Setting database class at run time in VB.Net - vb.net

Question

I have the follow classes for Sqlite and SqlServer:
Class for SQLite:
Imports System.Data.SQLite
Public Class clsOperDB_SQLite
Public Shared Function getValue(sql As String) As String
Try
Using conn As New SQLiteConnection(strConn_SQLITE)
Using cmd As New SQLiteCommand()
cmd.Connection = conn
conn.Open()
cmd.CommandText = sql
Return cmd.ExecuteScalar
End Using
End Using
Catch ex As Exception
MsgBox(ex.Message)
End Try
Return ""
End Function
End Class
Class for SQLSERVER:
Imports System.Data.SqlClient
Public Class clsOperDB_SQLSERVER
Public Shared Function getValue(sql As String) As String
Try
Using conn As New SqlConnection(strConn_SQLSERVER)
Using cmd As New SqlCommand()
cmd.Connection = conn
conn.Open()
cmd.CommandText = sql
Return cmd.ExecuteScalar
End Using
End Using
Catch ex As Exception
MsgBox(ex.Message)
End Try
Return ""
End Function
End Class
this is my test form:
Public Class Form1
'form level variable
Dim dbConnector
Private Sub Form1_Load(sender As Object, e As EventArgs) Handles MyBase.Load
Dim connectionType As String = "SQLITE"
' need something like this or any way to set form level variable
If connectionType = "SQLITE" Then
dbConnector = clsOperDB_SQLite
Else
dbConnector = clsOperDB_SQLSERVER
End If
End Sub
Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
'get value from SQLITE
Dim ValueFromDatabase As String = dbConnector.getValue("select .....")
End Sub
End Class
I need help to define dbConnector variable and set its value, also intellisense should show me class methods, using a parameter I want to change database and avoid using a conditional for every time I want to use one or the other database :
Private Sub Button2_Click(sender As Object, e As EventArgs) Handles Button2.Click
Dim Query As String = "Select ..."
Dim ValueFromDatabase As String = ""
' dont want to use if for each sql query
If connectionType = "SQLITE" Then
ValueFromDatabase = clsOperDB_SQLite.getValue(Query)
Else
ValueFromDatabase = clsOperDB_SQLSERVER.getValue(Query)
End If
End Sub
The rest of methods and params for both classes are the same only change class data objects (SQLiteConnection, SqlConnection, and so)
Thanks

Answer 1

You should define an interface that species all the common members. You can then create a class for each data source that implements that interface. In your application code, you can then declare a variable of that interface type and assign an instance of any class that implements it to that variable. You can then just use that variable and invoke any member of the interface without caring what type the actual class instance is.
The interface and the implementing classes would look something like this:
Public Interface IDataAccessProvider
Function GetValue(sql As String) As String
End Interface
Public Class SqliteDataAccessProvider
Implements IDataAccessProvider
Public Function GetValue(sql As String) As String Implements IDataAccessProvider.GetValue
'Provide SQLite-specific implementation here.
End Function
End Class
Public Class SqlServerDataAccessProvider
Implements IDataAccessProvider
Public Function GetValue(sql As String) As String Implements IDataAccessProvider.GetValue
'Provide SQL Server-specific implementation here.
End Function
End Class
Your application code might then look like this:
Private dataAccessProvider As IDataAccessProvider
Private Sub Form1_Load(sender As Object, e As EventArgs) Handles MyBase.Load
'Read a value that identifies the data source and store it here.
'The value might be stored in the config file or wherever else is appropriate.
Dim dataSourceIdentifier As String
Select Case dataSourceIdentifier
Case "SQLite"
dataAccessProvider = New SqliteDataAccessProvider()
Case "SQL Server"
dataAccessProvider = New SqlServerDataAccessProvider()
End Select
End Sub
You can then just call dataAccessProvider.GetValue in your code without any care for what the data source actually is, except to ensure that your SQL syntax is valid for that data source.
Please note that, while what you do is up to you, I have chosen to use a sane naming convention in this code. No one would last long working for me using class names like clsOperDB_SQLSERVER. There's a reason you don't see any types with names like that in the .NET Framework.