Question

Here’s my current set-up:

Public Interface IDatabase
   Function CreateParameter(name as String, dbType as <dbTypeEnumeration>)
End Interface

Public Class OracleDatabaseRepository
    Implements IDatabase
    Public Function CreateParameter(ByVal name As String, ByVal dbtype As <dbTypeEnumeration>) As Object
    End Function
End Class

Public Class SQLServerRepository
    Implements IDatabase
    Public Function CreateParameter(ByVal name As String, ByVal dbtype As <dbTypeEnumeration>) As Object
    End Function
End Class

Public Class DatabaseService
    Private _rep As IDatabase
    Sub New(ByRef rep As IDatabase)
        _rep = rep
    End Sub
    Public Function CreateParameter(ByVal name As String, ByRef dbtype As <dbTypeEnumeration>) As Object
       _rep.CreateParameter(name, dbtype)
    End Function
End Class

The problem here is what exactly is dbTypeEnumeration. In the example above, it’s simply a placeholder to what my problem is. Since we use both Oracle and SQL Server databases, the DbTypes are different depending on the database being used. For Oracle, the OracleClient object has its own OracleDbType enumeration with types. SQL Server also has its own enumeration.

My question is: is it possible to show those database-specific enumerations depending on which repository is injected into the DatabaseService constructor? If not, what’s the best way to go about this? I want to separate the two databases, share logic, and allow for future development, ala the interface as a code contract for that development.

Answer 1

Repository implementation correctness asside, I’d change the level of DbType abstraction.

This interface is db agnostic.

Public Interface IDatabase
    Function CreateParameter(ByVal name As String, ByVal type As Type) As Object
End Interface

The db constrained implementation should be in your concrete Repository itself. How else would you want to keep it generic but still differ between the two types?

Public Class SqlServerRepo : Implements IDatabase

    Private Shared typeLookup As Dictionary(Of Type, Data.SqlDbType) = New Dictionary(Of Type, SqlDbType)

    Shared Sub New()
       (...Fill the dict according to your specification ...)
    End Sub

    Public Function CreateParameter(name As String, type As Type) As Object Implements IDatabase.CreateParameter
        Dim concreteDbType = typeLookup(type)
        (.. your parameter creation here ..)
    End Function
End Class

Public Class OracleServerRepo : Implements IDatabase

    Private Shared typeLookup As Dictionary(Of Type, OracleType) = New Dictionary(Of Type, OracleType)

    Shared Sub New()
        (...Fill the dict according to your specification ...)
    End Sub

    Public Function CreateParameter(name As String, type As Type) As Object Implements IDatabase.CreateParameter
        Dim concreteDbType = typeLookup(type)
         (.. your parameter creation here ..)
    End Function
End Class