When custom controls or Windows Forms controls like Panel are resized, only their newly exposed portions are redrawn. However, this behaviour sometimes does not provide the desired results (see fig. 1 below).

This example shows how to ensure redrawing of the whole control after resizing by setting the control's Resi zeRedraw property.

Redrawing the whole control on resize

The ResizeRedraw protected property defined in the Control class indicates, whether the control redraws itself when resized. To ensure redrawing of the whole control on resize set this property to true. This will usually be done in the constructor of a derived class.

public MyControl()
{
InitializeComponent();
ResizeRedraw = true;
}

Alternatively, if you can not or don't want to create a derived class, set the property using reflection or simply call the Invalidate or Refresh method in the control's Resize event handler.

using System.Reflection;

public static void SetResizeRedraw(Control control)
{
typeof(Control).InvokeMember("ResizeRedraw",
BindingFlags.SetProperty | BindingFlags.Instance | BindingFlags.NonPublic,
null, control, new object[] { true });
}

myControl.Resize += new EventHandler(myControl_Resize);

private void myControl_Resize(object sender, EventArgs e)
{
((Control)sender).Invalidate();
}

A sample control

The following is a code of a user control displaying an ellipse that stretches and shrinks when the control's size changes:

public partial class MyControl : UserControl
{
public MyControl()
{
InitializeComponent();
ResizeRedraw = true;
}

protected override void OnPaintBackground(PaintEventArgs e)
{
base.OnPaintBackground(e);
// draw a blue ellipse into the control area
e.Graphics.FillEllipse(new SolidBrush(Color.Blue), 2, 2,
ClientRectangle.Width - 4, ClientRectangle.Height - 4);
}
}

Whithout the ResizeRedraw statement, the control is redrawn only partially and the result looks like in figure 1 above. When the ResizeRedraw = true statement is included, the control is redrawn properly as shown in the following figure 2.