public void textchange(object sender, EventArgs e)
{
   if(textbox1.Text.Contains("$"))
   {
     string valueToConvert = textbox1.Text.replace("$","");
     double newVal = Convert.ToDouble(valueToConvert);

     //Assign the new value with $ to textbox1
     textbox1.Text = "$"+Convert.ToString(newVal);
   }
}