Watermark TextBox in WinForms

Question

Can anyone point me to a good implementation of a basic Windows Forms TextBox that will initially show watermark text that disappears when the cursor enters it? I think I ca

Answer 1

With .Net Core 3 a property was introduced into TextBox: PlaceHolderText

If one is going to need this in a FrameWork application the required code parts can be taken from the official open source code and placed in a TextBox descendant (see license).

This supports multiline TextBox and also RTL text.

public class PlaceHolderTextBox : TextBox
{
    private const int WM_KILLFOCUS = 0x0008;
    private const int WM_PAINT = 0x000F;

    private string _placeholderText;

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

        if (this.ShouldRenderPlaceHolderText(m))
        {
            using Graphics g = this.CreateGraphics();
            this.DrawPlaceholderText(g);
        }
    }

    #region PlaceHolder

    /// 

    ///  Gets or sets the text that is displayed when the control has no text and does not have the focus.
    /// 
    /// The text that is displayed when the control has no text and does not have the focus.
    [Localizable(true), DefaultValue("")]
    public virtual string PlaceholderText
    {
        get => _placeholderText;
        set
        {
            if (value == null)
            {
                value = string.Empty;
            }

            if (_placeholderText != value)
            {
                _placeholderText = value;
                if (this.IsHandleCreated)
                {
                    this.Invalidate();
                }
            }
        }
    }

    //-------------------------------------------------------------------------------------------------

    /// 

    ///  Draws the  in the client area of the  using the default font and color.
    /// 
    private void DrawPlaceholderText(Graphics graphics)
    {
        TextFormatFlags flags = TextFormatFlags.NoPadding | TextFormatFlags.Top |
                                TextFormatFlags.EndEllipsis;
        Rectangle rectangle = this.ClientRectangle;

        if (this.RightToLeft == RightToLeft.Yes)
        {
            flags |= TextFormatFlags.RightToLeft;
            switch (this.TextAlign)
            {
                case HorizontalAlignment.Center:
                    flags |= TextFormatFlags.HorizontalCenter;
                    rectangle.Offset(0, 1);
                    break;
                case HorizontalAlignment.Left:
                    flags |= TextFormatFlags.Right;
                    rectangle.Offset(1, 1);
                    break;
                case HorizontalAlignment.Right:
                    flags |= TextFormatFlags.Left;
                    rectangle.Offset(0, 1);
                    break;
            }
        }
        else
        {
            flags &= ~TextFormatFlags.RightToLeft;
            switch (this.TextAlign)
            {
                case HorizontalAlignment.Center:
                    flags |= TextFormatFlags.HorizontalCenter;
                    rectangle.Offset(0, 1);
                    break;
                case HorizontalAlignment.Left:
                    flags |= TextFormatFlags.Left;
                    rectangle.Offset(1, 1);
                    break;
                case HorizontalAlignment.Right:
                    flags |= TextFormatFlags.Right;
                    rectangle.Offset(0, 1);
                    break;
            }
        }

        TextRenderer.DrawText(graphics, this.PlaceholderText, this.Font, rectangle, SystemColors.GrayText, this.BackColor, flags);
    }
    
    private bool ShouldRenderPlaceHolderText(in Message m) =>
        !string.IsNullOrEmpty(this.PlaceholderText) &&
        (m.Msg == WM_PAINT || m.Msg == WM_KILLFOCUS) &&
        !this.GetStyle(ControlStyles.UserPaint) &&
        !this.Focused && this.TextLength == 0;

    #endregion
}