Interfaces

[attributes] [accessmodifier] [Shadows] Interface Name [(Of typeparameters)]
    [Inherits InterfaceA, InterfaceB]
    member declarations
End Interface

Option Strict On

Public MustInherit Class CatalogItem
    Public Property Id As Integer
    Public Property Title As String = ""
    Public MustOverride ReadOnly Property Kind As String
End Class

Public Class Book
    Inherits CatalogItem

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

Public Class Magazine
    Inherits CatalogItem

    Public Overrides ReadOnly Property Kind As String
        Get
            Return "Magazine"
        End Get
    End Property
End Class

Public Interface ICatalogStore
    Function GetAll() As IReadOnlyList(Of CatalogItem)
    Sub Add(item As CatalogItem)
    Function Find(id As Integer) As CatalogItem
End Interface

Public Class InMemoryCatalogStore
    Implements ICatalogStore

    Private ReadOnly _items As New List(Of CatalogItem)

    Public Function GetAll() As IReadOnlyList(Of CatalogItem) Implements ICatalogStore.GetAll
        Return _items
    End Function

    Public Sub Add(item As CatalogItem) Implements ICatalogStore.Add
        _items.Add(item)
    End Sub

    Public Function Find(id As Integer) As CatalogItem Implements ICatalogStore.Find
        Return _items.FirstOrDefault(Function(item) item.Id = id)
    End Function
End Class

' A decorator: it is an ICatalogStore and it has one.
Public Class CountingCatalogStore
    Implements ICatalogStore

    Private ReadOnly _inner As ICatalogStore

    Public ReadOnly Property Calls As Integer

    Public Sub New(inner As ICatalogStore)
        _inner = inner
    End Sub

    Public Function GetAll() As IReadOnlyList(Of CatalogItem) Implements ICatalogStore.GetAll
        _Calls += 1
        Return _inner.GetAll()
    End Function

    Public Sub Add(item As CatalogItem) Implements ICatalogStore.Add
        _Calls += 1
        _inner.Add(item)
    End Sub

    Public Function Find(id As Integer) As CatalogItem Implements ICatalogStore.Find
        _Calls += 1
        Return _inner.Find(id)
    End Function
End Class

Public Class Catalog
    Private ReadOnly _store As ICatalogStore

    Public Sub New(store As ICatalogStore)
        _store = store
    End Sub

    Public Sub Add(item As CatalogItem)
        _store.Add(item)
    End Sub

    Public Function Count() As Integer
        Return _store.GetAll().Count
    End Function
End Class

Module Program
    Sub Main()
        Dim counting As New CountingCatalogStore(New InMemoryCatalogStore())
        Dim catalog As New Catalog(counting)

        catalog.Add(New Book With {.Id = 1, .Title = "Domain Modelling"})
        catalog.Add(New Magazine With {.Id = 2, .Title = "Shelf Monthly"})
        Console.WriteLine("items: " & catalog.Count())
        Console.WriteLine("calls on the decorator: " & counting.Calls)

        Dim store As ICatalogStore = counting
        Console.WriteLine("Find(2): " & store.Find(2).Title)
        Console.WriteLine("Find(9) is Nothing: " & (store.Find(9) Is Nothing))
        Console.WriteLine("calls now: " & counting.Calls)

        ' An interface variable lets the compiler check what may be called, nothing more.
        Console.WriteLine("variable type: " & store.GetType().Name)
        Console.WriteLine("is ICatalogStore: " & (TypeOf counting Is ICatalogStore))
        Dim asDisposable = TryCast(store, IDisposable)
        Console.WriteLine("is IDisposable: " & (asDisposable IsNot Nothing))
    End Sub
End Module

items: 2
calls on the decorator: 3
Find(2): Shelf Monthly
Find(9) is Nothing: True
calls now: 5
variable type: CountingCatalogStore
is ICatalogStore: True
is IDisposable: False

Public Class Report
    Inherits ReportBase                    ' at most one base class, first
    Implements INamed, IExporter           ' any number of interfaces
    Implements IPrinter                    ' more can follow on another line
    ...
End Class

Program.vb(9,16): error BC30149: Class 'MemoryStore' must implement 'Function Count() As Integer' for interface 'ICatalogStore'.

