Why do I get “No data exists for the row/column” error when I do have data?

问题

What's meant to happen is that when the a row is double clicked on the database the program should pull the data out of that row and fill it in the text boxes. This is the code I have so far, when double clicking a row I get "No data exists for the row/column".

Thanks,

Private Sub dtg_Email_DoubleClick(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles dtg_Email.DoubleClick
    Dim conn As New OleDbConnection
    Dim cmd As New OleDbCommand
    Dim dr As OleDbDataReader

    Try
      conn = New OleDbConnection(Get_Constring)
        conn.Open()
        cmd.Connection = conn
        cmd.CommandText = CommandType.Text
        ReadPlayerID = Team_Database.PlayerID
        cmd.CommandText = "SELECT Surname, Forename, Email FROM PlayerDatabase WHERE ID = " & ReadPlayerID
        dr = cmd.ExecuteReader


        Me.txt_Surname.Text = IIf(Not IsDBNull(dr("Surname")), dr("Surname"), "")
        Me.txt_Forename.Text = IIf(Not IsDBNull(dr("Forename")), dr("Forename"), "")
        Me.txt_Email.Text = IIf(Not IsDBNull(dr("Email")), dr("Email"), "")

    Catch ex As Exception
        MsgBox(ErrorToString)
    Finally
        conn.Close()
    End Try
End Sub

回答1:

You need to call dr.Read to advance to the next (first in this case) record before you can grab any data from the first row.

dr = cmd.ExecuteReader

dr.Read

Me.txt_Surname.Text = IIf(Not IsDBNull(dr("Surname")), dr("Surname"), "")

回答2:

At least for other DataReaders it is common that "ExecuteReader" only initializes the reader, while you have to iterate over the result set with dr.Read once. Maybe that's true for your case.