There are no items to select как исправить

Edit: Further investigation shows that you should be calling ssetObj(i) if you want to get indexed items of your selection set.

I’d not worry about trying to get the count of the selection set anyway if you plan on iterating through it. A For Each should suffice to walk though it. One of the problems with going from VBA/VB6 to VB.NET is the temptation to use the same methodology, when it can sometimes be invalid (at times it can be excellent, but .NET is very capable). Here’s my entire class that I tested your problem with, just to show how I’m connecting to AutoCAD and interfacing with it.

Public Class frmMain

    Private acApp As AcadApplication
    Private polyList As List(Of String)
    Const acProgId As String = "AutoCAD.Application.17"

    <DllImport("User32.dll")> _
    Private Shared Function SetForegroundWindow(ByVal hWnd As IntPtr) As Boolean
    End Function

    Private Sub btnStart_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnStart.Click
        Try
            acApp = DirectCast(Marshal.GetActiveObject(acProgId), AcadApplication)
        Catch
            Try
                Dim acType = Type.GetTypeFromProgID(acProgId)
                acApp = DirectCast(Activator.CreateInstance(acType), AcadApplication)
            Catch ex As Exception
                MsgBox("Unable to create AutoCAD application of type: " & acProgId)
            End Try
        End Try
    End Sub

    Private Sub btnSelect_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnSelect.Click
        If acApp Is Nothing Then Return
        acApp.Visible = True

        Dim acDoc As AcadDocument = acApp.ActiveDocument

        ' Kill all existing selection sets
        While (acDoc.SelectionSets.Count > 0)
            acDoc.SelectionSets(0).Delete()
        End While

        Dim mySS As AcadSelectionSet = acDoc.SelectionSets.Add("ss")
        SetForegroundWindow(acApp.HWND)

        mySS.SelectOnScreen()
        polyList = New List(Of String)
        Dim numVertices As Integer
        For Each ent As AcadEntity In mySS
            If ent.ObjectName = "AcDbLWPolyline" Or
            ent.ObjectName = "AcDbPolyline" Then
                numVertices = (ent.Coordinates.Length) / 2
                polyList.Add(String.Format("{0}  {1} Vertices", ent.ObjectID, numVertices))
            End If
        Next

    End Sub
End Class

External COM methods like this are going to be slower than you’re used to seeing via VBA. Therefore it’s definitely worth diving into the in-process AutoCAD .NET stuff to see great performance.

I figured out the answer while confirming a few behaviours of the mystery white box while writing the question. I’m posting and answering the question anyway since none of my searches turned up anything.

So it seems it’s the File Preview box: if you hit space in the ‘Open Files’ dialog with a file selected, a similar box gives you an enlarged preview of whatever file is selected.

In the ‘Save As’ dialog, however, you can’t select files (clicking just copies the file name), so it will always come up as ‘No items selected’ any time it comes up. While you can select folders in the Save As dialog, it doesn’t attempt to preview them and continues simply saying that nothing is selected.

Here’s a screen grab of it functioning by previewing the screen grab of itself not functioning:

Seems like it can never show anything other than ‘No items selected’ in the Save As dialog.