Option Strict On

Public Interface ICatalogStore
    Sub Add(title As String)
    Function Count() As Integer
End Interface

Public Class MemoryStore
    Implements ICatalogStore

    Public Sub Add(title As String) Implements ICatalogStore.Add
    End Sub
End Class

Module Program
    Sub Main()
    End Sub
End Module

Option Strict On

Public Interface ILender
    Sub [Return]()
End Interface

Public Interface IReturnable
    Sub [Return]()
End Interface

' Two interfaces declare a member with the same signature. The class
' picks, member by member, what each interface member runs.
Public Class Loan
    Implements ILender, IReturnable

    Public Sub LenderReturn() Implements ILender.Return
        Console.WriteLine("lender side: book is back on the shelf")
    End Sub

    Public Sub BorrowerReturn() Implements IReturnable.Return
        Console.WriteLine("borrower side: receipt printed")
    End Sub

    ' Not an implementation of anything: it is just a method called Return.
    Public Sub [Return]()
        Console.WriteLine("Loan.Return called through the class")
    End Sub
End Class

Public Class SimpleLoan
    Implements ILender, IReturnable

    ' One method can serve both members when the behaviour is the same.
    Public Sub [Return]() Implements ILender.Return, IReturnable.Return
        Console.WriteLine("SimpleLoan.Return")
    End Sub
End Class

Module Program
    Sub Main()
        Dim loan As New Loan()
        DirectCast(loan, ILender).Return()
        DirectCast(loan, IReturnable).Return()
        loan.Return()

        Dim simple As New SimpleLoan()
        Dim lender As ILender = simple
        Dim returnable As IReturnable = simple
        lender.Return()
        returnable.Return()
    End Sub
End Module

lender side: book is back on the shelf
borrower side: receipt printed
Loan.Return called through the class
SimpleLoan.Return
SimpleLoan.Return

Option Strict On

Public Interface IReadable
    Function Read(id As Integer) As String
End Interface

Public Interface IWritable
    Sub Write(id As Integer, text As String)
End Interface

' An interface can inherit several interfaces. Implementing it means implementing all members.
Public Interface IReadWriteStore
    Inherits IReadable, IWritable

    ReadOnly Property Count As Integer
End Interface

Public Class DictionaryStore
    Implements IReadWriteStore

    Private ReadOnly _data As New Dictionary(Of Integer, String)

    Public ReadOnly Property Count As Integer Implements IReadWriteStore.Count
        Get
            Return _data.Count
        End Get
    End Property

    ' The inherited members are named after the interface that declares them.
    Public Function Read(id As Integer) As String Implements IReadable.Read
        Dim text As String = Nothing
        Return If(_data.TryGetValue(id, text), text, "(missing)")
    End Function

    Public Sub Write(id As Integer, text As String) Implements IWritable.Write
        _data(id) = text
    End Sub
End Class

Module Program
    Sub Show(source As IReadable, id As Integer)
        Console.WriteLine(id & " -> " & source.Read(id))
    End Sub

    Sub Fill(target As IWritable)
        target.Write(1, "Domain Modelling")
        target.Write(2, "Shelf Monthly")
    End Sub

    Sub Main()
        Dim store As IReadWriteStore = New DictionaryStore()
        Fill(store)                 ' IReadWriteStore widens to IWritable
        Show(store, 1)              ' ... and to IReadable
        Show(store, 3)
        Console.WriteLine("count: " & store.Count)

        ' The inherited members are available through the derived interface.
        Console.WriteLine(store.Read(2))

        ' Narrowing needs a cast, and the cast can fail.
        Dim readOnlyView As IReadable = store
        Dim writable = TryCast(readOnlyView, IWritable)
        Console.WriteLine("back to IWritable: " & (writable IsNot Nothing))
    End Sub
End Module

1 -> Domain Modelling
3 -> (missing)
count: 2
Shelf Monthly
back to IWritable: True

Option Strict On

Public Interface IReadable
    ReadOnly Property Name As String
End Interface

Public Interface IWritable
    ReadOnly Property Name As String
End Interface

' Inheriting two interfaces with a member of the same name makes a call on the derived interface ambiguous.
Public Interface IReadWrite
    Inherits IReadable, IWritable
End Interface

' Shadows declares a new member that hides both inherited ones.
Public Interface IShadowedReadWrite
    Inherits IReadable, IWritable

