VB.NET Base Class/Derived Class Instance Assignment

[koschins]

Hi,

 

I was trying to write a class to encapsulate the differences in the OleDb/SQLReader/ADO.NET data access methodes. The main problem is that ADO.NET does not support the same basic methods/properties as the OleDB/SQLReader classes.

 

My approach was to implement a derived class of System.Data.DataTable and implement the missing core functions like HasRows() or Read():

 

Public Class DataTableRec
       Inherits System.Data.DataTable

       Protected recPos As Int64 = -1

       Public Function HasRows() As Boolean
           If MyBase.Rows.Count > 0 Then Return True Else Return False
       End Function

       Public Function Read() As Boolean
           If recPos > MyBase.Rows.Count Then Return False
           recPos += 1
           Return True
       End Function

   End Class

 

The problem is that you can't assign a base class instance to a variable of the derived class type. Here a simple code bit to demonstrate the problem:

 

Public Class _tBase

   Public bProp1 As String
   Public bProp2 As String

End Class

Public Class _tDerived
   Inherits _tBase

   Public cProp1 As String
   Public cProp2 As String

End Class

Sub _testInherit()

       Dim oBase As New _tBase
       Dim oDerived As New _tDerived

       ' This cast does not work (run-time error)
       oDerived = CType(oBase, _tDerived)

   End Sub

 

Is that a general restriction of VB.NET Inheritance/type cast ? Or am I thinking in the wrong direction ? Problem is that you have an existing instance of an System.Data.DataTable but want to extend the interface.

 

Any help is greatly appreciated !

[PlausiblyDamp]

You can only cast a derived to a base not the other way round. In your example _tDerived IS a _tBase and therefore can be cast into a variable of that type.

_tBase isn't however a _tDerived.

 

Could you give a bit more detail on what you are trying to acheive? There may be an alternate way.

[koschins]

The goal is to provide a consistent interface for all database access methods - a library should wrap the calls of sqlclient/oledb/ado.net database connections methods, so that you can develop a function like

 

Public Function db_recordset(sqlstring As String, ...) As Object
...
If <adoconfig=ado.net> Then
	Dim dsAdapter As Object, dsDataSet As New System.Data.DataSet, recTable As New DataTableRec

	If <dbconfig=mssql> Then
		dsAdapter = New System.Data.sqlclient.SqlDataAdapter(sqlstring, CType(connObj, System.data.SqlClient.SqlConnection))
	Else
		dsAdapter = New System.Data.OleDb.OleDbDataAdapter(sqlstring, CType(connObj, System.data.OleDb.OleDbConnection))
	End If
	dsAdapter.Fill(dsDataSet, "sqltab")
	objRec = dsDataSet.Tables("sqltab")
Else
	If <dbconfig=mssql> Then
		objCmd = New SqlClient.SqlCommand
	Else
		objCmd = New OleDb.OleDbCommand
	End If

	objCmd.CommandText = sqlstring
	objCmd.connection = connObj

	objRec = objCmd.ExecuteReader()
End If

db_recordset = objRec
End Function

 

A client using the function db_recordset() (latebinding through return type [object]) would have to have a consistent interface using methods/properties on the returned recordset object - primarily HasRows() and Read(). You could of course develop a whole new recordset class encapsulation all of the methods and properties (creating a "HAS A" relationship instead of "IS A" relationship) but then you'd have to reimplement all of the methods/properties and the client would loose the ability to use all of the methods/properties of the recordset through late binding (if it knows what type the db_recordset function is returning).

 

What would be your approach trying to implement such a wrapper class ?

 

Thanks for you reply !

[Optikal]

Take a look at the data layer in MS's .Net Pet Store sample application (it can be found on MSDN). That handles abstracting away the data-access code.