Chapter 13 Page 436 Single operator

caiman · September 8th, 2010, 03:33 PM

Hi Imar,

In attempting to reproduce your example of Single and SingleOrDefault operators on page 436, I get the following error message: "Data source is an invalid type. It must be either an IListSource, IEnumerable, or IDataSource." My assumption is that it is attempting to return a collection of results, but fails because I'm telling it to return a single result. I'm hoping to understand what needs to be done to make this work.

My code looks identical to what you have on the page:

Code:

Partial Class All
  Inherits BasePage

    Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load
        Using myDataContext As New PlanetWroxDataContext()
            Dim review22 = (From r In myDataContext.Reviews _
            Where r.Id = 22 _
            Select r).Single()

            GridView1.DataSource = review22
            GridView1.DataBind()

        End Using
    End Sub
End Class

Thanks,

-Chris

Imar · September 9th, 2010, 03:11 AM

Hi Chris,

Quote:

My assumption is that it is attempting to return a collection of results, but fails because I'm telling it to return a single result.

Nope, your LINQ query is fine. You're telling it to return a single instance, and that's exactly what it does.

The problem is what you do with the returned instance:

Quote:

GridView1.DataSource = review22

The GridView is designed to work with a collection, not with a single instance. So the error you get is from the GridView, not from the query.

Your code would work if you used the Review to populate, say a TextBox:

TextBox1.Text = review22.Title

Hope this helps,

Imar

caiman · September 9th, 2010, 09:52 AM

Yes, that helps!

Thanks as always.