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ASP.NET (snapshot 2017) Microsoft documentation and samples

Introduction to using tag helpers in forms in ASP.NET Core

By Rick Anderson, Dave Paquette, and Jerrie Pelser

This document demonstrates working with Forms and the HTML elements commonly used on a Form. The HTML Form element provides the primary mechanism web apps use to post back data to the server. Most of this document describes Tag Helpers and how they can help you productively create robust HTML forms. We recommend you read Introduction to Tag Helpers before you read this document.

In many cases, HTML Helpers provide an alternative approach to a specific Tag Helper, but it’s important to recognize that Tag Helpers do not replace HTML Helpers and there is not a Tag Helper for each HTML Helper. When an HTML Helper alternative exists, it is mentioned.

The Form Tag Helper

The Form Tag Helper:

Sample:

[!code-HTMLMain]

   1:  <form asp-controller="Demo" asp-action="Register" method="post">
   2:      <!-- Input and Submit elements -->
   3:  </form>

The Form Tag Helper above generates the following HTML:

The MVC runtime generates the action attribute value from the Form Tag Helper attributes asp-controller and asp-action. The Form Tag Helper also generates a hidden Request Verification Token to prevent cross-site request forgery (when used with the [ValidateAntiForgeryToken] attribute in the HTTP Post action method). Protecting a pure HTML Form from cross-site request forgery is difficult, the Form Tag Helper provides this service for you.

Using a named route

The asp-route Tag Helper attribute can also generate markup for the HTML action attribute. An app with a route named register could use the following markup for the registration page:

[!code-HTMLMain]

   1:  <form asp-route="register" method="post">
   2:      <!-- Input and Submit elements -->
   3:  </form>

Many of the views in the Views/Account folder (generated when you create a new web app with Individual User Accounts) contain the asp-route-returnurl attribute:

<form asp-controller="Account" asp-action="Login"
     asp-route-returnurl="@ViewData["ReturnUrl"]"
     method="post" class="form-horizontal" role="form">

[!NOTE] With the built in templates, returnUrl is only populated automatically when you try to access an authorized resource but are not authenticated or authorized. When you attempt an unauthorized access, the security middleware redirects you to the login page with the returnUrl set.

The Input Tag Helper

The Input Tag Helper binds an HTML <input> element to a model expression in your razor view.

Syntax:

The Input Tag Helper:

The Input Tag Helper sets the HTML type attribute based on the .NET type. The following table lists some common .NET types and generated HTML type (not every .NET type is listed).

.NET type Input Type
Bool type=”checkbox”
String type=”text”
DateTime type=”datetime”
Byte type=”number”
Int type=”number”
Single, Double type=”number”

The following table shows some common data annotations attributes that the input tag helper will map to specific input types (not every validation attribute is listed):

Attribute Input Type
[EmailAddress] type=”email”
[Url] type=”url”
[HiddenInput] type=”hidden”
[Phone] type=”tel”
[DataType(DataType.Password)] type=”password”
[DataType(DataType.Date)] type=”date”
[DataType(DataType.Time)] type=”time”

Sample:

[!code-csharpMain]

   1:  using System.ComponentModel.DataAnnotations;
   2:   
   3:  namespace FormsTagHelper.ViewModels
   4:  {
   5:      public class RegisterViewModel
   6:      {
   7:          [Required]
   8:          [EmailAddress]
   9:          [Display(Name = "Email Address")]
  10:          public string Email { get; set; }
  11:   
  12:          [Required]
  13:          [DataType(DataType.Password)]
  14:          public string Password { get; set; }
  15:      }
  16:  }

[!code-HTMLMain]

   1:  @model RegisterViewModel
   2:   
   3:  <form asp-controller="Demo" asp-action="RegisterInput" method="post">
   4:      Email:  <input asp-for="Email" /> <br />
   5:      Password: <input asp-for="Password" /><br />
   6:      <button type="submit">Register</button>
   7:  </form>

The code above generates the following HTML:

The data annotations applied to the Email and Password properties generate metadata on the model. The Input Tag Helper consumes the model metadata and produces HTML5 data-val-* attributes (see Model Validation). These attributes describe the validators to attach to the input fields. This provides unobtrusive HTML5 and jQuery validation. The unobtrusive attributes have the format data-val-rule="Error Message", where rule is the name of the validation rule (such as data-val-required, data-val-email, data-val-maxlength, etc.) If an error message is provided in the attribute, it is displayed as the value for the data-val-rule attribute. There are also attributes of the form data-val-ruleName-argumentName="argumentValue" that provide additional details about the rule, for example, data-val-maxlength-max="1024" .