    Shadows ReadOnly Property Name As String
End Interface

Public Class Store
    Implements IReadWrite, IShadowedReadWrite

    Private ReadOnly Property ReadName As String Implements IReadable.Name
        Get
            Return "read side"
        End Get
    End Property

    Private ReadOnly Property WriteName As String Implements IWritable.Name
        Get
            Return "write side"
        End Get
    End Property

    Private ReadOnly Property BothName As String Implements IShadowedReadWrite.Name
        Get
            Return "shadowing member"
        End Get
    End Property
End Class

Module Program
    Sub Main()
        Dim store As New Store()

        Dim ambiguous As IReadWrite = store
        ' ambiguous.Name is BC30521. Convert to the interface that declares the member you mean.
        Console.WriteLine(DirectCast(ambiguous, IReadable).Name)
        Console.WriteLine(DirectCast(ambiguous, IWritable).Name)

        Dim shadowed As IShadowedReadWrite = store
        Console.WriteLine(shadowed.Name)
        Console.WriteLine(DirectCast(shadowed, IReadable).Name)
    End Sub
End Module

read side
write side
shadowing member
read side

Program.vb(17,33): error BC30521: Overload resolution failed because no accessible 'Name' is most specific for these arguments:

Option Strict On

Public Interface IDescribable
    Function Describe() As String
End Interface

Public Class CatalogItem
    Implements IDescribable

    ' Overridable: the interface call is dispatched to the most derived override.
    Public Overridable Function Describe() As String Implements IDescribable.Describe
        Return "CatalogItem.Describe"
    End Function
End Class

Public Class Book
    Inherits CatalogItem

    Public Overrides Function Describe() As String
        Return "Book.Describe"
    End Function
End Class

Public Class ShadowingItem
    Inherits CatalogItem

    ' Shadows hides the base method for class callers, but the base class still implements the interface.
    Public Shadows Function Describe() As String
        Return "ShadowingItem.Describe (shadows)"
    End Function
End Class

Public Class Reimplemented
    Inherits CatalogItem
    Implements IDescribable        ' re-implementation: the interface is mapped again

    Public Shadows Function Describe() As String Implements IDescribable.Describe
        Return "Reimplemented.Describe"
    End Function
End Class

Module Program
    Sub Show(label As String, item As CatalogItem)
        Dim viaClass = item.Describe()
        Dim viaInterface = DirectCast(item, IDescribable).Describe()
        Console.WriteLine(label.PadRight(14) & viaClass.PadRight(28) & viaInterface)
    End Sub

    Sub Main()
        Console.WriteLine("variable".PadRight(14) & "via class".PadRight(28) & "via interface")
        Show("Book", New Book())
        Show("ShadowingItem", New ShadowingItem())
        Show("Reimplemented", New Reimplemented())

        ' With the exact derived type the shadowing methods are the ones seen.
        Dim shadowing As New ShadowingItem()
        Dim reimplemented As New Reimplemented()
        Console.WriteLine(shadowing.Describe())
        Console.WriteLine(reimplemented.Describe())
    End Sub
End Module

variable      via class                   via interface
Book          Book.Describe               Book.Describe
ShadowingItem CatalogItem.Describe        CatalogItem.Describe
Reimplemented CatalogItem.Describe        Reimplemented.Describe
ShadowingItem.Describe (shadows)
Reimplemented.Describe

Option Strict On

Public Interface ICounter
    Sub Increment()
    ReadOnly Property Value As Integer
End Interface

Public Structure Counter
    Implements ICounter

    Private _value As Integer

    Public Sub Increment() Implements ICounter.Increment
        _value += 1
    End Sub

    Public ReadOnly Property Value As Integer Implements ICounter.Value
        Get
            Return _value
        End Get
    End Property
End Structure

Module Program
    Sub Main()
        Dim a As New Counter()
        a.Increment()
        Console.WriteLine("a.Value after a.Increment(): " & a.Value)

        ' Converting to the interface boxes a copy of the structure.
        Dim boxed As ICounter = a
        boxed.Increment()
        boxed.Increment()
        Console.WriteLine("boxed.Value after two calls: " & boxed.Value)
        Console.WriteLine("a.Value is unchanged:       " & a.Value)

