Question

I have several different lists I want to call. They all have the same format for the class:
id, value, description, order. Instead of creating a bunch a classes to return the all of the many lists, I wanted to use generics and just TELL it what kind of list to return. However, I can not figure out how to populate the classes.

Here are 2 examples of function in my calling code. This should indicate the type of list and the stored proc used to get the data:

Public Function getTheEyeColors()
    Dim glEyeColors As New GenericList
    Return glEyeColors.GetALList(Of EyeColor)("GetAllEyeColors")
End Function

Public Function getTheHairColors()
    Dim glHairColors As New GenericList
    glHairColors.GetALList(Of HairColor)("GetAllHairColors")
End Function

And here is the code I am trying to use to build the generic list…

Public Function GetALList(Of t)(ByVal storedproc As String) As List(Of t)

    Dim lstGenericList As New List(Of t)
    Dim oGenericListItem As t
    Dim oProviderFactory As New ProviderFactory
    Dim oConnection As DbConnection
    Dim oReader As System.Data.IDataReader
    Dim oFactory As DbProviderFactory
    Dim oFileMgt As New FileMgt
    Dim oCmd As DbCommand

    oFactory = oProviderFactory.GetFactory
    oConnection = oProviderFactory.GetProviderConnection(oFactory)
    oCmd = oConnection.CreateCommand
    oCmd.CommandType = CommandType.StoredProcedure

    oCmd.CommandText = storedproc

    Using (oConnection)

        oConnection.Open()
        oReader = oCmd.ExecuteReader()

        While oReader.Read
            HERE IS WHERE I AM NOT SURE HOW TO POPULATE THE EYECOLOR OR HAIRCOLOR CLASS
            lstGenericList.Add(oGenericListItem)
        End While

        oConnection.Close()

    End Using

    Return lstGenericList

End Function

Answer 1

You could add two generic constraints; I don’t know how to express them in VB, but here’s the C# version:

• T : new() – there has to be a parameterless constructor
• T : ICommonInterface – T has to implement an interface

You then put the common properties (ID, Value, Description, Order) into the interface, and you’ll be able to create a new T(), set the properties and add it to the list.

EDIT:

The VB Syntax to specify that it must both be creatable and implement an interface is:

• (Of T As {ICommonInterface, New})