HTML Helper alternatives to Input Tag Helper

Html.TextBox, Html.TextBoxFor, Html.Editor and Html.EditorFor have overlapping features with the Input Tag Helper. The Input Tag Helper will automatically set the type attribute; Html.TextBox and Html.TextBoxFor will not. Html.Editor and Html.EditorFor handle collections, complex objects and templates; the Input Tag Helper does not. The Input Tag Helper, Html.EditorFor and Html.TextBoxFor are strongly typed (they use lambda expressions); Html.TextBox and Html.Editor are not (they use expression names).

HtmlAttributes

@Html.Editor() and @Html.EditorFor() use a special ViewDataDictionary entry named htmlAttributes when executing their default templates. This behavior is optionally augmented using additionalViewData parameters. The key “htmlAttributes” is case-insensitive. The key “htmlAttributes” is handled similarly to the htmlAttributes object passed to input helpers like @Html.TextBox().

Expression names

The asp-for attribute value is a ModelExpression and the right hand side of a lambda expression. Therefore, asp-for="Property1" becomes m => m.Property1 in the generated code which is why you don’t need to prefix with Model. You can use the “@” character to start an inline expression and move before the m.:

Generates the following:

With collection properties, asp-for="CollectionProperty[23].Member" generates the same name as asp-for="CollectionProperty[i].Member" when i has the value 23.

You can also navigate to child properties using the property path of the view model. Consider a more complex model class that contains a child Address property.

[!code-csharpMain]

   1:  using System.ComponentModel.DataAnnotations;
   2:   
   3:  namespace FormsTagHelper.ViewModels
   4:  {
   5:      public class AddressViewModel
   6:      {
   7:          public string AddressLine1 { get; set; }
   8:      }
   9:  }
  10:   

[!code-csharpMain]

   1:  using System.ComponentModel.DataAnnotations;
   2:   
   3:  namespace FormsTagHelper.ViewModels
   4:  { 
   5:      public class RegisterAddressViewModel
   6:      {
   7:          public string Email { get; set; }
   8:   
   9:          [DataType(DataType.Password)]
  10:          public string Password { get; set; }
  11:   
  12:          public AddressViewModel Address { get; set; }
  13:      }
  14:  }
  15:   

In the view, we bind to Address.AddressLine1:

[!code-HTMLMain]

   1:  @model RegisterAddressViewModel
   2:   
   3:  <form asp-controller="Demo" asp-action="RegisterAddress" method="post">
   4:      Email:  <input asp-for="Email" /> <br />
   5:      Password: <input asp-for="Password" /><br />
   6:      Address: <input asp-for="Address.AddressLine1" /><br />
   7:      <button type="submit">Register</button>
   8:  </form>

The following HTML is generated for Address.AddressLine1:

Expression names and Collections

Sample, a model containing an array of Colors:

[!code-csharpMain]

   1:  using System.Collections.Generic;
   2:   
   3:  namespace FormsTagHelper.ViewModels
   4:  {
   5:      public class Person
   6:      {
   7:          public List<string> Colors { get; set; }
   8:   
   9:          public int Age { get; set; }
  10:      }
  11:  }

The action method:

The following Razor shows how you access a specific Color element:

[!code-HTMLMain]

   1:  @model Person
   2:  @{
   3:      var index = (int)ViewData["index"];
   4:  }
   5:   
   6:  <form asp-controller="ToDo" asp-action="Edit" method="post">
   7:      @Html.EditorFor(m => m.Colors[index])
   8:      <label asp-for="Age"></label>
   9:      <input asp-for="Age" /><br />
  10:      <button type="submit">Post</button>
  11:  </form>

The Views/Shared/EditorTemplates/String.cshtml template:

[!code-HTMLMain]

   1:  @model string
   2:   
   3:  <label asp-for="@Model"></label>
   4:  <input asp-for="@Model" /> <br />

Sample using List<T>:

[!code-csharpMain]

   1:  namespace FormsTagHelper.ViewModels
   2:  {
   3:      public class ToDoItem
   4:      {
   5:          public string Name { get; set; }
   6:   
   7:          public bool IsDone { get; set; }
   8:      }
   9:  }

The following Razor shows how to iterate over a collection:

[!code-HTMLMain]

   1:  @model List<ToDoItem>
   2:   
   3:  <form asp-controller="ToDo" asp-action="Edit" method="post">
   4:      <table>
   5:          <tr> <th>Name</th> <th>Is Done</th> </tr>
   6:   
   7:          @for (int i = 0; i < Model.Count; i++)
   8:          {
   9:              <tr>
  10:                  @Html.EditorFor(model => model[i])
  11:              </tr>
  12:          }
  13:   
  14:      </table>
  15:      <button type="submit">Save</button>
  16:  </form>

