Properties

Properties are methods that act like Fields that allow for getting and setting its value.

They are used in place of fields when the value they contain must be calculated or guarded.
To the user of a property, they appear as a field.
Unlike fields, they are implemented with get/set accessors that define the statements executed when the property is accessed or assigned.
Properties cannot be passed as ref or out parameters.

Creating

Properties with get accessors only are read-only. Properties with both get and set are read-write. Write-only properties are rare and generally used to restrict access to sensitive data.

// This class has a hidden, parameterless constructor.
public class Person 
{
    // This is a backing field:
    private string _firstName;
    private string _location;

    // This is a manual read-only property:
    public string FirstName 
    {
        get { return _firstName; }

        // The *value* parameter holds the value passed to the constructor during instantiation.
        // It's type is the type of the property:
        set { _firstName = value; }
    }

    // Auto-implemented Properties  
    // If one accessor is auto, both must be. The compiler adds a hidden backing field:
    public string LastName { get; set; } // The compiler adds *readonly* to the get accessor.

    // This is an auto read-only property. It will be set to the default value for its type.
    public int Age { get; }

    // Auto-implemented properties can be initialized to a default value:
    public string FavoriteColor { get; set; } = "Red";

    // This property uses expression body syntax definitions:
    public string Location 
    {
        get => _location;
        set => _location = value;
    }
}

Validation

Validation is one of the key uses of properties:

public class Person 
{
    private string _firstName;

    public string FirstName 
    {
        get => _firstName;
        // Set accessors must return void, so errors must be reported by throwing an exception:
        set => _firstName = (!string.IsNullOrWhiteSpace(value)) ? value : throw new ArgumentException("First name must not be blank");
    }
}

Read-only vs. Init-only

Read-only properties can only be set in their constructor, but not via object-initializer syntax:

public string FirstName { get; }
// Illegal: Person p = new Person() { FirstName = "Jane" };
// Legal: Person p = new Person("Jane");

Init-only properties are read-only properties that support object-initializer syntax:

public string FirstName { get; init; }
// Legal: Person p = new Person() { FirstName = "Jane" };

Computed Properties

Properties can return computed values. They must be constructed carefully:

public class Person 
{
    private string _firstName;
    private string _lastName;
    private string _fullName;

    public string FirstName 
    {
        get => _firstName;
        // Each time FirstName is set, also set _fullName to null, which forces it to be computed again:
        set 
        {
            _firstName = value;
            _fullName = null;
        }
    }
    public string LastName 
    {
        get => _lastName;
        // Same as with FirstName:
        set 
        {
            _lastName = value;
            _fullName = null;
        }
    }
    public string FullName 
    {
        get 
        {
            if (_fullName is null) // If it's null, set it; if not, return it:
                _fullName = $"{FirstName} {LastName}";
            
            return _fullName;
        }
    }
}

Using Properties

MyList<string> names = new MyList<string>();
names.Capacity = 100; // Invokes set accessor
int i = names.Count; // Invokes get accessor
int j = names.Capacity; // Invokes get accessor

Property Modifiers

When a property is modified, so are its accessors.
Properties can be static or instance.
Properties can be modified with public, private, protected, internal, protected internal, or private protected.

Other modifiers:

virtual — The property can be overridden in a derived class.
override — Overrides the implementation of a virtual property from the base class.
sealed — A property overriding a virtual property can be sealed so that it is no longer virtual in further derived classes.
abstract — See below.

Required modifier

Availability: C# 11
The required modifier indicates that a field or property it’s applied to must be initialized by an object initializer, so any new instance of the type must initialize all required members.

Documentation: https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/keywords/required

Considerations:

Available on struct, record, record struct, and class types. Not available on interface types.
- A record’s position parameters cannot be modified with the required modifier.
The compiler issues an error if a required member is not initialized.
Required members must be at least as accessible as their containing type.
Required properties must have setters that are at least as accessible as their containing types.
Derived classes cannot hide a required member of the base class.
Derived types that override a required property must include the required modifier.

Accessors (set vs init vs private set)

set — property can be set within this type and consumers of the type.
private set — property can only be set within this type, but immutable to consumers.
- Cannot use an object initializer. Must use a constructor or factory method.
init — property can only be set during instantiation; supports object-initializer syntax.
required — property must be set when an object of the type is created.

Attributes

Required Members

This attribute notifies the compiler that this constructor sets all required members.
Warning: This disables the compiler’s checks that all required members are initialized.

public class Person 
{
    public Person() { }

    // This attribute notifies the compiler that this constructor sets all required members.
    [SetsRequiredMembers] 
    public Person(string firstName) => FirstName = firstName;

    public required string FirstName { get; init; }
}

Field Specifiers

A field specifier indicates that the attribute applies to the backing field of an auto-implemented property:

[field:NonSerialized]
public int Id { get; set; }

Accessor Access Modifiers

Accessors can have access modifiers…

…but only if the property has both get and set accessors…
…and only one of the two can have an access modifier.
If the property has an override modifier, the accessor modifier must match the accessor of the overridden accessor.
The accessibility level on the accessor must be more restrictive than that of the property itself.

Use the private modifier so that an accessor can only be used by methods in the class:

public string Firstname { get; private set; } // Firstname can only be changed by other methods in the class.

Abstract Properties

An abstract property does not provide an implementation of its accessors. This is done by the derived classes:

public abstract class Shape 
{
    private string name;

    public Shape(string s) 
    {
        Id = s; // Calls the set accessor of the Id property.
    }

    public string Id 
    {
        get => name;
        set => name = value;
    }

    // This abstract property must be implemented (overridden) in a derived class.
    public abstract double Area { get; }
}

public class Square : Shape 
{
    public Square(int side, string id) : base(id) 
    {
        this.side = side;
    }

    // Overrides abstract property Area from Shape to implement it.
    public override double Area
    { 
        get { return side * side; }
    }
}

Properties in Interfaces

Interface properties do not have a body.

Declaring the accessors without a body does not declare an auto property like it does in classes and structs.
Interface properties cannot have accessor access modifiers.

public interface ISampleInterface 
{
    string Name { get; set; } // The accessors must be implemented in the deriving type.
}

`INotifyPropertyChanged`

When the value of a property changes, the object raises the INotifyPropertyChanged.PropertyChanged event. Here’s how it’s implemented:

public class Person : INotifyPropertyChanged 
{
    private string _firstName;

    public string FirstName 
    {
        get => _firstName;
        set 
        {
            if (string.IsNullOrWhiteSpace(value))
                throw new ArgumentException("…");
            
            if (value != _firstName) 
            {
                _firstName = value;

                PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(nameof(FirstName)));
            }
        }
    }

    public event PropertyChangedEventHandler PropertyChanged;
}