why doesn't it add to the list

问题

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I haven't found an answer, but I am trying on a solution and resulted to another question..

Public Class markerRow
  Public MarkerName As String
  Public CameraID As Integer
  Public HostAddress As String
End Class

Public mr As New markerRow
Dim aList As New List(Of markerRow)

For Each dtrow In markerDtable.Rows
        mr.MarkerName = dtrow("MarkerName")
        mr.CameraID = dtrow("CameraID")
        mr.HostAddress = dtrow("HostAddress")
        aList.Add(New markerRow())
Next

from what I think, this code should add every data from the DB to the respective variables in the list.. But I still don't know what it does exactly.

I checked the aList.Count.ToString and it gave me 5 items, which is true since I have 5 rows in my DB..

For Each elem In aList
        listbox.Items.Add(mr.MarkerName)
        listbox.Items.Add(mr.CameraID)
    Next

to check what items are in the aList, I did that.. unfortunately.. it only gives me the last item, 5 times. the last item on MarkerName Column and Camera ID Column.

What am I missing? or what's wrong in my code?
the aList should have what's on the DB above. not only the last item.

回答1:

Public mr As New markerRow
Dim aList As New List(Of markerRow)

For Each dtrow In markerDtable.Rows
        mr.MarkerName = dtrow("MarkerName")
        mr.CameraID = dtrow("CameraID")
        mr.HostAddress = dtrow("HostAddress")
        aList.Add(New markerRow())
Next

Here, you are creating one markerRow named mr, and update it for every row in the DataTable. Also, for every row, you create another markerRow, and add that new one to aList.

For Each elem In aList
    listbox.Items.Add(mr.MarkerName)
    listbox.Items.Add(mr.CameraID)
Next

Then, for each element in aList, you add two items to the ListBox, and everytime it's MarkerName and CameraID from mr, and not the data from the elements of aList.

---

You're probably looking for something like:

Dim aList As New List(Of markerRow)

For Each dtrow In markerDtable.Rows 
    Dim mr = New markerRow() ' create a new markerRow for every row '
    mr.MarkerName = dtrow("MarkerName")
    mr.CameraID = dtrow("CameraID")
    mr.HostAddress = dtrow("HostAddress")
    aList.Add(mr)            ' add the new markerRow to the list '
Next

For Each elem In aList
    listbox.Items.Add(elem.MarkerName)
Next

来源：https://stackoverflow.com/questions/20513686/why-doesnt-it-add-to-the-list