The Views/Shared/EditorTemplates/ToDoItem.cshtml template:

[!code-HTMLMain]

   1:  @model ToDoItem
   2:   
   3:  <td>
   4:      <label asp-for="@Model.Name"></label>
   5:      @Html.DisplayFor(model => model.Name)
   6:  </td>
   7:  <td>
   8:      <input asp-for="@Model.IsDone" />
   9:  </td>
  10:   
  11:  @*
  12:      This template replaces the following Razor which evaluates the indexer three times.
  13:      <td>
  14:           <label asp-for="@Model[i].Name"></label>
  15:           @Html.DisplayFor(model => model[i].Name)
  16:       </td>
  17:       <td>
  18:           <input asp-for="@Model[i].IsDone" />
  19:       </td>
  20:  *@

[!NOTE] Always use for (and not foreach) to iterate over a list. Evaluating an indexer in a LINQ expression can be expensive and should be minimized.

 

[!NOTE] The commented sample code above shows how you would replace the lambda expression with the @ operator to access each ToDoItem in the list.

The Textarea Tag Helper

The Textarea Tag Helper tag helper is similar to the Input Tag Helper.

Sample:

[!code-csharpMain]

   1:  using System.ComponentModel.DataAnnotations;
   2:   
   3:  namespace FormsTagHelper.ViewModels
   4:  {
   5:      public class DescriptionViewModel
   6:      {
   7:          [MinLength(5)]
   8:          [MaxLength(1024)]
   9:          public string Description { get; set; }
  10:      }
  11:  }

[!code-HTMLMain]

   1:  @model DescriptionViewModel
   2:   
   3:  <form asp-controller="Demo" asp-action="RegisterTextArea" method="post">
   4:      <textarea asp-for="Description"></textarea>
   5:      <button type="submit">Test</button>
   6:  </form>

The following HTML is generated:

The Label Tag Helper

The Label Tag Helper provides the following benefits over a pure HTML label element:

Sample:

[!code-csharpMain]

   1:  using System.ComponentModel.DataAnnotations;
   2:   
   3:  namespace FormsTagHelper.ViewModels
   4:  {
   5:      public class SimpleViewModel
   6:      {
   7:          [Required]
   8:          [EmailAddress]
   9:          [Display(Name = "Email Address")]
  10:          public string Email { get; set; }
  11:      }
  12:  }
  13:   

[!code-HTMLMain]

   1:  @model SimpleViewModel
   2:   
   3:  <form asp-controller="Demo" asp-action="RegisterLabel" method="post">
   4:      <label asp-for="Email"></label>
   5:      <input asp-for="Email" /> <br />
   6:  </form>

The following HTML is generated for the <label> element:

The Label Tag Helper generated the for attribute value of “Email”, which is the ID associated with the <input> element. The Tag Helpers generate consistent id and for elements so they can be correctly associated. The caption in this sample comes from the Display attribute. If the model didn’t contain a Display attribute, the caption would be the property name of the expression.

The Validation Tag Helpers

There are two Validation Tag Helpers. The Validation Message Tag Helper (which displays a validation message for a single property on your model), and the Validation Summary Tag Helper (which displays a summary of validation errors). The Input Tag Helper adds HTML5 client side validation attributes to input elements based on data annotation attributes on your model classes. Validation is also performed on the server. The Validation Tag Helper displays these error messages when a validation error occurs.

The Validation Message Tag Helper

The Validation Message Tag Helper is used with the asp-validation-for attribute on a HTML span element.

The Validation Message Tag Helper will generate the following HTML:

You generally use the Validation Message Tag Helper after an Input Tag Helper for the same property. Doing so displays any validation error messages near the input that caused the error.

[!NOTE] You must have a view with the correct JavaScript and jQuery script references in place for client side validation. See Model Validation for more information.

When a server side validation error occurs (for example when you have custom server side validation or client-side validation is disabled), MVC places that error message as the body of the <span> element.

