HTML clipboard Indexers, another nifty feature of VB.NET, are similar to the overloaded [] (array
subscript) operator in C++. An indexer allows you to access a class instance in
terms of a member array.
An indexer declaration may include a set of attributes; a new modifier; a valid
combination of the public, private, protected, and internal access modifiers;
and one of the virtual, override, or abstract modifiers.
The type of an indexer declaration specifies the element type of the indexer
introduced by the declaration. Unless the indexer is an explicit interface
member implementation, the type is followed by the keyword this. For an explicit
interface member implementation, the type is followed by an interface type, a
period (.), and the keyword this. Unlike other members, indexers do not have
user defined names.
The formal parameter list of an indexer corresponds to that of a method, with
two differences: at least one parameter must be specified, and the ref and out
parameter modifiers are not permitted.
The accessors specify the executable statements associated with reading and
writing indexer elements.
Even though the syntax for accessing an indexer element is the same as that for
an array element, an indexer element is not classified as a variable. Thus, it
is not possible to pass an indexer element as a ref or out parameter.
It is an error for an indexer accessor to declare a local variable with the same
name as an indexer parameter. With these differences in mind, all rules defined
in apply to indexer accessors as well as property accessors.
Indexers and properties, although very similar in concept, differ in the
following ways:
-
A property is identified by its name whereas an indexer is identified by its signature.
-
A property is accessed through a simple-name or a member-access whereas an indexer element is accessed through an element-access.
-
A property can be a static member whereas an indexer is always an instance member.
-
A get accessor of a property corresponds to a method with no parameters whereas a get accessor of an indexer corresponds to a method with the same formal parameter list as the indexer.
-
A set accessor of a property corresponds to a method with a single parameter named value whereas a set accessor of an indexer corresponds to a method with the same formal parameter list as the indexer, plus an additional parameter named value.
To understand these concepts, refer to the code
in Listing 5.56.
Listing 5.56: Indexer Example
Imports
System
Imports
System.Net
Class GetDNSaliases
Private m_arrAlias
As String()
Public Sub
Fetch(ByVal strHost
As String)
Dim iphe As IPHostEntry =
Dns.GetHostByAddress(strHost)
m_arrAlias = iphe.Aliases
End Sub
Default
Public ReadOnly Property Item(ByVal
nIndex As Integer)
As String
'indexer
Get
Return m_arrAlias(nIndex)
End Get
End
Property
Public
ReadOnly Property Count()
As Integer
' property
Get
Return
m_arrAlias.GetUpperBound(0)
End Get
End Property
End Class
Class MyApp
Public
Shared str1 As
String
Public
Shared Sub Main()
Dim myGetDNSaliases
As New GetDNSaliases()
Do
Try
Console.WriteLine("write
a valid IP Address and press ENTER (to exit enter ""q"").")
If (str1 =
Console.ReadLine()) =
"q" Then
Exit Try
End If
myGetDNSaliases.Fetch(str1)
Dim nCount
As Int32 =
myGetDNSaliases.Count
Console.WriteLine("Found
{1} aliases for IP {0}", str1, nCount)
Dim i
As Int32 = 0
While i < nCount
Console.WriteLine(myGetDNSaliases(i))
System.Math.Max(System.Threading.Interlocked.Increment(i),
i - 1)
End
While
Catch e
As Exception
Console.WriteLine("Exception
occurred!{0}" & vbCr & vbLf, e.ToString())
Finally
'final code to be executed
End Try
Loop
While True
Console.ReadLine()
End Sub
End Class
The code in this listing generates the display in Figure 5.16.
Figure 5.16: Screen Output Generated from Listing 5.56
Conclusion
Hope this article would have helped you in understanding
indexer in VB.NET.