先上效果图

参考

1. 代码分享：给窗体添加水印 - 陈恩点 - 博客园 (cnblogs.com)
2. WinForm添加水印 - 白衣如花 - 博客园

思路

使用透明无框窗体覆盖需要添加水印的窗体，并设置owner为主窗体。然后在透明窗体绘制水印文本即可。

代码

  1 public class Watermark
  2 {
  3     private string text;
  4     private int gap;
  5     private Font font = new Font("微软雅黑", 16, FontStyle.Regular);
  6     private Color color = Color.FromArgb(255, 0, 0, 0);
  7 
  8     private Form form = new Form();
  9 
 10     public Watermark(Form ownerForm, string text = "Watermark", int gap = 75, double opacity = 0.1, Font font = null, Color? color = null)
 11     {
 12         this.text = text;
 13         this.gap = gap;
 14         if (font != null)
 15         {
 16             this.font = font;
 17         }
 18         if (color.HasValue)
 19         {
 20             this.color = color.Value;
 21         }
 22         form.Size = ownerForm.Size;
 23         ownerForm.SizeChanged += OwnerForm_SizeChanged;
 24         ownerForm.Move += OwnerForm_Move;
 25 
 26         form.Owner = ownerForm;
 27         form.FormBorderStyle = FormBorderStyle.None;
 28         form.Opacity = opacity;
 29         form.ShowInTaskbar = false;
 30         form.TransparencyKey = Color.White;
 31         form.BackColor = Color.White;
 32 
 33         form.Paint += Form_Paint;
 34         form.Show();
 35 
 36 
 37         GetWindowLong(form.Handle, GWL_EXSTYLE);
 38         SetWindowLong(form.Handle, GWL_EXSTYLE, WS_EX_TRANSPARENT | WS_EX_LAYERED);
 39     }
 40 
 41     private void OwnerForm_Move(object sender, EventArgs e)
 42     {
 43         form.Location = ((Form)sender).Location;
 44     }
 45 
 46     private void OwnerForm_SizeChanged(object sender, EventArgs e)
 47     {
 48         form.Size = ((Form)sender).Size;
 49     }
 50     private void Form_Paint(object sender, PaintEventArgs e)
 51     {
 52         const float cos30 = 0.866f;
 53         const float sin30 = 0.5f;
 54         var g = e.Graphics;
 55         g.SmoothingMode = SmoothingMode.AntiAlias;
 56         g.TextRenderingHint = System.Drawing.Text.TextRenderingHint.AntiAlias;
 57         //平移画布到需要画印章的位置
 58         g.TranslateTransform(0, gap);
 59         //逆时针旋转30度
 60         g.RotateTransform(-30);
 61         // 绘制画布区域
 62         //g.FillRectangle(new SolidBrush(Color.FromArgb(50, 100, 100, 100)), 0, 0, frm.Width, frm.Height);
 63         for (int x = 5; x < e.ClipRectangle.Right + gap; x += gap * 2)
 64         {
 65             for (int y = 5; y < e.ClipRectangle.Bottom + gap; y += gap * 2)
 66             {
 67                 // 计算文字起点位置
 68                 float x1 = cos30 * x - sin30 * y;
 69                 float y1 = sin30 * x + cos30 * y;
 70                 //画上文字
 71                 g.DrawString(text, font, new SolidBrush(color), x1, y1);
 72             }
 73         }
 74     }
 75 
 76     #region 在窗口结构中为指定的窗口设置信息
 77     /// <summary>
 78     /// 在窗口结构中为指定的窗口设置信息
 79     /// </summary>
 80     /// <param name="hwnd">欲为其取得信息的窗口的句柄</param>
 81     /// <param name="nIndex">欲取回的信息</param>
 82     /// <param name="dwNewLong">由nIndex指定的窗口信息的新值</param>
 83     /// <returns></returns>
 84     [DllImport("user32", EntryPoint = "SetWindowLong")]
 85     private static extern uint SetWindowLong(IntPtr hwnd, int nIndex, uint dwNewLong);
 86     #endregion
 87 
 88     #region 从指定窗口的结构中取得信息
 89     /// <summary>
 90     /// 从指定窗口的结构中取得信息
 91     /// </summary>
 92     /// <param name="hwnd">欲为其获取信息的窗口的句柄</param>
 93     /// <param name="nIndex">欲取回的信息</param>
 94     /// <returns></returns>
 95     [DllImport("user32", EntryPoint = "GetWindowLong")]
 96     private static extern uint GetWindowLong(IntPtr hwnd, int nIndex);
 97     #endregion
 98 
 99     private const uint WS_EX_LAYERED = 0x80000;
100     private const int WS_EX_TRANSPARENT = 0x20;
101     private const int GWL_EXSTYLE = (-20);
102 }

使用

1 public Form1()
2 {
3     InitializeComponent();
4     new Watermark(this);
5 }