How to Insert data into database in mvc using linq to sql

Try This

[HttpGet]
public JsonResult CheckUsername(string username)
{
  var allusers = db.Users.toList();

  var isDuplicate = false;

  foreach (var user in allusers)
  {
    if (user.Username == username)
      isDuplicate = true;
  }

  var jsonData = new { isDuplicate };

  return Json(jsonData, JsonRequestBehavior.AllowGet);
}

in javascript:

<script>
$("#UsernameTextBox").focusout(function () {
  $.getJSON('@Url.Action("CheckUsername", "MyController")', { username: $(this).val() }, function (data) {
    if (data.isDuplicate) {
      alert("That username is already taken.");
    }
});
</script>