Overview of Properties

Introduction

In C++ and Java, when creating the member variables of a class, programmers usually "hide" these members in private sections (C++) or create them as private (Java). This technique makes sure that a member variable is not accessible outside of the class so that the clients of the class cannot directly influence the value of the member variable. If you create a member variable as private but still want other classes to access or get the value of such a field, you must then create one or two "accessories", like a door in which the external classes must pass through to access the field.

A property is a member of a class that plays an intermediary role to a field of the class. For example, if you have a field of class and that member represents the salary of an employee, a property can be the "door" that other classes that need the salary must present their requests to. As such, these external classes cannot just change the salary or retrieve it as they wish. A property can be used to validate their request, to reject or to accept them.

A property is used to "filter" access to a field of a class. Therefore, you start by declaring a (private (if you don't make it private, you may be deceiving the purpose of creating a property)) field. Here is an example:

using System;

public class Square
{
    private double _side;
}

public class Exercise
{
    public static int Main()
    {
        return 0;
    }
}

Obviously this private field cannot be accessed by an outside class. To let the outside classes access this variable, you would/can create a property. To indicate that you are creating a property, there is a syntax you must follow. To start, you must create a member whose formula resembles a method without the parentheses. Since or if the property will be accessed by only by objects of the same program, you can mark it with the internal keyword. If the property will be accessed by objects of this and other programs, you should mark it as public. Therefore, you would start a property as follows:

public class Square
{
    private double _side;

    // This is a new property
    public double Side
    {
    }
}

With regards to their role, there are two types of properties.

1. Start Microsoft Visual C# and create a Console Application named DepartmentStore2
2. To create a new class, on the main menu, click Project -> Add Class...
3. Set the Name to ShoppingItem and press Enter
4. Change the content of the file as follows:
    

   using System; using System.Collections.Generic; using System.Linq; using System.Text; namespace DepartmentStore2 { class DepartmentStore { private long itemNo; private string nm; private string sz; private decimal price; } }

5. To create a property for each member variable, change the ShoppingItem class as follows:
    

   using System; using System.Collections.Generic; using System.Linq; using System.Text; namespace DepartmentStore2 { class ShoppingItem { private string itemNo; private string nm; private string sze; private decimal price; // A property for the stock number of an item public string ItemNumber { } // A property for the name of an item public string Name { } // A property for size of a merchandise public string Size { } // A property for the marked price of an item public decimal UnitPrice { } } }

6. Save the file

public class Square
{
    private double _side;

    // This is a new property
    public double Side
    {
        get
        {
        }
    }
}

In the body of the get clause, you can implement the behavior that would be used to make the field's value available outside. The simplest way consists of just returning the corresponding field. Here is an example:

public class Square
{
    private double _side;

    // This is a new property
    public double Side
    {
        get
        {
            return _side;
        }
    }
}

A read property is also referred to as read-only property because the clients of the class can only retrieve the value of the property but they cannot change it. Therefore, if you create (only) a read property, you should provide the users with the ability to primarily specify the value of the member variable. To do this, you can create an accessory method or a constructor for the class . Here is an example of such a constructor:

public class Square
{
    private double _side;

    // This is a new property
    public double Side
    {
        get
        {
            return _side;
        }
    }

    public Square(double s)
    {
        _side = s;
    }
}

Once a read property has been created, other classes can access it, for example they can read its value as follows:

using System;

public class Square
{
    private double _side;

    // This is a new property
    public double Side
    {
        get
        {
            return _side;
        }
    }

    public Square(double s)
    {
        _side = s;
    }
}

public class Exercise
{
    public static int Main()
    {
        var sq = new Square(-25.55);

        Console.WriteLine("Square Side: {0}", sq.Side);
        return 0;
    }
}

This would produce:

Square Side: -25.55

Press any key to continue...

