TextBox drawn in WM_PAINT flickers on mouse enter/

问题:

I have a custom TextBox in which I draw some place holder text when it's empty. It works pretty well, but it flickers when the mouse enters and leaves the TextBox. It seems related to the border becoming blue when the mouse hovers the control (I'm on Windows 8.1).

Any idea how I could fix this ?

I've tried various SetStyles flags without success.

class MyTextBox : TextBox
{
  public string PlaceHolder { get; set; }

  static readonly Brush sPlaceHolderBrush = new SolidBrush(Color.FromArgb(70, 70, 78));
  static readonly StringFormat sFormat = new StringFormat
  {
     Alignment = StringAlignment.Near,
     LineAlignment = StringAlignment.Center
  };

  private Font mPlaceHolderFont;

  [DllImport("user32")]
  private static extern IntPtr GetWindowDC(IntPtr hwnd);

  protected override void WndProc(ref Message m)
  {
     base.WndProc(ref m);

     if (m.Msg == 0x0F)   
     {
        if (string.IsNullOrEmpty(Text) && !Focused)
        {
           IntPtr dc = GetWindowDC(Handle);
           using (Graphics g = Graphics.FromHdc(dc))
           {
              if (mPlaceHolderFont == null)
                 mPlaceHolderFont = new Font(Font, FontStyle.Italic);

              var rect = new RectangleF(2, 2, Width - 4, Height - 4);
              g.FillRectangle(Brushes.White, rect);
              g.DrawString(PlaceHolder, mPlaceHolderFont, sPlaceHolderBrush, rect, sFormat);
           }
        }
     }
  }
}

---

I had other problems with overriding OnPaint. Here is the best solution I came up with :

class MyTextBox : TextBox
{
  public string PlaceHolder { get; set; }

  static readonly Brush sPlaceHolderBrush = new SolidBrush(Color.FromArgb(70, 70, 78));
  static readonly StringFormat sFormat = new StringFormat
  {
     Alignment = StringAlignment.Near,
     LineAlignment = StringAlignment.Near
  };

  private Font mPlaceHolderFont;
  private Brush mForegroundBrush;

  protected override void OnHandleCreated(EventArgs e)
  {
     base.OnHandleCreated(e);
     SetStyle(ControlStyles.OptimizedDoubleBuffer | ControlStyles.DoubleBuffer | ControlStyles.AllPaintingInWmPaint | ControlStyles.UserPaint, true);
  }

  protected override void OnPaint(PaintEventArgs e)
  {
     var bounds = new Rectangle(-2, -2, Width, Height);
     var rect = new RectangleF(1, 0, Width - 2, Height - 2);

     e.Graphics.FillRectangle(Brushes.White, rect);
     if (string.IsNullOrEmpty(Text) && !Focused)
     {
        if (mPlaceHolderFont == null)
           mPlaceHolderFont = new Font(Font, FontStyle.Italic);

        if (mForegroundBrush == null)
           mForegroundBrush = new SolidBrush(ForeColor);

        e.Graphics.DrawString(PlaceHolder, mPlaceHolderFont, sPlaceHolderBrush, rect, sFormat);
     }
     else
     {
        var flags = TextFormatFlags.Default | TextFormatFlags.TextBoxControl;
        if (!Multiline)
           flags |= TextFormatFlags.SingleLine | TextFormatFlags.NoPadding;

        TextBoxRenderer.DrawTextBox(e.Graphics, bounds, Text, Font, flags, TextBoxState.Selected);
     }
  }
}

回答1:

Is there a special reason for using WM_PAINT instead of OnPaint? In WM_PAINT you obtain a drawing context from the handle, which is always a direct access to the control. In OnPaint you already have a Graphics in the event args, which can be either a buffer or a direct context, depending on the styles.

You mentioned that you have tried a few styles with no success. Firstly I would say try these and move your paint logic into OnPaint:

SetStyle(ControlStyles.OptimizedDoubleBuffer | ControlStyles.DoubleBuffer | ControlStyles.AllPaintingInWmPaint | ControlStyles.UserPaint, true);

If it does not work (a focused control may behave strangely in Windows) and you must stick to WM_PAINT, then create a buffer manually. Your original code draws a white rectangle first, then some text, which causes flickering. You can avoid this by using a buffer:

IntPtr dc = GetWindowDC(Handle);
using (Graphics g = Graphics.FromHdc(dc))
{
    // creating a buffered context
    using (BufferedGraphicsContext context = new BufferedGraphicsContext())
    {
        // creating a buffer for the original Graphics
        using (BufferedGraphics bg = context.Allocate(e.Graphics, ClientRectangle))
        {
             if (mPlaceHolderFont == null)
                mPlaceHolderFont = new Font(Font, FontStyle.Italic);

             var gBuf = bg.Graphics;
             var rect = ClientRectangle;
             rect.Inflate(-1, -1);
             gBuf.FillRectangle(Brushes.White, rect);
             gBuf.DrawString(PlaceHolder, mPlaceHolderFont, sPlaceHolderBrush, rect, sFormat);

             // copying the buffer onto the original Graphics
             bg.Render(e.Graphics);
        }
    }
}