Microsoft KB Archive/829417

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

Article ID: 829417

Article Last Modified on 4/19/2007

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APPLIES TO


 * Microsoft Visual Studio 2005 Professional Edition
 * Microsoft Visual Studio 2005 Standard Edition
 * Microsoft Visual Studio .NET 2003 Enterprise Architect
 * Microsoft Visual Studio .NET 2003 Enterprise Developer
 * Microsoft Visual Studio .NET 2003 Academic Edition
 * Microsoft Visual Studio .NET 2002 Professional Edition
 * Microsoft Visual Studio .NET 2002 Enterprise Architect
 * Microsoft Visual Studio .NET 2002 Enterprise Developer
 * Microsoft Visual Studio .NET 2002 Academic Edition

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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:  Click Start, point to Programs, point to Microsoft Visual Studio .NET, and then click Microsoft Visual Studio .NET or Microsoft Visual Studio 2005. On the File menu, point to New, and then click Project. The New Project dialog box appears. Under Project Types, click Visual Basic Projects or click Visual C# Projects.

Note In Visual Studio 2005, click Visual C#. Under Templates, click Windows Application. In the Name box, type SampleWinApp, and then click OK. By default, a form that is named Form1 is created. Add a Button control and a Panel control to the Form1 form.

Note Do not insert the Button control inside the Panel control.</li> Right-click the Panel control, and then click Properties.</li> Set the AutoScroll property to True.</li>  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)); } </li> On the Debug menu, click Start.</li> In the Form1 form, click Button1 repeatedly to make the scroll bar appear in the Panel control.

Notice that the scroll position is maintained in the Panel control.</li></ol>

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STATUS
This behavior is by design.

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