        ' Two boxes are two copies; assigning a box copies the reference, not the structure.
        Dim alias1 As ICounter = boxed
        alias1.Increment()
        Console.WriteLine("boxed.Value through the alias: " & boxed.Value)

        ' A constrained generic call does not box.
        Dim b As New Counter()
        Bump(b)
        Console.WriteLine("b.Value after Bump(Of T) with ByRef: " & b.Value)
    End Sub

    Sub Bump(Of T As {Structure, ICounter})(ByRef counter As T)
        counter.Increment()
    End Sub
End Module

a.Value after a.Increment(): 1
boxed.Value after two calls: 3
a.Value is unchanged:       1
boxed.Value through the alias: 4
b.Value after Bump(Of T) with ByRef: 1

Option Strict On

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

Public Class Book
    Inherits CatalogItem
End Class

Public Class Magazine
    Inherits CatalogItem
End Class

' T must be a CatalogItem: the constraint is checked wherever the interface is closed.
Public Interface IRepository(Of T As CatalogItem)
    Sub Add(item As T)
    Function GetAll() As IEnumerable(Of T)
End Interface

Public Class ListRepository(Of T As CatalogItem)
    Implements IRepository(Of T)

    Private ReadOnly _items As New List(Of T)

    Public Sub Add(item As T) Implements IRepository(Of T).Add
        _items.Add(item)
    End Sub

    Public Function GetAll() As IEnumerable(Of T) Implements IRepository(Of T).GetAll
        Return _items
    End Function
End Class

' Out: T appears only in output positions. In: T appears only in input positions.
Public Interface IProducer(Of Out T)
    Function Produce() As T
End Interface

Public Interface IPrinter(Of In T)
    Sub Print(item As T)
End Interface

Public Class BookProducer
    Implements IProducer(Of Book)

    Public Function Produce() As Book Implements IProducer(Of Book).Produce
        Return New Book With {.Title = "Produced"}
    End Function
End Class

Public Class ItemPrinter
    Implements IPrinter(Of CatalogItem)

    Public Sub Print(item As CatalogItem) Implements IPrinter(Of CatalogItem).Print
        Console.WriteLine("printing " & item.GetType().Name & ": " & item.Title)
    End Sub
End Class

Module Program
    Sub Main()
        Dim books As IRepository(Of Book) = New ListRepository(Of Book)()
        books.Add(New Book With {.Title = "Shelf Design"})
        books.Add(New Book With {.Title = "Domain Modelling"})

        ' IEnumerable(Of Book) is covariant: it converts to IEnumerable(Of CatalogItem).
        Dim items As IEnumerable(Of CatalogItem) = books.GetAll()
        Console.WriteLine("items: " & items.Count())

        ' IProducer(Of Book) widens to IProducer(Of CatalogItem) (Out).
        Dim producer As IProducer(Of CatalogItem) = New BookProducer()
        Console.WriteLine("produced: " & producer.Produce().Title)

        ' IPrinter(Of CatalogItem) widens to IPrinter(Of Book) (In).
        Dim printer As IPrinter(Of Book) = New ItemPrinter()
        printer.Print(New Book With {.Title = "Async Cataloguing"})

        ' IRepository(Of T) is invariant: this line would be BC36757 under Option Strict On.
        ' Dim wrong As IRepository(Of CatalogItem) = books
        Console.WriteLine(GetType(IRepository(Of Book)).Name & " / " & GetType(IProducer(Of )).Name)
    End Sub
End Module

items: 2
produced: Produced
printing Book: Async Cataloguing
IRepository`1 / IProducer`1

Program.vb(17,52): error BC36757: 'IRepository(Of Book)' cannot be converted to 'IRepository(Of CatalogItem)'. Consider changing the 'T' in the definition of 'Interface IRepository(Of T)' to an Out type parameter, 'Out T'.

