can assign to them and read from them
Reading from a property will automatically call the get accessor for that property
- For example,
```
Console.WriteLine(
    $"The width is {myRectangle.Width}");
```
will call the get accessor inside the Width property, which in turn executes return width and returns the current value of the instance variable
- This is equivalent to
```
Console.WriteLine(
    $"The width is {myRectangle.GetWidth()}");
```
using the “old” Rectangle code
Assigning to (writing) a property will automatically call the set accessor for that property, with an argument equal to the right side of the = operator
- For example, myRectangle.Width = 15; will call the set accessor inside the Width property, with value equal to 15
- This is equivalent to myRectangle.SetWidth(15); using the “old” Rectangle code

In More Details

Note that in a property, value is what is called a contextual keyword: it is not a reserved word in C# (it could be used as an identifier), but inside a property it refers to something special, the name of the set method parameter.

In the following code:

private int width;
public int Width
{
    get
    {
        return width;
    }
    set
    {
        width = value;
    }
}

The attribute width is called the Width’s property backing field: it holds the data assigned to the property.

When the property’s get and set accessors are trivial (like the ones above), we can simply omit them their body completely. That is, the previous Width property could be implemented using
```
public int Width { get; set;  }
```
This is called auto-properties. Note that in this case, we do not need to declare the property’s backing field (that is, no need to have private int width;), but cannot refer to it!

Conversely, get and set accessor can contains arbitrarily convoluted code:

public int Length
{
    get { return length; }
    set {
        if (value < 0) {
            length = -value;
        }
        else if (value == 0) {
            length = 1;
        }
        else { 
            length = value;
        }
    }
}

Note however that if the set or get accessor is not the “trivial” one, then auto-properties cannot be used and the other accessor must be specified.
- For example, in the above code, simply writing get; instead of get { return length; } would give a compilation error.

Note that properties can exist without backing field, and they can be read-only (that is, without a set accessor) or write-only (that is, without a get accessor, but this is rarer).

An example of read-only property is as follows:

class Circle
{
    public decimal Diameter { get; set; }
    // The constructor below  sets the value
    // of the property's backing field through
    // the property's set accessor.
    public Circle(decimal dP)
    {
        Diameter = dP;
    }
    // The Radius property below is
    // 1. read-only (no set accessor),
    // 2. without a backing field.
    public decimal Radius {
        get { return Diameter / 2; }
    }
}

It is possible to set a “custom default value” for properties using a property initializer, as follows:
```
public double Width { get; set; } = -1;
```
In this case, the property’s backing field value will be -1 by default. Properties with initializer can be read-only:
```
public int MaximumValue { get; } = 999;
```

Finally, properties can be static as well:

public static string Explanation { get; set; } = "A Circle has for radius its diameter divided by 2.";

Such a property can be accessed using for example

Console.WriteLine(Circle.Explanation);

and its value can be changed, for instance by appending a string to it:

Circle.Explanation += "\nIts circumference is π multiplied by its diameter.";

Properties in UML Class Diagrams

Simple Notation

Since properties represent (or, rather, allow to access) attributes, they go in the “attributes” box (the second box)
If a property will simply “get” and “set” an instance variable of the same name, you do not need to write the instance variable in the box
- No need to write both the property Width and the instance variable width
Syntax: [+/-] <<property>> [name]: [type]
Note that the access modifier (+ or -) is for the property, not the instance variable, so it is + if the property is public (which it usually is)
Example for Rectangle, assuming we converted both attributes to use properties instead of getters and setters:
We no longer need to write all those setter and getter methods, since they are “built in” to the properties

More Accurate Notation

In general, instead of writing for example

+ <<properties>> Explanation: string

one can write

+ <<get, set>> Explanation: string

or even

+ <<set>> Explanation: string
+ <<get>> Explanation: string

The benefit of this notation is that read-only properties can easily be integrated in the UML class diagram, by simply omitting the <<set>> line:

+ <<get>> Radius : decimal

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Explorer

Properties