We described a property as serving as a door from outside to its corresponding field, preventing those outside classes to mess with the member variable. Notice that the Square class was given a negative value for the member variable, which is usually unrealistic for the side of a square. In this case and others, while still protecting the field as private, you can use the read property to reset the value of the field or even to reject it. To provide this functionality, you can create a conditional statement in the read property to perform a checking process. Here is an example:

using System;

public class Square
{
    private double _side;

    // This is a new property
    public double Side
    {
        get
        {
            // If the value given to the side is negative,
            // then set it to 0
            if (_side < 0)
                return 0;
            else
                return _side;
        }
    }

    public Square(double s)
    {
        _side = s;
    }
}

public class Exercise
{
    public static int Main()
    {
        var sq1 = new Square(-12.48);
        var sq2 = new Square(25.55);

        Console.WriteLine("First Square Characteristics");
        Console.WriteLine("Side:      {0}\n", sq1.Side);

        Console.WriteLine("Second Square Characteristics");
        Console.WriteLine("Side:      {0}\n", sq2.Side);
        return 0;
    }
}

This would produce:

First Square Characteristics
Side:      0

Second Square Characteristics
Side:      25.55

1. To create read properties, change the contents of the ShoppingStore.cs file as follows:
    

   using System; using System.Collections.Generic; using System.Linq; using System.Text; namespace DepartmentStore2 { class ShoppingItem { private long itemNo; private string nm; private string sz; private decimal price; // A property for the stock number of an item public long ItemNumber { get { return itemNo; } } // A property for the name of an item public string Name { get { return nm; } } // A property for size of a merchandise public string Size { get { if( sz == "0" ) return "Unknown Size or Fits All"; else return sz; } } // A property for the marked price of an item public decimal UnitPrice { get { return price; } } // A constructor used to initialize an item public ShoppingItem(long nbr, string nme, string siz, decimal prc) { itemNo = nbr; nm = nme; sz = siz; price = prc; } } }

2. Access the Program.cs file and change it as follows:
    

   using System; using System.Collections.Generic; using System.Linq; using System.Text; namespace DepartmentStore2 { class Program { static void Main() { long number; string name; string size; decimal price; Console.WriteLine("/-/Arrington Department Store/-/"); Console.Write("Enter the Item #: "); number = long.Parse(Console.ReadLine()); Console.Write("Enter the Item Name: "); name = Console.ReadLine(); Console.Write("Enter the Item Size: "); size = Console.ReadLine(); Console.Write("Enter the Unit Price: "); price = decimal.Parse(Console.ReadLine()); ShoppingItem store = new ShoppingItem(number, name, size, price); Console.WriteLine("\n================================"); Console.WriteLine("/-/Arrington Department Store/-/"); Console.WriteLine("--------------------------------"); Console.WriteLine("Customer Invoice"); Console.WriteLine("Item #: {0}", store.ItemNumber); Console.WriteLine("Description: {0}", store.Name); Console.WriteLine("Item Size: {0}", store.Size); Console.WriteLine("Unit Price: {0:C}", store.UnitPrice); Console.WriteLine("================================\n"); } } }

3. Execute the program:
    

   /-/Arrington Department Store/-/ Enter the Item #: 622805 Enter the Item Name: Black Leather Hand Bag Enter the Item Size: Medium Enter the Unit Price: 85.95 ================================ /-/Arrington Department Store/-/ -------------------------------- Customer Invoice Item #: 622805 Description: Black Leather Hand Bag Item Size: Medium Unit Price: $85.95 ================================ Press any key to continue . . .

4. Close the DOS window

To create a write property, type the set keyword followed by the curly bracket delimiters. Here is an example:

public class Square
{
    private double _side;

    // This is a new property
    public double Side
    {
        set
        {
        }
    }
}

The minimum assignment you can perform with a write property is to assign it a value that would be provided by the outside world. To support this, C# provides the value contextual keyword (contextual means the word is a keyword only in some cases, depending on how it is being used). Here is an example:

public class Square
{
    private double _side;

    // This is a new property
    public double Side
    {
        set
        {
            _side = value;
        }
    }
}

As you see, clients of a class can change the corresponding field of a member variable through the property writer. Based on this relationship, it is not unusual for a client of a class to make an attempt to "mess" with a field. For example, an external object can assign an invalid value to a member variable. Consider the following program:

using System;

public class Square
{
    public double _side;

    // This is a new property
    public double Side
    {
        set
        {
            _side = value;
        }
    }

    public Square()
    {
        _side = 0;
    }

    public Square(double s)
    {
        _side = s;
    }

    public double Perimeter()
    {
        return _side * 4;
    }

    public double Area()
    {
        return _side * _side;
    }
}

public class Exercise
{
    public static int Main()
    {
        var sq1 = new Square();
        var sq2 = new Square();

        sq1._side = -12.48;
        sq2._side = 25.55;

