Need explanation and example of VB.NET code

Can some help me give some sample code how to call this method?

Public Shared Function ReadObjects(Of T)(ByVal query As String, ByVal func As Func(Of IDataReader, T)) As IEnumerable(Of T)
    Dim outbound As New List(Of T)

    Using connection As OleDbConnection = NewConnection()
        connection.Open()
        Using command As OleDbCommand = New OleDbCommand(query, connection)
            command.CommandType = CommandType.Text
            command.CommandTimeout = 0
            Dim reader As OleDbDataReader = command.ExecuteReader
            While reader.Read()
                outbound.Add(func.Invoke(reader))
            End While
        End Using
        connection.Close()
    End Using

    Return outbound
End Function

Had no idea how to go about this one :)

Solution

First of all, I would write the function like this:

Public Shared Iterator Function ReadObjects(Of T)(query As String, transform As Func(Of IDataRecord, T), parameters As Action(Of OleDbParameterCollection)) As IEnumerable(Of T)

    Using connection As OleDbConnection = NewConnection()
    Using command As OleDbCommand = New OleDbCommand(query, connection)
        If parameters IsNot Nothing Then parameters(command.Parameters)
        command.CommandTimeout = 0

        connection.Open()
        Dim reader As OleDbDataReader = command.ExecuteReader()
        While reader.Read()
            Yield transform(reader)
        End While
    End Using
    End Using
End Function

There are several changes here, but the most important is the ability to accept query parameters separate from the command string. Otherwise, the prior code would have forced you put HUGE GAPING WIDE SECURITY VULNERABILITIES into your application.

That out of the way, it sounds like you're asking how to call the method, and it's likely the Func(Of IDataRecord, T) argument is the confusing part... and now also the Action(Of OleDbParameterCollection).

Here's how to do it.

First, let's get an example. Say you have an Employee table with columns for ID, LastName, and FirstName. You also have a class named Employee, with properties that match the column names. You have a variable named lastName, and you want to run a query to give you every record from the table where the value of the LastName column matches the value of the lastName variable. Then we loop through the results and write each Employee to the Console:

That would look like this:

Dim lastName As String = "Bilazon"
Dim SQL As String = "SELECT * FROM Employee WHERE LastName = ?"
Dim employees = ReadObjects(SQL, 
    Function(e) New Employee() With {ID = e("ID"), LastName = e("LastName"), FirstName = e("FirstName")},
    Sub(p) p.Add("?", OleDbType.VarWChar, 25).Value = lastName)

For Each emp As Employee In employees
    Console.WriteLine($"ID: {emp.ID}{vbTab}Last Name: {emp.LastName}{vbTab}First Name: {emp.FirstName}")
Next

The inline Function and Sub expressions above are called Lambda Expressions.

I might futher make this code a little easier to write and understand by adding a Shared method called FromDataRecord() to my Employee class:

Public Shared Function FromDataRecord(row As IDataRecord) As Employee
    Return New Employee() With {
        ID = row("ID"),
        LastName = row("LastName"),
        FirstName = row("FirstName")
    }
End Function

This is especially valuable as the class adds more properties or gets more complex. Now the function call can also be simplified a bit:

Dim lastName As String = "Bilazon"
Dim SQL As String = "SELECT * FROM Employee WHERE LastName = ?"
Dim employees = ReadObjects(SQL, Employee.FromDataRecord,
    Sub(p) p.Add("?", OleDbType.VarWChar, 25).Value = lastName)

For Each emp As Employee In employees
    Console.WriteLine($"ID: {emp.ID}{vbTab}Last Name: {emp.LastName}{vbTab}First Name: {emp.FirstName}")
Next

The next level up is moving the ReadObjects() method to it's own Module (and marking it private), so you also write a public method in the new Module for each of the queries you want to run.