The Validation Summary Tag Helper

The Validation Summary Tag Helper is used to display a summary of validation messages. The asp-validation-summary attribute value can be any of the following:

asp-validation-summary Validation messages displayed
ValidationSummary.All Property and model level
ValidationSummary.ModelOnly Model
ValidationSummary.None None

Sample

In the following example, the data model is decorated with DataAnnotation attributes, which generates validation error messages on the <input> element. When a validation error occurs, the Validation Tag Helper displays the error message:

[!code-csharpMain]

   1:  using System.ComponentModel.DataAnnotations;
   2:   
   3:  namespace FormsTagHelper.ViewModels
   4:  {
   5:      public class RegisterViewModel
   6:      {
   7:          [Required]
   8:          [EmailAddress]
   9:          [Display(Name = "Email Address")]
  10:          public string Email { get; set; }
  11:   
  12:          [Required]
  13:          [DataType(DataType.Password)]
  14:          public string Password { get; set; }
  15:      }
  16:  }

[!code-HTMLMain]

   1:  @model RegisterViewModel
   2:   
   3:  <form asp-controller="Demo" asp-action="RegisterValidation" method="post">
   4:      <div asp-validation-summary="ModelOnly"></div>
   5:      Email:  <input asp-for="Email" /> <br />
   6:      <span asp-validation-for="Email"></span><br />
   7:      Password: <input asp-for="Password" /><br />
   8:      <span asp-validation-for="Password"></span><br />
   9:      <button type="submit">Register</button>
  10:  </form>
  11:   
  12:  <script src="https://ajax.aspnetcdn.com/ajax/jquery/jquery-2.1.4.min.js"></script>
  13:  <script src="https://ajax.aspnetcdn.com/ajax/jquery.validate/1.14.0/jquery.validate.min.js">
  14:  </script>
  15:  <script src="https://ajax.aspnetcdn.com/ajax/mvc/5.2.3/jquery.validate.unobtrusive.min.js">
  16:  </script>

The generated HTML (when the model is valid):

The Select Tag Helper

The Select Tag Helper asp-for specifies the model property name for the select element and asp-items specifies the option elements. For example:

[!code-HTMLMain]

   1:  @model CountryViewModel
   2:   
   3:  <form asp-controller="Home" asp-action="Index" method="post">
   4:      <select asp-for="Country" asp-items="Model.Countries"></select> 
   5:      <br /><button type="submit">Register</button>
   6:  </form>

Sample:

[!code-csharpMain]

   1:  using Microsoft.AspNetCore.Mvc.Rendering;
   2:  using System.Collections.Generic;
   3:   
   4:  namespace FormsTagHelper.ViewModels
   5:  {
   6:      public class CountryViewModel
   7:      {
   8:          public string Country { get; set; }
   9:   
  10:          public List<SelectListItem> Countries { get; } = new List<SelectListItem>
  11:          {
  12:              new SelectListItem { Value = "MX", Text = "Mexico" },
  13:              new SelectListItem { Value = "CA", Text = "Canada" },
  14:              new SelectListItem { Value = "US", Text = "USA"  },
  15:          };
  16:      }
  17:  }

The Index method initializes the CountryViewModel, sets the selected country and passes it to the Index view.

[!code-csharpMain]

