ASP.NET Core 2.0 MVC [Remote] Validation

Problem

How to implement model validation using [Remote] attribute in ASP.NET Core MVC.

Solution

In Startup, configure the middleware and services for MVC.

public void ConfigureServices(
       IServiceCollection services)
   {
       services.AddMvc();
   }

   public void Configure(
       IApplicationBuilder app,
       IHostingEnvironment env)
   {
       app.UseStaticFiles();
       app.UseMvcWithDefaultRoute();
   }

Add a Model.

public class EmployeeInputModel
  {
      [Required]
      [Remote(action: "ValidateEmployeeNo", controller: "Home")]
      public string EmployeeNo { get; set; }
  }

Add a Controller.

public class HomeController : Controller
  {
      public IActionResult Index()
      {
          return View();
      }

      [HttpPost]
      [ValidateAntiForgeryToken]
      public IActionResult Index(EmployeeInputModel model)
      {
          if (!ModelState.IsValid)
          {
              return View(model);
          }

          var json = JsonConvert.SerializeObject(model);
          return Content(json);
      }

      public IActionResult ValidateEmployeeNo(string employeeNo)
      {
          if (employeeNo == "007")
              return Json(data: "007 is already assigned to James Bond!");

          return Json(data: true);
      }
  }

Add a Razor page and scripts for jQuery and its validation.

using Fiver.Mvc.ModelValidation.Remote.Models.Home
@model EmployeeInputModel

<strong>ASP.NET Core MVC Client Validation</strong>

<form asp-controller="Home" asp-action="Index" method="post">
    <div asp-validation-summary="All"></div>

    <label asp-for="EmployeeNo"></label>
    <input asp-for="EmployeeNo" />
    <span asp-validation-for="EmployeeNo"></span>

    <button type="submit">Save</button>
</form>

<script src="https://ajax.aspnetcdn.com/ajax/jQuery/jquery-1.11.3.min.js"></script>
<script src="https://ajax.aspnetcdn.com/ajax/jquery.validate/1.14.0/jquery.validate.min.js"></script>
<script src="https://ajax.aspnetcdn.com/ajax/jquery.validation.unobtrusive/3.2.6/jquery.validate.unobtrusive.min.js"></script>

Discussion

ASP.NET Core MVC gives a useful [Remote] attribute to make AJAX calls to Controller/Action to perform server-side validation, without the full post-back. The attribute uses jQuery and its validation JavaScript files to perform the AJAX requests.

We simply annotate the model property with [Remote] attribute, specifying Controller and Action. The action itself returns a JSON result with either validation message or true.

Source Code

GitHub