I have the VB.NET code as,

Imports OPCSystemsInterface
Public Class OPCselectserver
Public Sub ListAllServers()
Dim srv() As String = {}
Dim srvlist As OPCSystemsInterface.IOPCSystems ' Here I get the Null reference error
Try
srv = srvlist.GetListOfOPCServers("Localhost")
MsgBox(srv1.Length) ' it will show the number of available OPC servers
Catch ex As Exception
MsgBox(ex.ToString)
End Try
EndSub
End Class

Here I create object of OPCSystemsInterface.IOPCSystems as srvlist
It gives error as null reference exception.
so please tell me how to declare object of the interface 'IOPCSystems'
I want use this object for listing OPC servers in the PC.
OPCSystemsInterface is the NameSpace which contains interface IOPCSystems

In the simplest case, you need to finda class that implements the interface and do something like the following:

Dim srvlist As OPCSystemsInterface.IOPCSystems = New putNameOfAClassThatImplementsIOPCSystemsHere

Sometimes a constructor is not available and you need to call some method to get the instance instead of using the New keyword. The best thing to do at this point will be to check the documentation for th ecode you are trying to call.

In the simplest case, you need to finda class that implements the interface and do something like the following:

Dim srvlist As OPCSystemsInterface.IOPCSystems = New putNameOfAClassThatImplementsIOPCSystemsHere

Sometimes a constructor is not available and you need to call some method to get the instance instead of using the New keyword. The best thing to do at this point will be to check the documentation for th ecode you are trying to call.