   1:  using FormsTagHelper.ViewModels;
   2:  using Microsoft.AspNetCore.Mvc;
   3:   
   4:  namespace FormsTagHelper.Controllers
   5:  {
   6:      public class HomeController : Controller
   7:      {
   8:          public IActionResult Index()
   9:          {
  10:              var model = new CountryViewModel();
  11:              model.Country = "CA";
  12:              return View(model);
  13:          }
  14:   
  15:          [HttpPost]
  16:          [ValidateAntiForgeryToken]
  17:          public IActionResult Index(CountryViewModel model)
  18:          {
  19:              if (ModelState.IsValid)
  20:              {
  21:                  var msg = model.Country +  " selected";
  22:                  return RedirectToAction("IndexSuccess", new { message = msg});
  23:              }
  24:   
  25:              // If we got this far, something failed; redisplay form.
  26:              return View(model);
  27:          }
  28:   
  29:          public IActionResult IndexMultiSelect()
  30:          {
  31:              var model = new CountryViewModelIEnumerable();
  32:              return View(model);
  33:          }
  34:   
  35:          [HttpPost]
  36:          [ValidateAntiForgeryToken]
  37:          public IActionResult IndexMultiSelect(CountryViewModelIEnumerable model)
  38:          {
  39:              if (ModelState.IsValid)
  40:              {
  41:                  string strCountriesSelected="";
  42:                  foreach (string s in model.CountryCodes)
  43:                  {
  44:                      strCountriesSelected = strCountriesSelected + " " + s;
  45:                  }
  46:                  return RedirectToAction("IndexSuccess", new { message = strCountriesSelected });
  47:              }
  48:   
  49:              return View(model);
  50:          }
  51:   
  52:          public IActionResult IndexGroup()
  53:          {
  54:              var model = new CountryViewModelGroup();
  55:   
  56:              return View(model);
  57:          }
  58:   
  59:          [HttpPost]
  60:          [ValidateAntiForgeryToken]
  61:          public IActionResult IndexGroup(CountryViewModelGroup model)
  62:          {
  63:              if (ModelState.IsValid)
  64:              {
  65:                  var msg = model.Country + " selected";
  66:                  return RedirectToAction("IndexSuccess", new { message = msg});
  67:              }
  68:   
  69:              return View(model);
  70:          }
  71:   
  72:          public IActionResult IndexEnum()
  73:          {
  74:              var model = new CountryEnumViewModel();
  75:              model.EnumCountry = CountryEnum.Spain;
  76:              return View(model);
  77:          }
  78:   
  79:          [HttpPost]
  80:          [ValidateAntiForgeryToken]
  81:          public IActionResult IndexEnum(CountryEnumViewModel model)
  82:          {
  83:              if (ModelState.IsValid)
  84:              {
  85:                  var msg = model.EnumCountry + " selected";
  86:                  return RedirectToAction("IndexSuccess", new { message = msg});
  87:              }
  88:   
  89:              return View(model);
  90:          }
  91:   
  92:          public IActionResult IndexEmpty(int id)
  93:          {
  94:              var ViewPage = (id != 0) ? "IndexEmptyTemplate" : "IndexEmpty";
  95:   
  96:              return View(ViewPage, new CountryViewModel());
  97:          }
  98:   
  99:          [HttpPost]
 100:          [ValidateAntiForgeryToken]
 101:          public IActionResult IndexEmpty(CountryViewModel model)
 102:          {
 103:              if (ModelState.IsValid)
 104:              {
 105:                  var msg = !System.String.IsNullOrEmpty(model.Country) ? model.Country
 106:                      : "No slection";
 107:                  msg += " Selected";
 108:                  return RedirectToAction("IndexSuccess", new { message = msg });
 109:              }
 110:   
 111:              return View(model);
 112:          }
 113:   
 114:          public IActionResult IndexOption(int id)
 115:          {
 116:              var model = new CountryViewModel();
 117:              model.Country = "CA";
 118:              return View(model);
 119:          }
 120:   
 121:          public IActionResult MyModel()
 122:          {
 123:              return View();
 124:          }
 125:   
 126:          public IActionResult IndexSuccess(string message)
 127:          {
 128:              ViewData["Message"] = message;
 129:              return View();
 130:          }
 131:      }
 132:  }

The HTTP POST Index method displays the selection:

[!code-csharpMain]

   1:  using FormsTagHelper.ViewModels;
   2:  using Microsoft.AspNetCore.Mvc;
   3:   
   4:  namespace FormsTagHelper.Controllers
   5:  {
   6:      public class HomeController : Controller
   7:      {
   8:          public IActionResult Index()
   9:          {
  10:              var model = new CountryViewModel();
  11:              model.Country = "CA";
  12:              return View(model);
  13:          }
  14:   
  15:          [HttpPost]
  16:          [ValidateAntiForgeryToken]
  17:          public IActionResult Index(CountryViewModel model)
  18:          {
  19:              if (ModelState.IsValid)
  20:              {
  21:                  var msg = model.Country +  " selected";
  22:                  return RedirectToAction("IndexSuccess", new { message = msg});
  23:              }
  24:   
  25:              // If we got this far, something failed; redisplay form.
  26:              return View(model);
  27:          }
  28:   
  29:          public IActionResult IndexMultiSelect()
  30:          {
  31:              var model = new CountryViewModelIEnumerable();
  32:              return View(model);
  33:          }
  34:   
  35:          [HttpPost]
  36:          [ValidateAntiForgeryToken]
  37:          public IActionResult IndexMultiSelect(CountryViewModelIEnumerable model)
  38:          {
  39:              if (ModelState.IsValid)
  40:              {
  41:                  string strCountriesSelected="";
  42:                  foreach (string s in model.CountryCodes)
  43:                  {
  44:                      strCountriesSelected = strCountriesSelected + " " + s;
  45:                  }
  46:                  return RedirectToAction("IndexSuccess", new { message = strCountriesSelected });
  47:              }
  48:   
  49:              return View(model);
  50:          }
  51:   
  52:          public IActionResult IndexGroup()
  53:          {
  54:              var model = new CountryViewModelGroup();
  55:   
  56:              return View(model);
  57:          }
  58:   
  59:          [HttpPost]
  60:          [ValidateAntiForgeryToken]
  61:          public IActionResult IndexGroup(CountryViewModelGroup model)
  62:          {
  63:              if (ModelState.IsValid)
  64:              {
  65:                  var msg = model.Country + " selected";
  66:                  return RedirectToAction("IndexSuccess", new { message = msg});
  67:              }
  68:   
  69:              return View(model);
  70:          }
  71:   
  72:          public IActionResult IndexEnum()
  73:          {
  74:              var model = new CountryEnumViewModel();
  75:              model.EnumCountry = CountryEnum.Spain;
  76:              return View(model);
  77:          }
  78:   
  79:          [HttpPost]
  80:          [ValidateAntiForgeryToken]
  81:          public IActionResult IndexEnum(CountryEnumViewModel model)
  82:          {
  83:              if (ModelState.IsValid)
  84:              {
  85:                  var msg = model.EnumCountry + " selected";
  86:                  return RedirectToAction("IndexSuccess", new { message = msg});
  87:              }
  88:   
  89:              return View(model);
  90:          }
  91:   
  92:          public IActionResult IndexEmpty(int id)
  93:          {
  94:              var ViewPage = (id != 0) ? "IndexEmptyTemplate" : "IndexEmpty";
  95:   
  96:              return View(ViewPage, new CountryViewModel());
  97:          }
  98:   
  99:          [HttpPost]
 100:          [ValidateAntiForgeryToken]
 101:          public IActionResult IndexEmpty(CountryViewModel model)
 102:          {
 103:              if (ModelState.IsValid)
 104:              {
 105:                  var msg = !System.String.IsNullOrEmpty(model.Country) ? model.Country
 106:                      : "No slection";
 107:                  msg += " Selected";
 108:                  return RedirectToAction("IndexSuccess", new { message = msg });
 109:              }
 110:   
 111:              return View(model);
 112:          }
 113:   
 114:          public IActionResult IndexOption(int id)
 115:          {
 116:              var model = new CountryViewModel();
 117:              model.Country = "CA";
 118:              return View(model);
 119:          }
 120:   
 121:          public IActionResult MyModel()
 122:          {
 123:              return View();
 124:          }
 125:   
 126:          public IActionResult IndexSuccess(string message)
 127:          {
 128:              ViewData["Message"] = message;
 129:              return View();
 130:          }
 131:      }
 132:  }