        Console.WriteLine("First Square Characteristics");
        Console.WriteLine("Side:      {0}", sq1._side);
        Console.WriteLine("Perimeter: {0}", sq1.Perimeter());
        Console.WriteLine("Area:      {0}\n", sq1.Area());

        Console.WriteLine("Second Square Characteristics");
        Console.WriteLine("Side:      {0}", sq2._side);
        Console.WriteLine("Perimeter: {0}", sq2.Perimeter());
        Console.WriteLine("Area:      {0}", sq2.Area());

        return 0;
    }
}

This would produce:

First Square Characteristics
Side:      -12.48
Perimeter: -49.92
Area:      155.7504

Second Square Characteristics
Side:      25.55
Perimeter: 102.2
Area:      652.8025

Press any key to continue...

Because of this, and since it is through the writer that the external objects would change the value of the member variable, you can use the write property, rather than the reader, to validate or reject a new value assigned to the field. Remember that the client objects of the class can only read the value of the field through the read property. Therefore, there may be only little concern on that side.

1. To create property writers and complete the program, change the content of the ShoppingItem.cs file as follows:
    

   using System; using System.Collections.Generic; using System.Linq; using System.Text; namespace DepartmentStore2 { class ShoppingItem { private long itemNo; private string nm; private string sz; private decimal price; // A property for the stock number of an item public long ItemNumber { get { return itemNo; } set { if (itemNo <= 0) itemNo = 0; else itemNo = value; } } // A property for the name of an item public string Name { get { return nm; } set { if (nm == "") nm = "Item no Description"; else nm = value; } } // A property for size of a merchandise public string Size { get { if( sz == "0" ) return "Unknown Size or Fits All"; else return sz; } set { sz = value; } } // A property for the marked price of an item public decimal UnitPrice { get { return price; } set { if (price < 0) price = 0.00M; else price = value; } } public static ShoppingItem Read() { ShoppingItem shop = new ShoppingItem(); Console.Write("Item #: "); shop.itemNo = long.Parse(Console.ReadLine()); Console.Write("Item Name: "); shop.Name = Console.ReadLine(); Console.Write("Item Size (Enter 0 if unknown): "); shop.Size = Console.ReadLine(); Console.Write("Unit Price: "); shop.UnitPrice = decimal.Parse(Console.ReadLine()); return shop; } public static void Write(ShoppingItem item) { Console.WriteLine("Item #: {0}", item.ItemNumber); Console.WriteLine("Description: {0}", item.Name); Console.WriteLine("Item Size: {0}", item.Size); Console.WriteLine("Unit Price: {0:C}", item.UnitPrice); } } }

2. Access the Program.cs file and change it as follows:
    

   using System; using System.Collections.Generic; using System.Linq; using System.Text; namespace DepartmentStore2 { class Program { static void Main() { Console.WriteLine("/-/Arrington Department Store/-/"); Console.Write("Enter the following pieces of "); Console.WriteLine("information about the sale item"); ShoppingItem saleItem = ShoppingItem.Read(); Console.WriteLine("\n================================"); Console.WriteLine("/-/Arrington Department Store/-/"); Console.WriteLine("--------------------------------"); ShoppingItem.Write(saleItem); } } }

3. Execute the program. Here is an example:
    

   /-/Arrington Department Store/-/ Enter the following pieces of information about the sale item Item #: 114888 Item Name: North Hook Alpine Jacket Item Size (Enter 0 if unknown): Large Unit Price: 275.95 ================================ /-/Arrington Department Store/-/ -------------------------------- Item #: 114888 Description: North Hook Alpine Jacket Item Size: Large Unit Price: $275.95 Press any key to continue . . .

4. Close the DOS window

You can create a property as a Boolean type. To do this, first specify its data type as bool. When treating the property, make sure its get accessory returns a Boolean type. A Boolean property is not like the other regular properties. It must specify its value as true or false. As done for Boolean methods, a Boolean property must produce only a true or false value.

An enumeration is a technique of creating data type that mimics an integer. After creating it, you can treat it as a pseudo data type. You can declare a variable from it, you can pass it as an argument, and you can return it from a method. Based on these characteristics, you can create a property that is based on an enumeration.

