The Input Tag Helper

The Input tag helper generates appropriate name and id attribute values based on the PageModel property that is assigned to it. It will also generate an appropriate value for the type attribute, based on the property's meta data. The tag helper will also emit attributes that provide support for unobtrusive client-side validation.

The input tag helper has two attributes:

Attribute Description
for An expression to be evaluated against the current PageModel, usually a PageModel property name
format A format string used set the format of the value attribute


Although it only has two attributes, the input tag helper is quite powerful. It examines the meta data of the type that is passed to the for attribute, including any Data Annotations that have been applied to the property and emits HTML accordingly.

Here is a class with various property types and data annotation attributes applied to it:

public class Member
    public int PersonId { get; set; }
    public string Name { get; set; }
    public string Email { get; set; }
    public string Password { get; set; }
    public string Telephone { get; set; }
    [Display(Name="Date of Birth")]
    public DateTime DateOfBirth { get; set; }
    public decimal Salary { get; set; }
    public string Website { get; set; }
    [Display(Name="Send spam to me")]
    public bool SendSpam { get; set; }
    public int? NumberOfCats { get; set; }
    public IFormFile Selfie { get; set; }

This is then applied as a property to a PageModel for a page called Register.cshtml:

public class RegisterModel : PageModel
    public Member Member { get; set; }

    public void OnGet()

The properties of the model are applied to various input tag helpers in the Razor file:

<form method="post">
    <input asp-for="Member.PersonId" /><br />
    <input asp-for="Member.Name" /><br />
    <input asp-for="Member.Email" /><br />
    <input asp-for="Member.Password" /><br />
    <input asp-for="Member.Telephone" /><br />
    <input asp-for="Member.Website" /><br />
    <input asp-for="Member.DateOfBirth" /><br />
    <input asp-for="Member.Salary" /><br />
    <input asp-for="Member.SendSpam" /><br />
    <input asp-for="Member.NumberOfCats" /><br />
    <input asp-for="Member.Selfies" /><br />

And this generates the following HTML:

<form method="post">
    <input type="number" data-val="true" data-val-required="The PersonId field is required." id="Member_PersonId" name="Member.PersonId" value="" /><br />
    <input type="text" id="Member_Name" name="Member.Name" value="" /><br />
    <input type="email" data-val="true" data-val-email="The Email field is not a valid e-mail address." id="Member_Email" name="Member.Email" value="" /><br />
    <input type="password" id="Member_Password" name="Member.Password" /><br />
    <input type="tel" id="Member_Telephone" name="Member.Telephone" value="" /><br />
    <input type="url" data-val="true" data-val-url="The Website field is not a valid fully-qualified http, https, or ftp URL." id="Member_Website" name="Member.Website" value="" /><br />
    <input type="datetime-local" data-val="true" data-val-required="The Date of Birth field is required." id="Member_DateOfBirth" name="Member.DateOfBirth" value="" /><br />
    <input type="text" data-val="true" data-val-number="The field Salary must be a number." data-val-required="The Salary field is required." id="Member_Salary" name="Member.Salary" value="" /><br />
    <input data-val="true" data-val-required="The Send spam to me field is required." id="Member_SendSpam" name="Member.SendSpam" type="checkbox" value="true" /><br />
    <input type="number" id="Member_NumberOfCats" name="Member.NumberOfCats" value="" /><br />
    <input type="file" id="Member_Selfie" name="Member.Selfie" /><br />
    <input name="__RequestVerificationToken" type="hidden" value="CfDJ8I..." />
    <input name="Member.SendSpam" type="hidden" value="false" />

Type attribute based on data type

The data type of the property is taken into account when the input tag helper determines the type attribute's value to apply. HTML5 types are used wherever possible to take advantage of features provided by supporting browsers. They behave as type="text" in browsers that don't support the rendered HTML5 type:

.NET Type Input type
bool checkbox
byte, short, int, long number
decimal, double, float text1
string text
DateTime datetime-local
IFormFile file2
  1. Despite the fact that the input type is set to text for these data types, they will still be validated for numeric values.
  2. The IFormFile type is located in the Microsoft.AspNetCore.Http namespace. It is the .NET Core successor to the HttpPostedFile type.

Type attribute based on data annotations

Data annotation attributes applied to properties are also a determining factor for the selection of the input tag helper's type attribute. Most types are specified by a DataType enumeration value provided to the DataType attribute. Some types have their own attribute. The following table provides the DataType enumeration values, with equivalent attributes where they exist:

DataType Enumeration Attribute Input type
EmailAddress EmailAddress email
PhoneNumber Phone tel
Password password
Url Url url
Date date
Time time
DateTime, Duration datetime-local
HiddenInput1 hidden

All other DataType enumeration values (CreditCard, Currency, Html, ImageUrl, MultilineText, PostCode and Upload) result in type="text" being applied.

  1. The HiddenInput attribute requires a reference to Microsoft.AspNetCore.Mvc. All other attributes reside in the System.ComponentModel.DataAnnotations namespace.

Validation support

The input tag helper also emits data attributes that work with ASP.NET's Unobtrusive Client Validation framework (an extension to jQuery Validation). The validation rules are specified in data-val-* attributes and are calcuated from the data types and any data annotation attributes that have been applied to model properties.

The following attributes are designed for validation purposes and will result in appropriate data-val error messages and other attributes being generated:

  • Compare
  • MaxLength
  • MinLength
  • Range
  • Required 1
  • StringLength
  1. Non-nullable properties are treated as Required.

In addition, the following annotation attributes generate data-val attributes:

  • EmailAddress / DataType.EmailAddress
  • Phone / DataType.PhoneNumer
  • Url / DataType.Url

For further information on the validation attributes, see Validation.

Last updated: 26/10/2020 16:09:10

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