public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();

        }

        private void Form1_Load(object sender, EventArgs e)
        {
            RichEdit50 re = new RichEdit50();  
            //用RichEdit50来取代你的TextBox  这个就是透明文本框
            re.Text = "I am Transparent";
            this.Controls.Add(re);
            this.Show(); 
        }  
    }
    public class RichEdit50 : RichTextBox
    {
        [DllImport("kernel32.dll", CharSet = CharSet.Auto)]
        static extern IntPtr LoadLibrary(string lpFileName);

        protected override CreateParams CreateParams
        {
            get
            {
                CreateParams prams = base.CreateParams;
                if (LoadLibrary("msftedit.dll") != IntPtr.Zero)
                {
                    prams.ExStyle |= 0x020; // transparent 
                    prams.ClassName = "RICHEDIT50W";
                }
                return prams;
            }
        }    
    }