An Introduction to the .NET FCL, Part 3
Pages: 1, 2, 3, 4, 5

Example 1-6. Classes with the same name

Option Strict On
  
Namespace Extensions.Classes
  
   Public Class Person
      Dim sName As String
  
      Public Sub New(Name As String)
         sName = Name
      End Sub
  
      Public Property Name() As String
         Get
            Return sName
         End Get
         Set
            sName = Value
         End Set
      End Property
   End Class
End Namespace
  
Namespace Extensions.Demographics
   Public Enum Gender As Short
      Male = 2
      Female = 1
   End Enum
  
   Public Class Person
      Dim shAge As Short
      Dim chGender As Gender
  
      Public Property Age() As Short
         Get
            Return shAge
         End Get
         Set
            shAge = Value
         End Set
      End Property
  
      Public Property Gender() As Gender
         Get
            Return chGender
         End Get
         Set
            chGender = Value
         End Set
      End Property
   End Class
End Namespace

This code can be compiled into a dynamic link library. We can then attempt to access the Person class using code like that in Example 1-7.

Example 1-7. A type collision

Option Strict On
  
Imports Extensions.Classes
Imports Extensions.Demographics
Imports System
  
Module modMain
   Public Sub Main()
      Dim oPerson As New Person("John Doe")
      Console.WriteLine(oPerson.Name)
  
      Dim oPerson2 As New Person
      oPerson2.Age = 32
      oPerson2.Gender = Gender.Female
      Console.WriteLine(oPerson2.Gender.ToString)
   End Sub
End Module

However, when we attempt to compile this code, the VB.NET command-line compiler raises two instances of the following compiler error:

error BC30561: 'Person' is ambiguous, imported from the namespaces or types 'Extensions.Demographics, Extensions.Classes'.

To resolve this problem of type collisions, two solutions are available. The first is to use the fully qualified namespace name to indicate the namespace containing the type we want to instantiate, just as if we hadn't used the Imports statement. The second is to assign an alias to a namespace and to use that alias to identify the namespace containing the type we want to instantiate. To do this, we also use the Imports directive, which then has the following syntax:

Imports aliasname = namespace 

where aliasname is the alias by which the namespace will be referenced in code, and namespace is the fully qualified namespace name.

We can then modify our code example from Example 1-7 to take advantage of aliasing. The result is shown in Example 1-8 (again, modified lines are shown in boldface).

Example 1-8. Using aliasing to prevent type naming conflicts

Option Strict On
  
Imports cl = Extensions.Classes
Imports Extensions.Demographics
Imports System
  
Module modMain
   Public Sub Main()
      Dim oPerson As New cl.Person("John Doe")
      Console.WriteLine(oPerson.Name)
  
      Dim oPerson2 As New Person
      oPerson2.Age = 32
      oPerson2.Gender = Gender.Female
      Console.WriteLine(oPerson2.Gender.ToString)
   End Sub
End Module

Note that we have aliased a single namespace, which has magically resolved the ambiguous reference to both namespaces. The use of an alias, however, means that all further references to types in the Extensions.Classes namespace must use the alias in order to resolve the reference to the type, or we must use the fully qualified name of the type.

In the next installment, learn the types of a .NET Namespaces