The Index view:

[!code-cshtmlMain]

   1:  @model CountryViewModel
   2:   
   3:  <form asp-controller="Home" asp-action="Index" method="post">
   4:      <select asp-for="Country" asp-items="Model.Countries"></select> 
   5:      <br /><button type="submit">Register</button>
   6:  </form>

Which generates the following HTML (with “CA” selected):

[!NOTE] We do not recommend using ViewBag or ViewData with the Select Tag Helper. A view model is more robust at providing MVC metadata and generally less problematic.

The asp-for attribute value is a special case and doesn’t require a Model prefix, the other Tag Helper attributes do (such as asp-items)

[!code-HTMLMain]

   1:  @model CountryViewModel
   2:   
   3:  <form asp-controller="Home" asp-action="Index" method="post">
   4:      <select asp-for="Country" asp-items="Model.Countries"></select> 
   5:      <br /><button type="submit">Register</button>
   6:  </form>

Enum binding

It’s often convenient to use <select> with an enum property and generate the SelectListItem elements from the enum values.

Sample:

[!code-csharpMain]

   1:  namespace FormsTagHelper.ViewModels
   2:  {
   3:      public class CountryEnumViewModel
   4:      {
   5:          public CountryEnum EnumCountry { get; set; }
   6:      }
   7:  }

[!code-csharpMain]

   1:  using System.ComponentModel.DataAnnotations;
   2:   
   3:  namespace FormsTagHelper.ViewModels
   4:  {
   5:      public enum CountryEnum
   6:      {
   7:          [Display(Name = "United Mexican States")]
   8:          Mexico,
   9:          [Display(Name = "United States of America")]
  10:          USA,
  11:          Canada,
  12:          France,
  13:          Germany,
  14:          Spain
  15:      }
  16:  }

The GetEnumSelectList method generates a SelectList object for an enum.

[!code-HTMLMain]

   1:  @model CountryEnumViewModel
   2:   
   3:  <form asp-controller="Home" asp-action="IndexEnum" method="post">
   4:      <select asp-for="EnumCountry" 
   5:              asp-items="Html.GetEnumSelectList&lt;CountryEnum>()"> >
   6:      </select> 
   7:      <br /><button type="submit">Register</button>
   8:  </form>

You can decorate your enumerator list with the Display attribute to get a richer UI:

[!code-csharpMain]

   1:  using System.ComponentModel.DataAnnotations;
   2:   
   3:  namespace FormsTagHelper.ViewModels
   4:  {
   5:      public enum CountryEnum
   6:      {
   7:          [Display(Name = "United Mexican States")]
   8:          Mexico,
   9:          [Display(Name = "United States of America")]
  10:          USA,
  11:          Canada,
  12:          France,
  13:          Germany,
  14:          Spain
  15:      }
  16:  }

The following HTML is generated:

Option Group

The HTML <optgroup> element is generated when the view model contains one or more SelectListGroup objects.