To create an enumeration property, you use the same formula as one of the primitive data types we have used previously. Keep it mind that the property is of the type of an enumeration. This means that you cannot request its value like that of a primitive type and, when manipulating it, you must process it appropriately.

1. Access the ShoppingStore.cs file and change it as follows:
    

   using System; using System.Collections.Generic; using System.Linq; using System.Text; namespace DepartmentStore2 { public enum ItemCategory { Unspecified, Women, Men, Girls, Boys, Babies } class ShoppingItem { private long itemNo; private ItemCategory cat; private string nm; private string sz; private decimal price; // A property for the stock number of an item public long ItemNumber { get { return itemNo; } set { if (itemNo <= 0) itemNo = 0; else itemNo = value; } } // A property for the category of item public ItemCategory Category { get { return cat; } set { cat = value; } } // A property for the name of an item public string Name { get { return nm; } set { if (nm == "") nm = "Item no Description"; else nm = value; } } // A property for size of a merchandise public string Size { get { if( sz == "0" ) return "Unknown Size or Fits All"; else return sz; } set { sz = value; } } // A property for the marked price of an item public decimal UnitPrice { get { return price; } set { if (price < 0) price = 0.00M; else price = value; } } public static ShoppingItem Read() { int category = 0; ShoppingItem shop = new ShoppingItem(); Console.Write("Item #: "); shop.itemNo = long.Parse(Console.ReadLine()); Console.WriteLine("Store Items Categories"); Console.WriteLine("\t1. Women"); Console.WriteLine("\t2. Men"); Console.WriteLine("\t3. Girls"); Console.WriteLine("\t4. Boys"); Console.WriteLine("\t5. Babies"); Console.Write("Enter the Category: "); category = int.Parse(Console.ReadLine()); if (category == 1) shop.Category = ItemCategory.Women; else if (category == 2) shop.Category = ItemCategory.Men; else if (category == 3) shop.Category = ItemCategory.Girls; else if (category == 4) shop.Category = ItemCategory.Boys; else if (category == 5) shop.Category = ItemCategory.Babies; else shop.Category = ItemCategory.Unspecified; Console.Write("Item Name: "); shop.Name = Console.ReadLine(); Console.Write("Item Size (Enter 0 if unknown): "); shop.Size = Console.ReadLine(); Console.Write("Unit Price: "); shop.UnitPrice = decimal.Parse(Console.ReadLine()); return shop; } public static void Write(ShoppingItem item) { Console.WriteLine("Item #: {0}", item.ItemNumber); Console.WriteLine("Category: {0}", item.Category); Console.WriteLine("Description: {0}", item.Name); Console.WriteLine("Item Size: {0}", item.Size); Console.WriteLine("Unit Price: {0:C}", item.UnitPrice); } } }

2. Execute the application and test it. Here is an example:
    

   /-/Arrington Department Store/-/ Enter the following pieces of information about the sale item Item #: 624008 Store Items Categories 1. Women 2. Men 3. Girls 4. Boys 5. Babies Enter the Category: 3 Item Name: Scotta Miniskirt Item Size (Enter 0 if unknown): 11 Unit Price: 35.95 ================================ /-/Arrington Department Store/-/ -------------------------------- Item #: 624008 Category: Girls Description: Scotta Miniskirt Item Size: 11 Unit Price: $35.95 Press any key to continue . . .

3. Close the DOS window

Remember that, after creating a class, it becomes a data type in its own right. We have seen that you could declare a variable from it, you could pass it as argument, and you could return it from a method. As a normal data type, a class can be validated. This means that its value can be evaluated, rejected, or retrieved. Based on these characteristics of a class, you can create a property from it.

To create a property that is based on a class, primarily follow the same formulas we have applied to the other properties. The most important aspect to remember is that the class is composite. That is, it is (likely) made of fields of various types.

