Properties and Default Properties

[<attributes>] [Default] [accessmodifier] [Shared | Overridable | Overrides | MustOverride | NotOverridable | Shadows]
   [ReadOnly | WriteOnly] Property Name[(parameters)] [As Type] [Implements InterfaceName.Member]
    [<attributes>] [accessmodifier] Get
        ' statements
    End Get
    [<attributes>] [accessmodifier] Set[(ByVal value As Type)]
        ' statements
    End Set
End Property

Option Strict On

Public Class Loan
    Private _borrower As String = ""
    Private _dueOn As Date

    ' A read/write property with validation in both accessors.
    Public Property Borrower As String
        Get
            Return _borrower
        End Get
        Set(value As String)
            If String.IsNullOrWhiteSpace(value) Then
                Throw New ArgumentException("A borrower name is required.", NameOf(value))
            End If
            _borrower = value.Trim()
        End Set
    End Property

    ' A read-only property computed from other state: no backing field.
    Public ReadOnly Property IsOverdue(today As Date) As Boolean
        Get
            Return today.Date > _dueOn.Date
        End Get
    End Property

    Public WriteOnly Property DueOn As Date
        Set(value As Date)
            _dueOn = value
        End Set
    End Property

    ' Older style: assign to the property name inside Get, then Exit Property.
    Public ReadOnly Property Label As String
        Get
            If _borrower.Length = 0 Then
                Label = "(nobody)"
                Exit Property
            End If
            Label = "Loan to " & _borrower
        End Get
    End Property
End Class

Module Program
    Sub Main()
        Dim loan As New Loan()
        Console.WriteLine(loan.Label)
        loan.Borrower = "  Ada Lovelace "
        loan.DueOn = New Date(2026, 10, 1)
        Console.WriteLine(loan.Label)
        Console.WriteLine(loan.IsOverdue(New Date(2026, 10, 19)))
        Try
            loan.Borrower = " "
        Catch ex As ArgumentException
            Console.WriteLine(ex.GetType().Name & ": " & ex.Message.Replace(Environment.NewLine, " | "))
        End Try
        Console.WriteLine("[" & loan.Borrower & "]")
    End Sub
End Module

(nobody)
Loan to Ada Lovelace
True
ArgumentException: A borrower name is required. (Parameter 'value')
[Ada Lovelace]

Option Strict On

Imports System.Reflection

Public Class Author
    Public Property Name As String
    Public Property BornOn As Date?
End Class

Public Class Book
    ' Auto-implemented: initializer, As New, ReadOnly with initializer.
    Public Property Title As String = "Untitled"
    Public Property Year As Integer = 2000
    Public ReadOnly Property Authors As New List(Of Author)
    Public ReadOnly Property Created As Date = New Date(2026, 10, 19)
    Public Property Copies As Integer
    Public ReadOnly Property Shelf As String

    Public Sub New(shelf As String)
        Me.Shelf = shelf          ' a ReadOnly auto-property can be set in the constructor
    End Sub
End Class

Module Program
    Sub Main()
        Dim book As New Book("A-12")
        Console.WriteLine(book.Title & " / " & book.Year & " (" & book.Year.GetType().Name & ") / " & book.Shelf)
        Console.WriteLine(book.Authors.Count & " authors, copies " & book.Copies)

        ' The compiler generates a backing field named _ + property name.
        Dim fields = GetType(Book).GetFields(BindingFlags.Instance Or BindingFlags.NonPublic)
        For Each f In fields.OrderBy(Function(x) x.Name)
            Console.WriteLine("field " & f.Name & " : " & f.FieldType.Name & If(f.IsInitOnly, " (initonly)", ""))
        Next
        Console.WriteLine(GetType(Book).GetProperty("Year")?.PropertyType.Name)
    End Sub
End Module

Untitled / 2000 (Int32) / A-12
0 authors, copies 0
field _Authors : List`1
field _Copies : Int32
field _Created : DateTime
field _Shelf : String
field _Title : String
field _Year : Int32
Int32

Option Strict On

Public Structure Point
    Public X As Integer
    Public Y As Integer
End Structure

Public Class Shelf
    Private _count As Integer

    ' Public Get, Private Set: callers read, only the class writes.
    Public Property Count As Integer
        Get
            Return _count
        End Get
        Private Set(value As Integer)
            _count = value
        End Set
    End Property

    ' An auto-property cannot have accessor blocks, so a narrower setter needs the full form above.
    Public Property Origin As Point
    Public ReadOnly Property Fixed As Point

    Public Sub Add()
        Count += 1
    End Sub

    Public Sub BumpByRef()
        Program.Increment(Count)   ' ByRef to a property: copy-in, call, copy-out through the setter
    End Sub
End Class

Public Module Program
    Friend Sub Increment(ByRef value As Integer)
        value += 1
    End Sub

    Sub Main()
        Dim s As New Shelf()
        s.Add()
        s.Add()
        Console.WriteLine("Count = " & s.Count)