The CountryViewModelGroup groups the SelectListItem elements into the “North America” and “Europe” groups:

[!code-csharpMain]

   1:  using Microsoft.AspNetCore.Mvc.Rendering;
   2:  using System.Collections.Generic;
   3:   
   4:  namespace FormsTagHelper.ViewModels
   5:  {
   6:      public class CountryViewModelGroup
   7:      {
   8:          public CountryViewModelGroup()
   9:          {
  10:              var NorthAmericaGroup = new SelectListGroup { Name = "North America" };
  11:              var EuropeGroup = new SelectListGroup { Name = "Europe" };
  12:   
  13:              Countries = new List<SelectListItem>
  14:              {
  15:                  new SelectListItem
  16:                  {
  17:                      Value = "MEX",
  18:                      Text = "Mexico",
  19:                      Group = NorthAmericaGroup
  20:                  },
  21:                  new SelectListItem
  22:                  {
  23:                      Value = "CAN",
  24:                      Text = "Canada",
  25:                      Group = NorthAmericaGroup
  26:                  },
  27:                  new SelectListItem
  28:                  {
  29:                      Value = "US",
  30:                      Text = "USA",
  31:                      Group = NorthAmericaGroup
  32:                  },
  33:                  new SelectListItem
  34:                  {
  35:                      Value = "FR",
  36:                      Text = "France",
  37:                      Group = EuropeGroup
  38:                  },
  39:                  new SelectListItem
  40:                  {
  41:                      Value = "ES",
  42:                      Text = "Spain",
  43:                      Group = EuropeGroup
  44:                  },
  45:                  new SelectListItem
  46:                  {
  47:                      Value = "DE",
  48:                      Text = "Germany",
  49:                      Group = EuropeGroup
  50:                  }
  51:            };
  52:          }
  53:   
  54:          public string Country { get; set; }
  55:   
  56:          public List<SelectListItem> Countries { get; }
  57:      }
  58:  }

The two groups are shown below:

option group example
option group example

The generated HTML:

Multiple select

The Select Tag Helper will automatically generate the multiple = “multiple” attribute if the property specified in the asp-for attribute is an IEnumerable. For example, given the following model:

[!code-csharpMain]

   1:  using Microsoft.AspNetCore.Mvc.Rendering;
   2:  using System.Collections.Generic;
   3:   
   4:  namespace FormsTagHelper.ViewModels
   5:  {
   6:      public class CountryViewModelIEnumerable
   7:      {
   8:          public IEnumerable<string> CountryCodes { get; set; }
   9:   
  10:          public List<SelectListItem> Countries { get; } = new List<SelectListItem>
  11:          {
  12:              new SelectListItem { Value = "MX", Text = "Mexico" },
  13:              new SelectListItem { Value = "CA", Text = "Canada" },
  14:              new SelectListItem { Value = "US", Text = "USA"    },
  15:              new SelectListItem { Value = "FR", Text = "France" },
  16:              new SelectListItem { Value = "ES", Text = "Spain"  },
  17:              new SelectListItem { Value = "DE", Text = "Germany"}
  18:           };
  19:      }
  20:  }

With the following view:

[!code-HTMLMain]

   1:  @model CountryViewModelIEnumerable
   2:   
   3:  <form asp-controller="Home" asp-action="IndexMultiSelect" method="post">
   4:      <select asp-for="CountryCodes" asp-items="Model.Countries"></select> 
   5:      <br /><button type="submit">Register</button>
   6:  </form>

Generates the following HTML:

No selection

If you find yourself using the “not specified” option in multiple pages, you can create a template to eliminate repeating the HTML:

[!code-HTMLMain]

   1:  @model CountryViewModel
   2:   
   3:  <form asp-controller="Home" asp-action="IndexEmpty" method="post">
   4:      @Html.EditorForModel()
   5:      <br /><button type="submit">Register</button>
   6:  </form>

The Views/Shared/EditorTemplates/CountryViewModel.cshtml template:

[!code-HTMLMain]

   1:  @model CountryViewModel
   2:   
   3:  <select asp-for="Country" asp-items="Model.Countries">
   4:      <option value="">--none--</option>
   5:  </select>
   6:   

Adding HTML <option> elements is not limited to the No selection case. For example, the following view and action method will generate HTML similar to the code above:

[!code-csharpMain]

