Interface Statement (Visual Basic)
Declares the name of an interface and introduces the definitions of the members that the interface comprises.
[ <attributelist> ] [ accessmodifier ] [ Shadows ] _ Interface name [ ( Of typelist ) ] [ Inherits interfacenames ] [ [ modifiers ] Property membername ] [ [ modifiers ] Function memberame ] [ [ modifiers ] Sub memberame ] [ [ modifiers ] Event memberame ] [ [ modifiers ] Interface memberame ] [ [ modifiers ] Class memberame ] [ [ modifiers ] Structure memberame ] End Interface
Optional. See Attribute List.
Optional. Can be one of the following:
Optional. See Shadows.
Required. Name of this interface. See.
Optional. Specifies that this is a generic interface.
Required if you use the Type List.keyword. List of type parameters for this interface. See
Optional. Indicates that this interface inherits the attributes and members of another interface or interfaces. See.
Required if you use the Inherits statement. The names of the interfaces from which this interface derives.
Optional. Appropriate modifiers for the interface member being defined.
Optional. Defines a property that is a member of the interface.
Optional. Defines a Function procedure that is a member of the interface.
Optional. Defines a Sub procedure that is a member of the interface.
Optional. Defines an event that is a member of the interface.
Optional. Defines an interface that is a nested within this interface. The nested interface definition must terminate with an End Interface statement.
Optional. Defines a class that is a member of the interface. The member class definition must terminate with an End Class statement.
Optional. Defines a structure that is a member of the interface. The member structure definition must terminate with an End Structure statement.
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.
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, seeand .
For a comparison of interfaces and classes, see.
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.
Interfaces default toaccess. You can adjust their access levels with the access modifiers. For more information, see .
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 specifyor any procedure modifier except . You can declare any member with , and you can use when defining a property, as well as or .
Inheritance. If the interface uses the, 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 thestatement 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 themodifier. For more information, see .
Notice that this means that an interface can define a default property only if it inherits none.
Access Level. All interface members implicitly haveaccess. 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.
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.