        ' A property of a structure type returns a copy; the compiler rejects
        ' s.Origin.X = 5 (see the error example below). Copy, modify, assign back:
        Dim p = s.Origin
        p.X = 5
        s.Origin = p
        Console.WriteLine("Origin.X = " & s.Origin.X)

        s.BumpByRef()
        Console.WriteLine("after BumpByRef = " & s.Count)

        ' Passing a property ByRef: copy-in/copy-out.
        Dim copy As Integer = s.Count
        Increment(copy)
        Console.WriteLine("local copy " & copy & ", property still " & s.Count)
    End Sub
End Module

Count = 2
Origin.X = 5
after BumpByRef = 3
local copy 4, property still 3

Option Strict On

Public Class Box
    Private _name As String = "stored"
    Public Property Name As String
        Get
            Dim seen = Name
            Return If(seen, "(Nothing)")
        End Get
        Set(value As String)
            Name = value
            _name = value
        End Set
    End Property
End Class

Module Program
    Sub Main()
        Dim b As New Box()
        b.Name = "x"
        Console.WriteLine(b.Name)
    End Sub
End Module

Program.vb(7,24): warning BC42104: Variable 'Name' is used before it has been assigned a value. A null reference exception could result at runtime.
Program.vb(11,13): warning BC42026: Expression recursively calls the containing property 'Public Property Name As String'.

Stack overflow.

Option Strict On

Imports System.Reflection

Public Class Book
    Public ReadOnly Property Title As String
    Public ReadOnly Property Isbn As String

    Public Sub New(title As String, isbn As String)
        Me.Title = title
        Me.Isbn = isbn
    End Sub

    Public Overrides Function ToString() As String
        Return Title
    End Function
End Class

Public Class Shelf
    Private ReadOnly _books As New List(Of Book)

    Public Sub Add(book As Book)
        _books.Add(book)
    End Sub

    ' The Default property: shelf(0) is shelf.Item(0).
    Default Public Property Item(index As Integer) As Book
        Get
            Return _books(index)
        End Get
        Set(value As Book)
            If value Is Nothing Then Throw New ArgumentNullException(NameOf(value))
            _books(index) = value
        End Set
    End Property

    ' An overload of the default property with a String key: shelf!Dune and shelf("Dune") both work.
    Default Public ReadOnly Property Item(isbn As String) As Book
        Get
            Return _books.FirstOrDefault(Function(b) b.Isbn = isbn)
        End Get
    End Property

    ' A non-default parameterized property: always written with its name.
    Public ReadOnly Property ByTitle(text As String, Optional ignoreCase As Boolean = True) As IEnumerable(Of Book)
        Get
            Dim comparison = If(ignoreCase, StringComparison.OrdinalIgnoreCase, StringComparison.Ordinal)
            Return _books.Where(Function(b) b.Title.Contains(text, comparison))
        End Get
    End Property

    ' Two-dimensional parameterized property.
    Public ReadOnly Property Slot(row As Integer, column As Integer) As String
        Get
            Return "R" & row & "C" & column
        End Get
    End Property
End Class

Module Program
    Sub Main()
        Dim shelf As New Shelf()
        shelf.Add(New Book("Dune", "9780441013593"))
        shelf.Add(New Book("Emma", "9780141439587"))

        Console.WriteLine(shelf(0))                       ' default property by index
        Console.WriteLine(shelf.Item(1))                  ' the same, spelled out
        Console.WriteLine(shelf("9780141439587"))         ' overload by String
        shelf(1) = New Book("Persuasion", "9780141439686")
        Console.WriteLine(shelf(1).Title)

        For Each b In shelf.ByTitle("per")
            Console.WriteLine("found " & b.Title)
        Next
        Console.WriteLine(shelf.ByTitle("PER", ignoreCase:=False).Count())
        Console.WriteLine(shelf.Slot(2, 3))

        Try
            Console.WriteLine(shelf(5))
        Catch ex As ArgumentOutOfRangeException
            Console.WriteLine(ex.GetType().Name)
        End Try

        Dim member = GetType(Shelf).GetCustomAttribute(Of DefaultMemberAttribute)()
        Console.WriteLine("DefaultMember = " & member?.MemberName)
        Dim shelves As Shelf() = {shelf}
        Console.WriteLine(shelves(0)(0))                  ' array index first, then the default property
    End Sub
End Module

Dune
Emma
Emma
Persuasion
found Persuasion
0
R2C3
ArgumentOutOfRangeException
DefaultMember = Item
Dune

Option Strict On

Public Interface IPriced
    ReadOnly Property Price As Decimal
    Property Note As String
End Interface

Public MustInherit Class CatalogItem
    Public Property Title As String = ""

    ' MustOverride property: derived classes must supply it.
    Public MustOverride ReadOnly Property Kind As String

    ' Overridable property with a protected setter.
    Public Overridable Property Rating As Integer
        Get
            Return _rating
        End Get
        Protected Set(value As Integer)
            _rating = Math.Clamp(value, 0, 5)
        End Set
    End Property
    Private _rating As Integer

