Properties

In the previous level of programming, you learned that:

• fields define the attributes of objects of the type.
• fields are declared with private access, following the Object-Oriented Programming principle of Encapsulation.
• reading, writing and computing field values (object state) happens in accessor and mutator methods.

C#

public class Employee
{
    private int identificationNumber;

    public int GetIdentificationNumber()
    {
        return this.identificationNumber;
    }

    public void SetIdentificationNumber(int identificationNumber)
    {
        this.identificationNumber = identificationNumber;
    }
}

A C# property is a class member that provides a mechanism to read, write or compute the value of a private field. In many cases, properties eliminate the need for accessor and mutator methods.

Declaring and Defining Properties

A property is defined using get and set accessors:

C#

public class Employee
{
    private int identificationNumber;

    public int IdentificationNumber
    {
        get
        {
            return this.identificationNumber;
        }

        set
        {
            this.identificationNumber = value;
        }
    }
}

get Accessor

To access an object's state, a property must include a get accessor. A get accessor can include any type of statements, but must have a return.

C#

get
{
    return this.identificationNumber;
}

set Accessor

To modify an object's state, a property must include a set accessor. A set accessor can include any type or number of statements. Like an mutator method, to update the state, the field of the class will be assigned a new value. Because properties are not methods, there is no parameter to store the value to assign to the field. Rather, the keyword value is used to store the value assigned to the property.

C#

set
{
    this.identificationNumber = value;
}

Expression Body Definition

The implementation of a property's accessor is often a single-line expression. When the accessor implementation is a single-line expression, the accessor can be defined using a => symbol followed by the expression to assign or retrieve from the property.

C#

public class Employee
{
    private int identificationNumber;

    public int IdentificationNumber
    {
        get => this.identificationNumber;
        set => this.identificationNumber = value;
    }
}

Note

The return keyword is not used with an Expression Body Definition.

Read-only Properties

Properties can be defined with one or two accessors:

1. Read and Write: Access to both accessors.
2. Read-only: Only access to the get accessor.
3. Write-only: Only access to the set accessor.

In the examples above, you seen how properties can be defined with both get and set accessors. If one of the accessors is not required, it can be omitted.

C#

public class Employee
{
    private string name;

    public string Name
    {
        get => this.name;
    }
}

This example defines the Name property as a read-only property. The property is considered read-only because without the set accessor a new state cannot be written to the object. Defining the property this way, the class itself won't be able to set the property.

Danger

The C# language contains the keyword readonly, which has a different meaning than the read-only described above. You won't use readonly in this course.

If an accessor is only required within the class, the private accessor modifier can be used.

C#

public class Employee
{
    private string name;

    public string Name
    {
        get => this.name;
        private set => this.name = value;
    }
}

This example also defines the Name property as read-only, but the class itself will be able to set the property.

Note

It is very rare for a property to be write-only.

Using Properties

Using properties is like using a public field. Assigning a value to the property invokes the set accessor. Using the property in all other expressions will invoke the get accessor.

C#

class Program
{
    static void Main(string[] args)
    {
        Employee employee;

        employee = new Employee();

        // Invokes the set accessor
        employee.IdentificationNumber = 5;

        // Invokes the get accessor
        Console.WriteLine(employee.IdentificationNumber);
    }
}

Auto-Implemented Properties

When the implementation of a property is only to assign a value or retrieve a value from a field without any additional logic, the property can be auto-implemented. An auto-implemented property's accessors will not have a block. Also, a field (called a Backing Field) is automatically created for the class. When the class is compiled, a backing field and implementation is created for the property.

C#

public class Employee
{
    public string Name
    {
        get;
        set;
    }
}

Warning

When a property is auto-implemented, you will not have access to the generated backing field. This includes in the class the property is declared within.

Danger

If you are required to declare a field within the class, you will not be able to auto-implement the associated property.

The previous example is equivalent to:

C#

public class Employee
{
    private string name;

    public string Name
    {
        get
        {
            return this.name;
        }

        set
        {
            this.name = value;
        }
    }
}

Further Reading

• Properties
• Using Properties