Winform窗体增加水印

新增水印类：

public class Watermark
{
    private string text;
    private int gap;
    private Font font = new Font("微软雅黑", 16, FontStyle.Regular);
    private Color color = Color.FromArgb(255, 0, 0, 0);


    private Form form = new Form();

    public Watermark(Form ownerForm, string text = "水印信息", int gap = 75, double opacity = 0.1, Font font = null, Color? color = null)
    {
        this.text = text;
        this.gap = gap;
        if (font != null)
        {
            this.font = font;
        }
        if (color.HasValue)
        {
            this.color = color.Value;
        }
        form.Size = ownerForm.Size;
        ownerForm.SizeChanged += OwnerForm_SizeChanged;
        ownerForm.Move += OwnerForm_Move;

        form.Owner = ownerForm;
        form.FormBorderStyle = FormBorderStyle.None;
        form.Opacity = opacity;
        form.ShowInTaskbar = false;
        form.TransparencyKey = Color.White;
        form.BackColor = Color.White;

        form.Paint += Form_Paint;
        form.Show();


        GetWindowLong(form.Handle, GWL_EXSTYLE);
        SetWindowLong(form.Handle, GWL_EXSTYLE, WS_EX_TRANSPARENT | WS_EX_LAYERED);
    }

    private void OwnerForm_Move(object sender, EventArgs e)
    {
        form.Location = ((Form)sender).Location;
    }

    private void OwnerForm_SizeChanged(object sender, EventArgs e)
    {
        form.Size = ((Form)sender).Size;
    }
    private void Form_Paint(object sender, PaintEventArgs e)
    {
        const float cos30 = 0.866f;
        const float sin30 = 0.5f;
        var g = e.Graphics;
        g.SmoothingMode = SmoothingMode.AntiAlias;
        g.TextRenderingHint = System.Drawing.Text.TextRenderingHint.AntiAlias;
        //平移画布到需要画印章的位置
        g.TranslateTransform(0, gap);
        //逆时针旋转30度
        g.RotateTransform(-30);
        // 绘制画布区域
        //g.FillRectangle(new SolidBrush(Color.FromArgb(50, 100, 100, 100)), 0, 0, frm.Width, frm.Height);
        for (int x = 5; x < e.ClipRectangle.Right + gap; x += gap * 2)
        {
            for (int y = 5; y < e.ClipRectangle.Bottom + gap; y += gap * 2)
            {
                // 计算文字起点位置
                float x1 = cos30 * x - sin30 * y;
                float y1 = sin30 * x + cos30 * y;
                //画上文字
                g.DrawString(text, font, new SolidBrush(color), x1, y1);
            }
        }
    }

    #region 在窗口结构中为指定的窗口设置信息
    /// <summary>
    /// 在窗口结构中为指定的窗口设置信息
    /// </summary>
    /// <param name="hwnd">欲为其取得信息的窗口的句柄</param>
    /// <param name="nIndex">欲取回的信息</param>
    /// <param name="dwNewLong">由nIndex指定的窗口信息的新值</param>
    /// <returns></returns>
    [DllImport("user32", EntryPoint = "SetWindowLong")]
    private static extern uint SetWindowLong(IntPtr hwnd, int nIndex, uint dwNewLong);
    #endregion

    #region 从指定窗口的结构中取得信息
    /// <summary>
    /// 从指定窗口的结构中取得信息
    /// </summary>
    /// <param name="hwnd">欲为其获取信息的窗口的句柄</param>
    /// <param name="nIndex">欲取回的信息</param>
    /// <returns></returns>
    [DllImport("user32", EntryPoint = "GetWindowLong")]
    private static extern uint GetWindowLong(IntPtr hwnd, int nIndex);
    #endregion

    private const uint WS_EX_LAYERED = 0x80000;
    private const int WS_EX_TRANSPARENT = 0x20;
    private const int GWL_EXSTYLE = (-20);
}

调用：

public Form1()
 {
     InitializeComponent();
     new Watermark(this);
 }