    Public Sub Rate(value As Integer)
        Rating = value
    End Sub
End Class

Public Class Book
    Inherits CatalogItem
    Implements IPriced

    ' ReadOnly in the interface, read/write in the class: allowed (an extra accessor is fine).
    Public Property Price As Decimal Implements IPriced.Price

    ' Different name than the interface member.
    Private _note As String = ""
    Public Property Remark As String Implements IPriced.Note
        Get
            Return _note
        End Get
        Set(value As String)
            _note = value
        End Set
    End Property

    Public Overrides ReadOnly Property Kind As String
        Get
            Return "Book"
        End Get
    End Property

    Public Overrides Property Rating As Integer
        Get
            Return MyBase.Rating
        End Get
        Protected Set(value As Integer)
            MyBase.Rating = value
            Console.WriteLine("rated " & value & " -> " & MyBase.Rating)
        End Set
    End Property
End Class

Module Program
    Sub Main()
        Dim b As New Book() With {.Title = "Dune", .Price = 9.5D, .Remark = "signed"}
        b.Rate(9)
        Console.WriteLine(b.Kind & ": " & b.Title & " " & b.Rating)
        Dim priced As IPriced = b
        Console.WriteLine(priced.Price & " " & priced.Note)
        priced.Note = "reprint"
        Console.WriteLine(b.Remark)
    End Sub
End Module

rated 9 -> 5
Book: Dune 5
9.5 signed
reprint

using System.Diagnostics.CodeAnalysis;

namespace CsLib;

public class Reader
{
    public string Name { get; init; } = "";
    public required string CardNumber { get; init; }
    public int Visits { get; set; }

    public Reader() { }

    [SetsRequiredMembers]
    public Reader(string cardNumber) { CardNumber = cardNumber; }

    // C# indexer: compiled as a property named Item with DefaultMember("Item").
    public string this[int index] => $"slot {index}";
}

public record Member(string Name, int Id);

Option Strict On

Imports CsLib

Module Program
    Sub Main()
        ' Init-only and required properties are set in an object initializer.
        Dim reader As New Reader() With {.Name = "Ada", .CardNumber = "C-1"}

        ' This constructor is marked [SetsRequiredMembers] in the C# source.
        Dim viaCtor As New Reader("C-2")
        Console.WriteLine(reader.Name & " " & reader.CardNumber & " " & viaCtor.CardNumber)

        reader.Visits += 1                    ' an ordinary setter is still fine
        Console.WriteLine(reader.Visits)

        ' A C# indexer is seen as the default property.
        Console.WriteLine(reader(3))

        Dim m As New Member("Grace", 7)
        Console.WriteLine(m.Name & " #" & m.Id)
    End Sub
End Module

Ada C-1 C-2
1
slot 3
Grace #7

BC37311: Init-only property 'Name' can only be assigned by an object member initializer, or on 'Me', 'MyClass` or 'MyBase' in an instance constructor.
BC37321: Required member 'Public Overloads Property CardNumber As String' must be set in the object initializer or attribute arguments.

Option Strict On

Imports System.ComponentModel
Imports System.Runtime.CompilerServices

Public MustInherit Class ObservableObject
    Implements INotifyPropertyChanged

    Public Event PropertyChanged As PropertyChangedEventHandler Implements INotifyPropertyChanged.PropertyChanged

    Protected Sub OnPropertyChanged(<CallerMemberName> Optional propertyName As String = Nothing)
        RaiseEvent PropertyChanged(Me, New PropertyChangedEventArgs(propertyName))
    End Sub

    ' Returns True when the value changed; ByRef lets one helper serve every backing field.
    Protected Function SetField(Of T)(ByRef field As T, value As T,
                                      <CallerMemberName> Optional propertyName As String = Nothing) As Boolean
        If EqualityComparer(Of T).Default.Equals(field, value) Then Return False
        field = value
        OnPropertyChanged(propertyName)
        Return True
    End Function
End Class

Public Class LoanViewModel
    Inherits ObservableObject

    Private _borrower As String = ""
    Private _daysLate As Integer

    Public Property Borrower As String
        Get
            Return _borrower
        End Get
        Set(value As String)
            SetField(_borrower, value)
        End Set
    End Property

    Public Property DaysLate As Integer
        Get
            Return _daysLate
        End Get
        Set(value As Integer)
            If SetField(_daysLate, value) Then
                OnPropertyChanged(NameOf(Fine))        ' dependent, computed property
            End If
        End Set
    End Property

    Public ReadOnly Property Fine As Decimal
        Get
            Return _daysLate * 0.25D
        End Get
    End Property
End Class

Module Program
    Sub Main()
        Dim vm As New LoanViewModel()
        AddHandler vm.PropertyChanged,
            Sub(sender, e)
                Console.WriteLine("changed: " & e.PropertyName)
            End Sub

        vm.Borrower = "Ada"
        vm.Borrower = "Ada"          ' same value: no notification
        vm.DaysLate = 4
        Console.WriteLine("Fine = " & vm.Fine.ToString("N2", Globalization.CultureInfo.InvariantCulture))
    End Sub
End Module

changed: Borrower
changed: DaysLate
changed: Fine
Fine = 1.00