C# 实现水印

直接上源码

   public class WaterTextBox : TextBox
    {
        //private const int EM_SETCUEBANNER = 0x1501;
        //[DllImport("user32.dll", CharSet = CharSet.Auto)]
        //private static extern Int32 SendMessage
        // (IntPtr hWnd, int msg, int wParam, [MarshalAs(UnmanagedType.LPWStr)] string lParam);
        string waterText = string.Empty;
        [Category("外观")]
        [Description("设置水印文本")]
        [DefaultValue(typeof(String), "提示文字")]
        public string WaterText
        {
            get { return waterText; }
            set
            {
                waterText = value;
                this.Invalidate();
            }
        }
        public WaterTextBox()
        {
            //        base.SetStyle(
            //ControlStyles.UserPaint |                      // 控件将自行绘制，而不是通过操作系统来绘制  
            //ControlStyles.OptimizedDoubleBuffer |          // 该控件首先在缓冲区中绘制，而不是直接绘制到屏幕上，这样可以减少闪烁  
            //ControlStyles.AllPaintingInWmPaint |           // 控件将忽略 WM_ERASEBKGND 窗口消息以减少闪烁  
            //ControlStyles.ResizeRedraw |                   // 在调整控件大小时重绘控件  
            //ControlStyles.SupportsTransparentBackColor,    // 控件接受 alpha 组件小于 255 的 BackColor 以模拟透明  
            //true);                                         // 设置以上值为 true  
            //        base.UpdateStyles();
            base.SetStyle(ControlStyles.SupportsTransparentBackColor, true);
            this.BorderStyle = BorderStyle.FixedSingle;
            //this.BorderStyle = Forms.BorderStyle.None;
            this.SuspendLayout();
            this.Font = new System.Drawing.Font("微软雅黑", 9f);
            this.ResumeLayout(false);
            this.WaterText = "水印文字";
            this.AutoSize = false;
            this.Height = 18;
            HelpText = "选择";
            HelpWidth = 40;

        }
        [DllImport("kernel32.dll", CharSet = CharSet.Auto)]
        static extern IntPtr LoadLibrary(string lpFileName);

        /// <summary>
        /// 支持透明背景
        /// </summary>
        protected override CreateParams CreateParams
        {
            get
            {
                if (BackColor != Color.Transparent) return base.CreateParams;
                CreateParams prams = base.CreateParams;
                if (LoadLibrary("msftedit.dll") != IntPtr.Zero)
                {
                    prams.ExStyle |= 0x020;
                    prams.ClassName = "RICHEDIT50W";
                }
                return prams;
            }
        }

        public int HelpWidth { get; set; }
        protected override void WndProc(ref Message m)
        {
            base.WndProc(ref m);
            DrawBorder(ref m);
            if (m.Msg == 0xF)
            {
                WmPaint(ref m);
            }


        }
        Point mPoint = Point.Empty;
        protected override void OnMouseMove(MouseEventArgs e)
        {
            mPoint = e.Location;
            base.OnMouseMove(e);
        }

        // public override Cursor Cursor
        //{
        //    get
        //    {
        //        var rect = ClientRectangle;
        //        rect.X = ClientRectangle.Width - HelpWidth;
        //        rect.Width = HelpWidth;
        //        if (rect.Contains(mPoint))
        //            return Cursors.Hand;
        //        return base.Cursor;

        //    }
        //    set
        //    {
        //        base.Cursor = value;
        //    }
        //}
        public Color BorderColor
        {
            get { return borderColor; }
            set { borderColor = value; }
        }
        private Color borderColor = Color.Orange;   // default border color is black
        private static int WM_NCPAINT = 0x0085;    // WM_NCPAINT message
        private static int WM_ERASEBKGND = 0x0014; // WM_ERASEBKGND message
        private static int WM_PAINT = 0x000F;      // WM_PAINT message
        public event Action<Point> OnDrawText;
        public bool IsSelect { get; set; }
        public string HelpText { get; set; }
        StringFormat sf = new StringFormat() { LineAlignment = StringAlignment.Center, Alignment = StringAlignment.Center };
        [DllImport("user32.dll")]
        static extern int SetWindowRgn(IntPtr hWnd, IntPtr hRgn, bool bRedraw);

        [DllImport("gdi32.dll")]
        static extern IntPtr CreateRoundRectRgn(int x1, int y1, int x2, int y2, int cx, int cy);
        public void DrawBorder(ref Message message)
        {
            if (message.Msg == WM_NCPAINT || message.Msg == WM_ERASEBKGND ||
                message.Msg == WM_PAINT)
            {

                using (Graphics graphics = Graphics.FromHwnd(this.Handle))
                {
                    if (this.BorderStyle == BorderStyle.FixedSingle)
                    {
                        Rectangle rectangle = this.ClientRectangle;
                        ControlPaint.DrawBorder(graphics, rectangle,
                                     borderColor, ButtonBorderStyle.Solid);
                        graphics.Dispose();
                        message.Result = (IntPtr)1;
                    }

                    //SetWindowRgn(this.Handle, CreateRoundRectRgn(0, 0, this.Width + 1, Height + 1, 5, 5), true);

                }


            }
        }
        protected override void OnPaintBackground(PaintEventArgs e)
        {

            base.OnPaintBackground(e);
            e.Graphics.SmoothingMode = SmoothingMode.AntiAlias;
            using (GraphicsPath path = new GraphicsPath())
            {
                path.AddArc(0, 0, Height - 1, Height - 1, 90, 180);
                path.AddArc(Width - Height, 0, Height - 1, Height - 1, 270, 180);
                path.CloseFigure();

                e.Graphics.FillPath(Brushes.Orange, path);
                using (Pen pen = new Pen(Color.Green))
                {
                    e.Graphics.DrawPath(pen, path);
                }
            }
        }

        private void WmPaint(ref Message m)
        {
            using (Graphics graphics = Graphics.FromHwnd(base.Handle))
            {
                if (string.IsNullOrEmpty(Text))
                {
                    TextFormatFlags format =
                        TextFormatFlags.EndEllipsis |
                         TextFormatFlags.VerticalCenter;
                    if (Multiline)
                    {
                        format =
                 TextFormatFlags.EndEllipsis |
                         TextFormatFlags.VerticalCenter |

                  TextFormatFlags.Left;
                    }
                    if (RightToLeft == RightToLeft.Yes)
                    {
                        format |= TextFormatFlags.RightToLeft | TextFormatFlags.Right;
                    }
                    TextRenderer.DrawText(
                        graphics,
                        WaterText,
                        Font,
                        ClientRectangle,
                        Color.Gray,
                        format);
                }
                //if (IsSelect)
                //{
                //    var rect = ClientRectangle;
                //    rect.X = ClientRectangle.Width - HelpWidth;
                //    rect.Width = HelpWidth;
                //    graphics.FillRectangle(Brushes.Goldenrod, rect);
                //    using (var brush = new SolidBrush(Color.White))
                //        graphics.DrawString(HelpText, Font, brush, rect, sf);
                //     if(OnDrawText!=null)
                //     {
                //         OnDrawText(rect.Location);
                //     }
                //}

            }

        }

    }

把这个放到项目中，然后重新生成项目，最后直接在工具箱上拖拽WaterTextBox就可以用