Interface Statement (Visual Basic)

Article
09/29/2021

Declares the name of an interface and introduces the definitions of the members that the interface comprises.

Syntax

[ <attributelist> ] [ accessmodifier ] [ Shadows ] _  
Interface name [ ( Of typelist ) ]  
    [ Inherits interfacenames ]  
    [ [ modifiers ] Property membername ]  
    [ [ modifiers ] Function membername ]  
    [ [ modifiers ] Sub membername ]  
    [ [ modifiers ] Event membername ]  
    [ [ modifiers ] Interface membername ]  
    [ [ modifiers ] Class membername ]  
    [ [ modifiers ] Structure membername ]  
End Interface

Parts

Term	Definition
`attributelist`	Optional. See Attribute List.
`accessmodifier`	Optional. Can be one of the following: - Public - Protected - Friend - Private - Protected Friend - Private Protected See Access levels in Visual Basic.
`Shadows`	Optional. See Shadows.
`name`	Required. Name of this interface. See Declared Element Names.
`Of`	Optional. Specifies that this is a generic interface.
`typelist`	Required if you use the Of keyword. List of type parameters for this interface. Optionally, each type parameter can be declared variant by using `In` and `Out` generic modifiers. See Type List.
`Inherits`	Optional. Indicates that this interface inherits the attributes and members of another interface or interfaces. See Inherits Statement.
`interfacenames`	Required if you use the `Inherits` statement. The names of the interfaces from which this interface derives.
`modifiers`	Optional. Appropriate modifiers for the interface member being defined.
`Property`	Optional. Defines a property that is a member of the interface.
`Function`	Optional. Defines a `Function` procedure that is a member of the interface.
`Sub`	Optional. Defines a `Sub` procedure that is a member of the interface.
`Event`	Optional. Defines an event that is a member of the interface.
`Interface`	Optional. Defines an interface that is a nested within this interface. The nested interface definition must terminate with an `End Interface` statement.
`Class`	Optional. Defines a class that is a member of the interface. The member class definition must terminate with an `End Class` statement.
`Structure`	Optional. Defines a structure that is a member of the interface. The member structure definition must terminate with an `End Structure` statement.
`membername`	Required for each property, procedure, event, interface, class, or structure defined as a member of the interface. The name of the member.
`End Interface`	Terminates the `Interface` definition.

Remarks

An interface defines a set of members, such as properties and procedures, that classes and structures can implement. The interface defines only the signatures of the members and not their internal workings.

A class or structure implements the interface by supplying code for every member defined by the interface. Finally, when the application creates an instance from that class or structure, an object exists and runs in memory. For more information, see Objects and Classes and Interfaces.

You can use Interface only at namespace or module level. This means the declaration context for an interface must be a source file, namespace, class, structure, module, or interface, and cannot be a procedure or block. For more information, see Declaration Contexts and Default Access Levels.

Interfaces default to Friend access. You can adjust their access levels with the access modifiers. For more information, see Access levels in Visual Basic.

Rules

Nesting Interfaces. You can define one interface within another. The outer interface is called the containing interface, and the inner interface is called a nested interface.
Member Declaration. When you declare a property or procedure as a member of an interface, you are defining only the signature of that property or procedure. This includes the element type (property or procedure), its parameters and parameter types, and its return type. Because of this, the member definition uses only one line of code, and terminating statements such as End Function or End Property are not valid in an interface.

In contrast, when you define an enumeration or structure, or a nested class or interface, it is necessary to include their data members.
Member Modifiers. You cannot use any access modifiers when defining module members, nor can you specify Shared or any procedure modifier except Overloads. You can declare any member with Shadows, and you can use Default when defining a property, as well as ReadOnly or WriteOnly.
Inheritance. If the interface uses the Inherits Statement, you can specify one or more base interfaces. You can inherit from two interfaces even if they each define a member with the same name. If you do so, the implementing code must use name qualification to specify which member it is implementing.

An interface cannot inherit from another interface with a more restrictive access level. For example, a Public interface cannot inherit from a Friend interface.

An interface cannot inherit from an interface nested within it.
Implementation. When a class uses the Implements statement to implement this interface, it must implement every member defined within the interface. Furthermore, each signature in the implementing code must exactly match the corresponding signature defined in this interface. However, the name of the member in the implementing code does not have to match the member name as defined in the interface.

When a class is implementing a procedure, it cannot designate the procedure as Shared.
Default Property. An interface can specify at most one property as its default property, which can be referenced without using the property name. You specify such a property by declaring it with the Default modifier.

Notice that this means that an interface can define a default property only if it inherits none.

Behavior

Access Level. All interface members implicitly have Public access. You cannot use any access modifier when defining a member. However, a class implementing the interface can declare an access level for each implemented member.

If you assign a class instance to a variable, the access level of its members can depend on whether the data type of the variable is the underlying interface or the implementing class. The following example illustrates this.
```
Public Interface IDemo
    Sub DoSomething()
End Interface
Public Class implementIDemo
    Implements IDemo
    Private Sub DoSomething() Implements IDemo.DoSomething
    End Sub
End Class
Dim varAsInterface As IDemo = New implementIDemo()
Dim varAsClass As implementIDemo = New implementIDemo()
```
If you access class members through varAsInterface, they all have public access. However, if you access members through varAsClass, the Sub procedure doSomething has private access.
Scope. An interface is in scope throughout its namespace, class, structure, or module.

The scope of every interface member is the entire interface.
Lifetime. An interface does not itself have a lifetime, nor do its members. When a class implements an interface and an object is created as an instance of that class, the object has a lifetime within the application in which it is running. For more information, see "Lifetime" in Class Statement.

Example

The following example uses the Interface statement to define an interface named thisInterface, which must be implemented with a Property statement and a Function statement.

Public Interface thisInterface
    Property ThisProp(ByVal thisStr As String) As Char
    Function ThisFunc(ByVal thisInt As Integer) As Integer
End Interface

Note that the Property and Function statements do not introduce blocks ending with End Property and End Function within the interface. The interface defines only the signatures of its members. The full Property and Function blocks appear in a class that implements thisInterface.

Share via