1. Access the ShoppingItem.cs file and change it as follows:
    

   using System; using System.Collections.Generic; using System.Linq; using System.Text; namespace DepartmentStore2 { public enum ItemCategory { Unspecified, Women, Men, Girls, Boys, Babies } class ShoppingItem { private long itemNo; private ItemCategory cat; private string nm; private string sz; private decimal price; // A property for the stock number of an item public long ItemNumber { get { return itemNo; } set { itemNo = value; } } // A property for the category of item public ItemCategory Category { get { return cat; } set { cat = value; } } // A property for the name of an item public string Name { get { return nm; } set { if (nm == "") nm = "Item no Description"; else nm = value; } } // A property for size of a merchandise public string Size { get { if (sz == "0") return "Unknown Size or Fits All"; else return sz; } set { sz = value; } } // A property for the marked price of an item public decimal UnitPrice { get { return price; } set { if (price < 0) price = 0.00M; else price = value; } } } }

2. To create a new class, in the Solution Explorer, right-click the name of the project, position the mouse on Add and click Class...
3. Set the Name to DepartmentStore and click Add
4. Change the file as follows:
    

   using System; using System.Collections.Generic; using System.Linq; using System.Text; namespace DepartmentStore2 { class DepartmentStore { private int qty; private ShoppingItem itm; public int Quantity { get { return qty; } set { if (qty <= 0) qty = 0; else qty = value; } } public ShoppingItem SaleItem { get { return itm; } set { if (itm == null) { itm.ItemNumber = 0; itm.Category = ItemCategory.Unspecified; itm.Name = "Unknown"; itm.Size = "0"; itm.UnitPrice = 0.00M; } else itm = value; } } public DepartmentStore() { itm = new ShoppingItem(); } public void ProcessOrder() { int category; Console.WriteLine("/-/Arrington Department Store/-/"); Console.WriteLine("Enter the following pieces of"); Console.WriteLine("information about the sale item"); Console.Write("Item #: "); itm.ItemNumber = long.Parse(Console.ReadLine()); Console.WriteLine("Store Items Categories"); Console.WriteLine("\t1. Women"); Console.WriteLine("\t2. Men"); Console.WriteLine("\t3. Girls"); Console.WriteLine("\t4. Boys"); Console.WriteLine("\t5. Babies"); Console.Write("Enter the Category: "); category = int.Parse(Console.ReadLine()); if (category == 1) itm.Category = ItemCategory.Women; else if (category == 2) itm.Category = ItemCategory.Men; else if (category == 3) itm.Category = ItemCategory.Girls; else if (category == 4) itm.Category = ItemCategory.Boys; else if (category == 5) itm.Category = ItemCategory.Babies; else itm.Category = ItemCategory.Unspecified; Console.Write("Item Name: "); itm.Name = Console.ReadLine(); Console.Write("Item Size (Enter 0 if unknown): "); itm.Size = Console.ReadLine(); Console.Write("Unit Price: "); itm.UnitPrice = decimal.Parse(Console.ReadLine()); Console.Write("How many samples of "); Console.Write(itm.Name); Console.Write(": "); qty = int.Parse(Console.ReadLine()); } public void DisplayReceipt() { decimal totalPrice = itm.UnitPrice * Quantity; Console.WriteLine("\n================================"); Console.WriteLine("/-/Arrington Department Store/-/"); Console.WriteLine("--------------------------------"); Console.WriteLine("Item #: {0}", itm.ItemNumber); Console.WriteLine("Category: {0}", itm.Category); Console.WriteLine("Description: {0}", itm.Name); Console.WriteLine("Item Size: {0}", itm.Size); Console.WriteLine("Unit Price: {0:C}", itm.UnitPrice); Console.WriteLine("Quantity: {0}", Quantity); Console.WriteLine("Total Price: {0:C}\n", totalPrice); Console.WriteLine("\n================================"); } } }

5. Access the Program.cs file and change it as follows:
    

   using System; using System.Collections.Generic; using System.Linq; using System.Text; namespace DepartmentStore2 { class Program { static void Main() { DepartmentStore store = new DepartmentStore(); store.ProcessOrder(); store.DisplayReceipt(); } } }

6. Execute the application and test it. Here is an example:
    

   /-/Arrington Department Store/-/ Enter the following pieces of information about the sale item Item #: 444412 Store Items Categories 1. Women 2. Men 3. Girls 4. Boys 5. Babies Enter the Category: 1 Item Name: Stretch Cotton Shirt Item Size (Enter 0 if unknown): 14 Unit Price: 55.95 How many samples of Stretch Cotton Shirt: 2 ================================ /-/Arrington Department Store/-/ -------------------------------- Item #: 444412 Category: Women Description: Stretch Cotton Shirt Item Size: 14 Unit Price: $55.95 Quantity: 2 Total Price: $111.90 ================================ Press any key to continue . . .

7. Close the DOS window