how can i show custom exception next to textbox? - asp.net-mvc-3

Question

I have register form. I want to check new username that is in db or not and if there is in DB , exception show next to it's textbox "UserName already exist...", what should I do?
this my method with exception that I have used it in Register action.:
public void InsertNewUser(MemberRegisterModel mm)
{
EShopThemeDBEntities context = new EShopThemeDBEntities(idbconnection.ConnStr);
using (context)
{
var listUsers = (from o in context.Users
select o.Username).ToList();
var a = listUsers.Count();
foreach (var item in listUsers)
{
if (mm.Username == item.ToString())
{
throw new Exception("UserName already exist...");
}
User mmr = new User();
mmr.FName = mm.FName;
mmr.LName = mm.LName;
mmr.Username = mm.Username;
mmr.Password = mm.Password;
mmr.Email = mm.Email;
mmr.Phone = mm.Phone;
mmr.Mobile = mm.Mobile;
mmr.CreateDate = DateTime.Now;
mmr.RoleId = 2;
context.AddToUsers(mmr);
context.SaveChanges();
}
}

Answer 1

You can set the Model error and return the model object back to view.
if(mm.Username == item.ToString())
{
ModelState.AddModelError("UserName","Username already taken";)
return View(model);
}
Also You do not need to get a list of usrs from database and do a loop to check whether the user entered user name exist or not. You can use the FirstOrDefault method to atleast one is there.
using (context)
{
var user=(from o in context.Users
where o.UserName==mm.UserName).FirstOrDefault();
if(user!=null)
{
ModelState.AddModelError("UserName","Username already taken";)
return View(model);
}
else
{
//Save new user info
}
}
Make sure you have the validation fields in your view, adjacent to the text box
#Html.TextBoxFor(m => m.UserName)
#Html.ValidationMessageFor(m => m.UserName)
But, Ideally, I would also do it asynchronosly with ajax to provide a rich user experience to the user. For that what you have to do is to look for the blur event of the text box and get the value of the textbox, make an ajax call to an action method which checks the availability of user name and return appropriate result.
<script type="text/javascript">
$(function(){
$("#UserName").blur(){
var userName=$(this).val();
$.getJSON("#Url.Action("Check","User")/"+userName,function(response){
if(response.status=="Available")
{
//It is available to register. May be show a green signal in UI
}
else
{
//not available. Show the message to user
$("#someMsgDIv").html("User name not available");
}
});
});
});
</script>
Now we should have an action method called Check in UserController to handle the ajax request
public ActionResult Check(string id)
{
bool isAvailable=false;
string userName=id;
//Check the user name is availabe here
if(isAvailable)
return Json(new { status="Available"},
JsonRequestBehaviour.AllowGet);
else
return Json(new { status="Not Available"},
JsonRequestBehaviour.AllowGet);
}
Note: Never do the client side approach only. Always do the server side checking no matter whether you have client side checking or not.

Answer 2

Shyju's answer is a thorough answer. However, based on your comments about handling the exception, here's a sample:
public void InsertNewUser(MemberRegisterModel mm)
{
// Some code...
if (userExists)
{
throw new ArgumentException("User name not available");
}
}
in your action method:
public ActionResult AddUser(MemberRegisterModel newUser)
{
try
{
var userManager = new MembersSrv();
userManager.InsertNewUser(newUser);
}
catch (ArgumentException ex)
{
if (ex.Message == "User name not available")
{
ModelState.AddModelError("UserName","Username already taken";)
return View(model);
}
}
}
Please note that the better way is to define a class which derives from Exception class (e.g. DuplicateUserNameException) and throw/catch that exception in your code. This sample code has been simplified.