private string Notes = "提示文本";
private void textBox1_Leave(object sender, EventArgs e)
{
    //  退出失去焦点，重新显示
    if (string.IsNullOrEmpty(textBox1.Text))
    {
        textBox1.ForeColor = Color.DarkGray;
        this.textBox1.Text = Notes;
    }
}
 
private void textBox1_Enter(object sender, EventArgs e)
{
    //  进入获得焦点，清空
    if (textBox1.Text == Notes)
    {
        textBox1.ForeColor = Color.Black;
        this.textBox1.Text = "";
    }
}
此方法只针对提示框未被选中时，在多个文本框在进入时就显示还未解决