   1:  using FormsTagHelper.ViewModels;
   2:  using Microsoft.AspNetCore.Mvc;
   3:   
   4:  namespace FormsTagHelper.Controllers
   5:  {
   6:      public class HomeController : Controller
   7:      {
   8:          public IActionResult Index()
   9:          {
  10:              var model = new CountryViewModel();
  11:              model.Country = "CA";
  12:              return View(model);
  13:          }
  14:   
  15:          [HttpPost]
  16:          [ValidateAntiForgeryToken]
  17:          public IActionResult Index(CountryViewModel model)
  18:          {
  19:              if (ModelState.IsValid)
  20:              {
  21:                  var msg = model.Country +  " selected";
  22:                  return RedirectToAction("IndexSuccess", new { message = msg});
  23:              }
  24:   
  25:              // If we got this far, something failed; redisplay form.
  26:              return View(model);
  27:          }
  28:   
  29:          public IActionResult IndexMultiSelect()
  30:          {
  31:              var model = new CountryViewModelIEnumerable();
  32:              return View(model);
  33:          }
  34:   
  35:          [HttpPost]
  36:          [ValidateAntiForgeryToken]
  37:          public IActionResult IndexMultiSelect(CountryViewModelIEnumerable model)
  38:          {
  39:              if (ModelState.IsValid)
  40:              {
  41:                  string strCountriesSelected="";
  42:                  foreach (string s in model.CountryCodes)
  43:                  {
  44:                      strCountriesSelected = strCountriesSelected + " " + s;
  45:                  }
  46:                  return RedirectToAction("IndexSuccess", new { message = strCountriesSelected });
  47:              }
  48:   
  49:              return View(model);
  50:          }
  51:   
  52:          public IActionResult IndexGroup()
  53:          {
  54:              var model = new CountryViewModelGroup();
  55:   
  56:              return View(model);
  57:          }
  58:   
  59:          [HttpPost]
  60:          [ValidateAntiForgeryToken]
  61:          public IActionResult IndexGroup(CountryViewModelGroup model)
  62:          {
  63:              if (ModelState.IsValid)
  64:              {
  65:                  var msg = model.Country + " selected";
  66:                  return RedirectToAction("IndexSuccess", new { message = msg});
  67:              }
  68:   
  69:              return View(model);
  70:          }
  71:   
  72:          public IActionResult IndexEnum()
  73:          {
  74:              var model = new CountryEnumViewModel();
  75:              model.EnumCountry = CountryEnum.Spain;
  76:              return View(model);
  77:          }
  78:   
  79:          [HttpPost]
  80:          [ValidateAntiForgeryToken]
  81:          public IActionResult IndexEnum(CountryEnumViewModel model)
  82:          {
  83:              if (ModelState.IsValid)
  84:              {
  85:                  var msg = model.EnumCountry + " selected";
  86:                  return RedirectToAction("IndexSuccess", new { message = msg});
  87:              }
  88:   
  89:              return View(model);
  90:          }
  91:   
  92:          public IActionResult IndexEmpty(int id)
  93:          {
  94:              var ViewPage = (id != 0) ? "IndexEmptyTemplate" : "IndexEmpty";
  95:   
  96:              return View(ViewPage, new CountryViewModel());
  97:          }
  98:   
  99:          [HttpPost]
 100:          [ValidateAntiForgeryToken]
 101:          public IActionResult IndexEmpty(CountryViewModel model)
 102:          {
 103:              if (ModelState.IsValid)
 104:              {
 105:                  var msg = !System.String.IsNullOrEmpty(model.Country) ? model.Country
 106:                      : "No slection";
 107:                  msg += " Selected";
 108:                  return RedirectToAction("IndexSuccess", new { message = msg });
 109:              }
 110:   
 111:              return View(model);
 112:          }
 113:   
 114:          public IActionResult IndexOption(int id)
 115:          {
 116:              var model = new CountryViewModel();
 117:              model.Country = "CA";
 118:              return View(model);
 119:          }
 120:   
 121:          public IActionResult MyModel()
 122:          {
 123:              return View();
 124:          }
 125:   
 126:          public IActionResult IndexSuccess(string message)
 127:          {
 128:              ViewData["Message"] = message;
 129:              return View();
 130:          }
 131:      }
 132:  }

[!code-HTMLMain]

   1:  @model CountryViewModel
   2:   
   3:  <form asp-controller="Home" asp-action="IndexEmpty" method="post">
   4:      <select asp-for="Country">
   5:          <option value="">&lt;none&gt;</option>
   6:          <option value="MX">Mexico</option>
   7:          <option value="CA">Canada</option>
   8:          <option value="US">USA</option>
   9:      </select> 
  10:      <br /><button type="submit">Register</button>
  11:  </form>

The correct <option> element will be selected ( contain the selected="selected" attribute) depending on the current Country value.

Additional Resources



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