Validate field after typing in ASP.NET MVC

You can use ASP.Net MVC Partial View and JQuery Ajax.

In your Controller create a method that returns a PartialView:

    public PartialViewResult CheckNickname(string nickname)
    {
        if (Nicknames.GetNicknames().Exists(x => x == nickname))
            ViewBag.StatusNickName = "Nickname existe";
        else
            ViewBag.StatusNickName = "Nickname não existe";

        return PartialView();
    }

Right click on the CheckNickname method and click on "add view". When you open the View settings window check the "Create as partial view" box.

InyourPartialViewyoucanputthevalidationmessageofthenickname,IusedaViewBagtoreturnthemessage(onlyhasthiscodebelowinthepartialview):

<h1>@Html.ViewBag.StatusNickName</h1>

NowyouneedtorenderPartialViewsomewhereinyourRegistrationView:

<divid="checkNicknameDiv">
@{
    Html.RenderPartial("CheckNickname");
}

This is Ajax that will call the method that returns the partial view:

<script type="text/javascript">

function checkNickname() {
    $.ajax(
    {
        type: 'GET',
        url: '/User/CheckNickname',
        data: { nickname: $('#nickname').val() },
        dataType: 'html',
        cache: false,
        async: true,
        success: function (data) {
            $('#checkNicknameDiv').html(data);
        }
    });
}</script>

In the parameter data of ajax put the code that will get the value of the indication field.

To trigger the checkNickname() function, I created a timer. When you start typing in the field, the timer waits for a second without typing to trigger the function, if you continue typing before completing one second, the timer will reset and start again.

    var typingTimer;                //timer
    var doneTypingInterval = 1000;  //tempo ms, 1 segundo por exemplo
    var $input = $('#nickname'); //campo de indicação

    //no keyup, cria o contador e começa a contagem
    $input.on('keyup', function () {
        clearTimeout(typingTimer);
        typingTimer = setTimeout(doneTyping, doneTypingInterval);
    });

    //no keydown, elimina o contador
    $input.on('keydown', function (e) {
        var keyCode = e.keyCode || e.which;
        //se teclar tab
        if (keyCode == 9) {
            checkNickname();
        }
        else {
            clearTimeout(typingTimer);
        }        
    });

    //se o tempo acabar, chama a função
    function doneTyping() {
        checkNickname();
    }

Any questions, just talk ... hugs!

You can put the following attribute in the username template field

[Remote("validaExiste", "Account", HttpMethod = "POST", ErrorMessage = "User name already exists. Please enter a different user name.")]
public string Username

....

Then create the method in the controller something like this:

[HttpPost]
public JsonResult validaExiste(string UserName) {

    var user = Membership.GetUser(UserName);

    return Json(user == null);
}

Note: you need jquery.validate.min.js and jquery.validate.unobtrusive.min.js

From what I understand you want to check if the value exists in the database. If that value does not exist, show a warning.

I'd rather use Ajax.beginForm , easier and cleaner. But you need one more plugin to install.

Follow NuGet: link

Here is an example in the view or partialview:

@using (Ajax.BeginForm("Colocar nome da ação", "Colocar controller aqui", new AjaxOptions { HttpMethod = "POST", OnSuccess = "OnSuccess" }, new { @role = "form" }))
{
@Html.AntiForgeryToken()

<div class="form-group">
    <label>Exemplo</label>
    @Html.TextBoxFor(m => m.Exemplo, new { @class = "form-control", @maxlength = "15", @placeholder = "Digite o seu nickname", @autocomplete = "off" })
    @Html.ValidationMessageFor(m => m.Exemplo, "", new { @class = "text-danger" })
</div> 

<button type="submit" class="btn btn-success">Submit</button>
}

//JS
<script>
function OnSuccess(responsive) {
     alert(responsive.result);
}
</script>

Controller:

[HttpPost]
[ValidateAntiForgeryToken]
public ActionResult NomeDaSuaAção(SeuModel model)
{
    using (var ctx = new Entities())
    {
        var result = ctx.SuaTabela.Where(x => x.SeuCampo == "valor").FirstOrDefault();

        if (result == null)
        {
            return Json(new { result = true }, JsonRequestBehavior.AllowGet);
        }
    }
}