How do I project Datareader into a pre-defined object

Question

I need to know how to convert the following to VB.net for an Async/Await project:

public static class Extensions {

    public static IEnumerable<T> Select<T>(
        this SqlDataReader reader, Func<SqlDataReader, T> projection) {

        while (reader.Read()) {
            yield return projection(reader);
        }
    }
}

Usage:

'call the database asynchronously... and 'await' the results
Using reader = Await Db.GetReaderAsync("spGetDashboard", ParamList)
    Return reader.Select(Function(r)
                             Return New DashboardInfo With {.RowNum = CType(r.Item("RowNum"), Long?)}
                         End Function)

Here is what I have so far:

<Extension()>
Public Function [Select](Of T)(reader As SqlDataReader, projection As Func(Of SqlDataReader, T)) As IEnumerable(Of T)
    While reader.Read()
        Return CType(projection(reader), IEnumerable(Of T))
    End While
End Function

Here is the exception:

System.InvalidCastException: Unable to cast object of type 'WindowsApplication1.DashboardInfo' to type 'System.Collections.Generic.IEnumerable`1[WindowsApplication1.DashboardInfo]'. at WindowsApplication1.Extensions.Select[T](SqlDataReader reader, Func`2 projection) in C:\Dev\AsyncDbTesting\AsyncDbTesting\Library\Extensions.vb:line 523
at WindowsApplication1.Process.VB$StateMachine_1_GetDashboardAsync.MoveNext() in C:\Dev\AsyncDbTesting\AsyncDbTesting\Process.vb:line 30 --- End of stack trace from previous location where exception was thrown --- at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task) at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task) at System.Runtime.CompilerServices.TaskAwaiter`1.GetResult() at WindowsApplication1.Form1.VB$StateMachine_0_Button1_Click.MoveNext() in C:\Dev\AsyncDbTesting\AsyncDbTesting\Form1.vb:line 17

While the vb.net code (at the bottom) complies, it throws an 'converting' error at runtime. — Bryan, Dec 10 '12 at 21:14
It didn't thrown an "error". It threw an "Exception". Please post the full exception - the Output of ex.ToString() after you catch the exception. — John Saunders, Dec 10 '12 at 21:23
Please show us the full exception. Otherwise you're making us guess. — John Saunders, Dec 10 '12 at 21:49

prprcupofcoffee · Accepted Answer · 2012-12-10T22:03:30.787

0

The exception is being thrown because you're trying to convert an item of type T into type IEnumerable(Of T).

Assuming you're using .Net framework 4.5, the code you want is

<Extension()>
Public Iterator Function [Select](Of T)(reader As SqlDataReader, projection As Func(Of SqlDataReader, T)) As IEnumerable(Of T)
    While reader.Read()
        Yield projection(reader)
    End While
End Function

Note that Yield and the concept of iterators don't exist in VB.NET before .Net framework 4.5.

edited Dec 10 '12 at 22:03

answered Dec 10 '12 at 21:29

prprcupofcoffee

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I believe it's just `Yield`, not `Yield Return`. – Stephen Cleary Dec 10 '12 at 21:32
Close... the Yield (without Return) compiles an runs but now my exception is: Invalid attempt to call Read when reader is closed. – Bryan Dec 10 '12 at 21:38
My Exception had to do with my Using statement, inadvertently closing the reader before my loop was finished. – Bryan Dec 10 '12 at 21:56
Edited code to remove `Return`. Thanks for spotting it, @StephenCleary – prprcupofcoffee Dec 10 '12 at 22:18

How do I project Datareader into a pre-defined object

1 Answers1