How to Programmatically Scroll a Panel

I have a System.Windows.Forms.Panel with some content.

I am trying to programmatically scroll the panel (vertically) either up or down.

I have tried setting the AutoScrollPosition property to a new Point on the panel but that doesn't seem to do it.

I have the AutoScroll property set to true.

I even tried to set the VerticalScroll.Value twice as suggested here, but that doesn't seem to work either.

This is what I am currently doing:

//I have tried passing both positive and negative values.
panel.AutoScrollPosition = new Point(5, 10);

The X and Y values on AutoScrollPosition remain 0 and 0.

Any help or direction on this would be greatly appreciated it.

Thanks in advance,

Marwan

Solution 1:

Here is a solution. I guess you can scroll your Panel by arbitrary position using Win32 however there is a simple trick to help you achieve your requirement here:

public void ScrollToBottom(Panel p){
  using (Control c = new Control() { Parent = p, Dock = DockStyle.Bottom })
     {
        p.ScrollControlIntoView(c);
        c.Parent = null;
     }
}
//use the code
ScrollToBottom(yourPanel);

Or use extension method for convenience:

public static class PanelExtension {
   public static void ScrollToBottom(this Panel p){
      using (Control c = new Control() { Parent = p, Dock = DockStyle.Bottom })
      {
         p.ScrollControlIntoView(c);
         c.Parent = null;
      }
   }
}
//Use the code
yourPanel.ScrollToBottom();

UPDATE

If you want to set the exact position, modifying the code above a little can help:

//This can help you control the scrollbar with scrolling up and down.
//The position is a little special.
//Position for scrolling up should be negative.
//Position for scrolling down should be positive
public static class PanelExtension {
    public static void ScrollDown(this Panel p, int pos)
    {
        //pos passed in should be positive
        using (Control c = new Control() { Parent = p, Height = 1, Top = p.ClientSize.Height + pos })
        {
            p.ScrollControlIntoView(c);                
        }
    }
    public static void ScrollUp(this Panel p, int pos)
    {
        //pos passed in should be negative
        using (Control c = new Control() { Parent = p, Height = 1, Top = pos})
        {
            p.ScrollControlIntoView(c);                
        }
    }
}
//use the code, suppose you have 2 buttons, up and down to control the scrollbar instead of clicking directly on the scrollbar arrows.
int i = 0;
private void buttonUp_Click(object sender, EventArgs e)
{
   if (i >= 0) i = -1;
   yourPanel.ScrollUp(i--);
}
private void buttonDown_Click(object sender, EventArgs e)
{
   if (i < 0) i = 0;
   yourPanel.ScrollDown(i++);
}

Another solution you may want to use is using Panel.VerticalScroll.Value. However I think you need more research to make it work as you expect. Because I can see once changing the Value, the scrollbar position and control position don't sync well. Notice that Panel.VerticalScroll.Value should be between Panel.VerticalScroll.Minimum and Panel.VerticalScroll.Maximum.

Solution 2:

This surprisingly works! NOTE THE MINUS SIGN in the code. There is strange behavior in setting scroll position. If you set the position to exact value (50), it goes negative when you read it next time (-50). So you have to invert it before setting new scroll value.

Scroll down:

private void ButtonScrollDown_OnClick(object sender, EventArgs e)
{
    Point current = yourScrollPanel.AutoScrollPosition;
    Point scrolled = new Point(current.X, -current.Y + 10);
    yourScrollPanel.AutoScrollPosition = scrolled;
}

Scroll up similarly, (-current.Y - 10)

Solution 3:

If you have a class that derives from Panel, then call these two protected methods to scroll the panel:

// The bottom is off screen; scroll down. These coordinates must be negative or zero.
SetDisplayRectLocation(0, AutoScrollPosition.Y - item.BoundingRect.Bottom + ClientRectangle.Bottom);
AdjustFormScrollbars(true);

In my example, item.BoundingRect.Bottom is the Y coordinate of the bottom of a thumbnail, and I need to scroll the panel down so that the whole thumbnail is visible.

@King King's solution of creating a temporary Control just so that scrolling could be done seemed "heavy" to me. And @Hans Passant's suggestion of setting AutoScrollMinSize and AutoScrollPosition didn't work for me.

Leave AutoScroll to its default value of 'true'.