In Windows Forms, the DoubleBuffered property does not affect child controls such as text boxes. Instead it affects just the Form or Panel it is set for.
If you want double-buffering for child elements on a form, you will need to implement manual double-buffering.
Bob Powell has written a good article (and others) on how to do this.
Also, from a forum answer Bob also says:
The ownership of a window means that
they will flicker uncontrollably
because you cannot double-buffer
outside of the target windows area. A
panel with child controls cannot be
made to double buffer itself and
it's children for example.
The only way to do this correctly is
to create a single control that does
all the drawing using a form of
retained mode graphics system.
Therefore, to get flicker-free textbox resize using manual double-buffering you would need to somehow render the textbox to your back buffer and then display it as part of the buffered update. If even possible: I do not expect this would be easy.
[Update]
Some other answers have said this is a problem with Windows Forms specifically. This is not correct, it is actually deeper than that and is caused by Windows GDI. As an example, open Notepad/Wordpad etc. and paste a large chunk of text, resize the window, and notice the same flickering issue.
Here is a basic solution I used years ago to do something similar. It is a simple form containing a multiline textbox and a custom class inheriting from Panel. Both controls have the same location and size. It uses the Forms ResizeBegin and ResizeEnd to show the panel when resizing, and the textbox otherwise. It's not perfect but it does eliminate the flickering.
public partial class Form1 : Form
{
public Form1()
{
InitializeComponent();
}
Bitmap bm = null;
private void textBox1_Resize(object sender, EventArgs e)
{
Graphics g = textBox1.CreateGraphics();
if (g.VisibleClipBounds.IsEmpty == false)
{
bm = new Bitmap((int)g.VisibleClipBounds.Width, (int)g.VisibleClipBounds.Height);
textBox1.DrawToBitmap(bm, new Rectangle(0, 0, (int)g.VisibleClipBounds.Width, (int)g.VisibleClipBounds.Height));
}
g.Dispose();
}
private void panelDB1_Paint(object sender, PaintEventArgs e)
{
if (bm != null)
{
e.Graphics.DrawImageUnscaled(bm, 0, 0,bm.Width,bm.Height );
}
}
private void Form1_ResizeBegin(object sender, EventArgs e)
{
panelDB1.BringToFront();
}
private void Form1_ResizeEnd(object sender, EventArgs e)
{
panelDB1.SendToBack();
}
}
class PanelDB : Panel
{
public PanelDB()
{
this.SetStyle(ControlStyles.AllPaintingInWmPaint | ControlStyles.UserPaint | ControlStyles.OptimizedDoubleBuffer,true);
//this.DoubleBuffered = true;
}
protected override void OnPaint(PaintEventArgs e)
{
base.OnPaint(e);
}
}
