Knowledge Base

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

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

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.
Right-click the Panel control, and then click Properties.
Set the AutoScroll property to True.

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)); 
}

On the Debug menu, click Start.
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.

STATUS

This behavior is by design.

REFERENCES

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

http://msdn2.microsoft.com/en-us/library/system.windows.forms.scrollablecontrol.autoscrollposition(vs.71).aspx

Keywords: kbvs2005swept kbvs2005applies kbprb kbscrollbar kbwindowsforms KB829417

Microsoft KB Archive/829417

Contents