if (TextBox1.Text.Split('.').Length > 1)
{    // alert message for multiple dots
}
           if (TextBox1.Text.Trim() == ".")
           {
               // alert message for only dot
           }

尝试：

Try:

if (myTextBox.Text.Trim() == ".")
   {
   ...
   }

namespace WindowsFormsApplication1
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        private void button1_Click(object sender, EventArgs e)
        {
            Regex r1 = new Regex(@"^.