List of IEnumerables

Question

Please see the code below, which was written by someone else and works very well:

Public Function GetMembers(Optional ByVal sortExpression As String = "MemberId ASC") As List(Of Member) Implements IMemberDao.GetMembers
            Dim sql As String =
                " SELECT MemberId, Email, CompanyName, City, Country" &
                "   FROM [Member] ".OrderBy(sortExpression)

            Return db.Read(sql, Make).ToList()
        End Function

Public Iterator Function Read(Of T)(ByVal sql As String, ByVal make As Func(Of IDataReader, T), ParamArray ByVal parms() As Object) As IEnumerable(Of T)
            Using connection = CreateConnection()
                Using command = CreateCommand(sql, connection, parms)
                    Using reader = command.ExecuteReader()
                        Do While reader.Read()
                            Yield make(reader)
                        Loop
                    End Using
                End Using
            End Using
        End Function

Private Shared Make As Func(Of IDataReader, Member) =
            Function(reader) _
                New Member() With {
                    .MemberId = Extensions.AsId(reader("MemberId")),
                    .Email = Extensions.AsString(reader("Email")),
                    .CompanyName = Extensions.AsString(reader("CompanyName")),
                    .City = Extensions.AsString(reader("City")),
                    .Country = Extensions.AsString(reader("Country"))
                }

I understand that Make is a delegate that populates the Member objects with values, but I do not understand how a list of Persons is returned by the Read function? (a list is returned and works very well).

Answer 1

it does this by the yield keyword. This is generally how it works when you are iterating over a collection. Take a look here:

http://msdn.microsoft.com/en-us/library/vstudio/hh156729.aspx

This should give you a good/simple understanding.