Option Strict On

Public Interface IConverter(Of TIn, TOut)
    Function Convert(value As TIn) As TOut
End Interface

' The same generic interface can be implemented several times with different arguments.
Public Class FormatConverter
    Implements IConverter(Of Integer, String), IConverter(Of Date, String)

    Public Function ConvertNumber(value As Integer) As String Implements IConverter(Of Integer, String).Convert
        Return "#" & value.ToString("D4")
    End Function

    Public Function ConvertDate(value As Date) As String Implements IConverter(Of Date, String).Convert
        Return value.ToString("yyyy-MM-dd", Globalization.CultureInfo.InvariantCulture)
    End Function
End Class

Module Program
    Sub Main()
        Dim converter As New FormatConverter()
        Dim numbers As IConverter(Of Integer, String) = converter
        Dim dates As IConverter(Of Date, String) = converter

        Console.WriteLine(numbers.Convert(42))
        Console.WriteLine(dates.Convert(New Date(2026, 10, 19)))
        Console.WriteLine(GetType(FormatConverter).GetInterfaces().Length & " interfaces")
        For Each iface In GetType(FormatConverter).GetInterfaces().OrderBy(Function(t) t.ToString())
            Console.WriteLine("  " & iface.ToString())
        Next
    End Sub
End Module

#0042
2026-10-19
2 interfaces
  ShelfDocs.IConverter`2[System.DateTime,System.String]
  ShelfDocs.IConverter`2[System.Int32,System.String]

Option Strict On

Public Interface IShelfLabels
    Event Changed As EventHandler
    Default ReadOnly Property Item(index As Integer) As String
    ReadOnly Property Count As Integer
    Sub Add(label As String)
End Interface

Public Class LabelList
    Implements IShelfLabels

    Private ReadOnly _labels As New List(Of String)

    Public Event Changed As EventHandler Implements IShelfLabels.Changed

    Default Public ReadOnly Property Item(index As Integer) As String Implements IShelfLabels.Item
        Get
            Return _labels(index)
        End Get
    End Property

    Public ReadOnly Property Count As Integer Implements IShelfLabels.Count
        Get
            Return _labels.Count
        End Get
    End Property

    Public Sub Add(label As String) Implements IShelfLabels.Add
        _labels.Add(label)
        RaiseEvent Changed(Me, EventArgs.Empty)
    End Sub
End Class

Module Program
    Sub Main()
        Dim labels As IShelfLabels = New LabelList()
        AddHandler labels.Changed, Sub(sender, e) Console.WriteLine("labels changed")

        labels.Add("Fiction")
        labels.Add("Reference")
        ' The default property is visible through the interface variable.
        Console.WriteLine(labels(1) & " of " & labels.Count)

        ' WithEvents also works with an interface-typed field; see events-and-handlers.
    End Sub
End Module

labels changed
labels changed
Reference of 2

namespace CsShelf;

public interface IShelfLabel
{
    string Text { get; }

    // Default interface method: C# 8, runtime feature.
    string Describe() => "Label: " + Text;
}

public interface ILabelFactory
{
    // Static abstract member: C# 11.
    static abstract IShelfLabel Create(string text);
}

public sealed class SimpleLabel : IShelfLabel
{
    public SimpleLabel(string text) => Text = text;
    public string Text { get; }
}

<Project Sdk="Microsoft.NET.Sdk">
  <PropertyGroup>
    <OutputType>Exe</OutputType>
    <TargetFramework>net10.0</TargetFramework>
    <RootNamespace>ShelfDocs</RootNamespace>
    <OptionStrict>On</OptionStrict>
    <TreatWarningsAsErrors>true</TreatWarningsAsErrors>
    <NuGetAudit>false</NuGetAudit>
  </PropertyGroup>
  <ItemGroup>
    <ProjectReference Include="..\cs\cs.csproj" />
  </ItemGroup>
</Project>

Option Strict On

Imports CsShelf

Module Program
    Sub Main()
        ' SimpleLabel is written in C# and does not define Describe: the interface supplies it.
        Dim simple As New SimpleLabel("Maps")
        Dim label As IShelfLabel = simple
        Console.WriteLine(label.Text & " -> " & label.Describe())
        Console.WriteLine(DirectCast(simple, IShelfLabel).Describe())
    End Sub
End Module

/w/g3-interfaces-13/vb/Program.vb(7,16): error BC30149: Class 'PlainLabel' must implement 'Function Describe() As String' for interface 'IShelfLabel'. [/w/g3-interfaces-13/vb/vb.vbproj]
/w/g3-interfaces-13/vb/Program.vb(7,16): error BC37315: Class 'PlainLabel' cannot implement interface 'IShelfLabel' because it contains shared abstract or virtual 'Function Create(text As String) As IShelfLabel'. [/w/g3-interfaces-13/vb/vb.vbproj]

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