用win32API 实现TextBox水印特效

using System;
using System.Collections.Generic;
using System.Linq;
using System.Runtime.InteropServices;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;

namespace 水印特效
{
    public static class SetWaterText
    {
        [DllImport("user32.dll", CharSet = CharSet.Auto)]
        private static extern Int32 SendMessage(IntPtr hWnd, int msg,
            int wParam, [MarshalAs(UnmanagedType.LPWStr)] string lParam);

        [DllImport("user32.dll")]
        private static extern bool SendMessage(IntPtr hwnd, int msg, int wParam, StringBuilder lParam);

        [DllImport("user32.dll")]
        private static extern bool GetComboBoxInfo(IntPtr hwnd, ref COMBOBOXINFO pcbi);

        [StructLayout(LayoutKind.Sequential)]
        private struct COMBOBOXINFO
        {
            public int cbSize;
            public RECT rcItem;
            public RECT rcButton;
            public IntPtr stateButton;
            public IntPtr hwndCombo;
            public IntPtr hwndItem;
            public IntPtr hwndList;
        }

        [StructLayout(LayoutKind.Sequential)]
        private struct RECT
        {
            public int left;
            public int top;
            public int right;
            public int bottom;
        }

        private const int EM_SETCUEBANNER = 0x1501;
        private const int EM_GETCUEBANNER = 0x1502;

        public static void SetCueText(Control control, string text)
        {
            if (control is ComboBox)
            {
                COMBOBOXINFO info = GetComboBoxInfo(control);
                SendMessage(info.hwndItem, EM_SETCUEBANNER, 0, text);
            }
            else
            {
                SendMessage(control.Handle, EM_SETCUEBANNER, 0, text);
            }
        }

        private static COMBOBOXINFO GetComboBoxInfo(Control control)
        {
            COMBOBOXINFO info = new COMBOBOXINFO();
            //a combobox is made up of three controls, a button, a list and textbox;
            //we want the textbox
            info.cbSize = Marshal.SizeOf(info);
            GetComboBoxInfo(control.Handle, ref info);
            return info;
        }

        public static string GetCueText(Control control)
        {
            StringBuilder builder = new StringBuilder();
            if (control is ComboBox)
            {
                COMBOBOXINFO info = new COMBOBOXINFO();
                //a combobox is made up of two controls, a list and textbox;
                //we want the textbox
                info.cbSize = Marshal.SizeOf(info);
                GetComboBoxInfo(control.Handle, ref info);
                SendMessage(info.hwndItem, EM_GETCUEBANNER, 0, builder);
            }
            else
            {
                SendMessage(control.Handle, EM_GETCUEBANNER, 0, builder);
            }
            return builder.ToString();
        }
        //SetWaterText.SetCueText(TextBox1,"fdsadf")
    }
}

 private void button1_Click(object sender, EventArgs e)
        {
            SetWaterText.SetCueText(textBox1, "请填写用户名");
            SetWaterText.SetCueText(textBox2, "请填写密码");
        }