The scroll position is not maintained in an auto-scrollable Panel control in a Windows Forms application


Symptoms


In a Microsoft Windows Forms application, if you clear and then redraw an auto-scrollable Panel control that contains child controls, the scroll position is not maintained.

Cause


Sometimes, you must clear the contents of the Panel control and then redraw the contents of the Panel control. For example, you must do this if the auto-scrollable Panel control contains a collection of controls that have a specific order. Typically, these controls are user controls.

However, the application does not store the value of the
AutoScrollPosition property of the Panel control. Therefore, the scroll position is not maintained when the contents of the Panel control are redrawn.

Workaround


To work around this behavior, use a System.Drawing.Point structure to store value of the AutoScrollPosition property of the Panel control.

After the Panel control is redrawn, you can retrieve the value of the AutoScrollPosition property by using a new instance of the System.Drawing.Point structure.

The get method of the Panel.AutoScrollPosition.X property and the get method of the Panel.AutoScrollPosition.Y property return negative values. However, positive values are required. You can use the Math.Abs function to obtain a positive value from the Panel.AutoScrollPosition.X property and the Panel.AutoScrollPosition.Y property, as in the following line of code:

Visual Basic .NET or Visual Basic 2005 code
Panel1.AutoScrollPosition = New Point(Math.Abs(Panel1.AutoScrollPosition.X), Math.Abs(CurrentPoint.Y))
Visual C# .NET Visual C# 2005 code
panel1.AutoScrollPosition = new Point(Math.Abs(panel1.AutoScrollPosition.X), Math.Abs(CurrentPoint.Y));

Sample application

To use this workaround in a sample application, follow these steps:
  1. Click Start, point to
    Programs, point to Microsoft Visual Studio .NET, and then click Microsoft Visual Studio .NET or Microsoft Visual Studio 2005.
  2. On the File menu, point to
    New, and then click Project. The New Project dialog box appears.
  3. Under Project Types, click Visual Basic Projects or click Visual C# Projects.
    Note In Visual Studio 2005, click Visual C#.
  4. Under Templates, click Windows Application.
  5. In the Name box, type
    SampleWinApp, and then click OK. By default, a form that is named Form1 is created.
  6. Add a Button control and a Panel control to the Form1 form.

    Note Do not insert the Button control inside the Panel control.
  7. Right-click the Panel control, and then click Properties.
  8. Set the AutoScroll property to True.
  9. In the Form1.vb file, add the following code before the End Class statement:

    Visual Basic .NET or Visual Basic 2005 code
    Private count As Integer
    Private arrayctl As New ArrayList


    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
    Dim newtxt As New TextBox
    newtxt.Text = count
    count += 1
    arrayctl.Add(newtxt)
    DrawControls()
    End Sub

    Private Sub DrawControls()
    Dim txt As TextBox

    Dim CurrentPoint As System.Drawing.Point
    CurrentPoint = Panel1.AutoScrollPosition()

    Dim i As Integer = 0
    Panel1.Controls.Clear()
    Panel1.SuspendLayout()
    For Each txt In arrayctl
    Panel1.Controls.Add(txt)

    txt.Width = Panel1.ClientRectangle.Width
    txt.Top = i
    i += txt.Height
    Next
    Panel1.ResumeLayout()
    Panel1.AutoScrollPosition = New Point(Math.Abs(Panel1.AutoScrollPosition.X), Math.Abs(CurrentPoint.Y))

    End Sub
    Visual C# .NET or Visual C# 2005 code
    private int count; 
    private ArrayList arrayctl = new ArrayList();
    private void button1_Click(object sender, System.EventArgs e)
    {
    TextBox newtxt = new TextBox();
    newtxt.Text = count.ToString();
    count++; arrayctl.Add(newtxt);
    DrawControls();
    }

    private void DrawControls()
    {
    System.Drawing.Point CurrentPoint; CurrentPoint = panel1.AutoScrollPosition;
    int i = 0;
    panel1.Controls.Clear();
    panel1.SuspendLayout();
    foreach (TextBox txt in arrayctl)
    {
    panel1.Controls.Add(txt);
    txt.Width = panel1.ClientRectangle.Width;
    txt.Top = i; i += txt.Height;
    }
    panel1.ResumeLayout();
    panel1.AutoScrollPosition = new Point(Math.Abs(panel1.AutoScrollPosition.X), Math.Abs(CurrentPoint.Y));
    }
  10. On the Debug menu, click
    Start.
  11. In the Form1 form, click Button1repeatedly to make the scroll bar appear in the Panel control.

    Notice that the scroll position is maintained in the Panel control.

Status


This behavior is by design.

References


For more information, visit the following Microsoft Developer Network (MSDN) Web site: