asp.net-coreTag Helpers

Parameters

NameInfo
asp-actionThe name of the action method to which the form should be posted to
asp-controllerThe name of the controller where the action method specified in asp-action exists
asp-route-*Custom route values you want to add as querystring to the form action attribute value. Replace 8 with the querystring name you want

Form Tag Helper - Basic example

<form asp-action="create" asp-controller="Home">
     <!--Your form elements goes here--> 
</form>

Form Tag Helper - With custom route attributes

<form asp-action="create" 
      asp-controller="Home" 
      asp-route-returnurl="dashboard" 
      asp-route-from="google">
     <!--Your form elements goes here--> 
</form>

This will generate the below markup

<form action="/Home/create?returnurl=dashboard&amp;from=google" method="post">
    <!--Your form elements goes here--> 
</form>

Input Tag Helper

Assuming your view is strongly typed to a view model like

public class CreateProduct
{
   public string Name { set; get; }
}

And you are passing an object of this to the view from your action method.

@model CreateProduct
<form asp-action="create" asp-controller="Home" >

    <input type="text" asp-for="Name"/>   
    <input type="submit"/>

</form>

This will generate the below markup.

<form action="/Home/create" method="post"> 

    <input type="text" id="Name" name="Name" value="" />
    <input type="submit"/>
    <input name="__RequestVerificationToken" type="hidden" value="ThisWillBeAUniqueToken" />

</form>

If you want the input field to be rendered with a default value, you can set the Name property value of your view model in the action method.

public IActionResult Create()
{
  var vm = new CreateProduct { Name="IPhone"};
  return View(vm);
} 

Form submission & Model binding

Model binding will work fine if you use CreateProduct as your HttpPost action method parameter/a parameter named name

Select Tag Helper

Assuming your view is strongly typed to a view model like this

public class CreateProduct
{       
   public IEnumerable<SelectListItem> Categories { set; get; }
   public int SelectedCategory { set; get; }
}

And in your GET action method, you are creating an object of this view model, setting the Categories property and sending to the view

public IActionResult Create()
{
    var vm = new CreateProduct();
    vm.Categories = new List<SelectListItem>
    {
        new SelectListItem {Text = "Books", Value = "1"},
        new SelectListItem {Text = "Furniture", Value = "2"}
    };
    return View(vm);
}

and in your view

@model CreateProduct
<form asp-action="create" asp-controller="Home">
    <select asp-for="SelectedCategory" asp-items="@Model.Categories">
        <option>Select one</option>
    </select>
    <input type="submit"/>
</form>

This will render the below markup(included only relevant parts of form/fields)

<form action="/Home/create" method="post">  
    <select data-val="true" id="SelectedCategory" name="SelectedCategory">
        <option>Select one</option>
        <option value="1">Shyju</option>
        <option value="2">Sean</option>
    </select>
    <input type="submit"/>
</form>

Getting the selected dropdown value in form submission

You can use the same view model as your HttpPost action method parameter

[HttpPost]
public ActionResult Create(CreateProduct model)
{
  //check model.SelectedCategory value
  / /to do : return something
}

Set an option as the selected one

If you want to set an option as the selected option, you may simply set the SelectedCategory property value.

public IActionResult Create()
{
    var vm = new CreateProduct();
    vm.Categories = new List<SelectListItem>
    {
        new SelectListItem {Text = "Books", Value = "1"},
        new SelectListItem {Text = "Furniture", Value = "2"},
        new SelectListItem {Text = "Music", Value = "3"}
    };
    vm.SelectedCategory = 2;
    return View(vm);
}

Rendering a Multi select dropdown/ListBox

If you want to render a multi select dropdown, you can simply change your view model property which you use for asp-for attribute in your view to an array type.

public class CreateProduct
{       
   public IEnumerable<SelectListItem> Categories { set; get; }
   public int[] SelectedCategories { set; get; }
}

In the view

@model CreateProduct
<form asp-action="create" asp-controller="Home" >
    <select asp-for="SelectedCategories" asp-items="@Model.Categories">
        <option>Select one</option>
    </select>
    <input type="submit"/>
</form>

This will generate the SELECT element with multiple attribute

<form action="/Home/create" method="post">
     <select id="SelectedCategories" multiple="multiple" name="SelectedCategories">
        <option>Select one</option>
        <option value="1">Shyju</option>
       <option value="2">Sean</option>
     </select>
    <input type="submit"/>
</form>

Custom Tag Helper

You can create your own tag helpers by implementing ITagHelper or deriving from the convenience class TagHelper.

  • The default convention is to target an html tag that matches the name of the helper without the optional TagHelper suffix. For example WidgetTagHelper will target a <widget> tag.
  • The [HtmlTargetElement] attribute can be used to further control the tag being targetted
  • Any public property of the class can be given a value as an attribute in the razor markup. For example a public property public string Title {get; set;} can be given a value as <widget title="my title">
  • By default, tag helpers translates Pascal-cased C# class names and properties for tag helpers into lower kebab case. For example, if you omit using [HtmlTargetElement] and the class name is WidgetBoxTagHelper, then in Razor you'll write <widget-box></widget-box>.
  • Process and ProcessAsync contain the rendering logic. Both receive a context parameter with information about the current tag being rendered and an output parameter used to customize the rendered result.

Any assembly containing custom tag helpers needs to be added to the _ViewImports.cshtml file (Note it is the assembly being registered, not the namespace):

@addTagHelper *, MyAssembly

Sample Widget Custom Tag Helper

The following example creates a custom widget tag helper that will target razor markup like:

<widget-box title="My Title">This is my content: @ViewData["Message"]</widget-box>

Which will be rendered as:

<div class="widget-box">
    <div class="widget-header">My Title</div>
    <div class="widget-body">This is my content: some message</div>
</div>

The coded needed to create such a tag helper is the following:

[HtmlTargetElement("widget-box")]
public class WidgetTagHelper : TagHelper
{
    public string Title { get; set; }

    public override async Task ProcessAsync(TagHelperContext context, TagHelperOutput output)
    {
        var outerTag = new TagBuilder("div");
        outerTag.Attributes.Add("class", output.TagName);
        output.MergeAttributes(outerTag);
        output.TagName = outerTag.TagName;

        //Create the header
        var header = new TagBuilder("div");
        header.Attributes.Add("class", "widget-header");
        header.InnerHtml.Append(this.Title);
        output.PreContent.SetHtmlContent(header);

        //Create the body and replace original tag helper content
        var body = new TagBuilder("div");
        body.Attributes.Add("class", "widget-body");
        var originalContents = await output.GetChildContentAsync();
        body.InnerHtml.Append(originalContents.GetContent());
        output.Content.SetHtmlContent(body);
    }
}

Label Tag Helper

Label Tag Helper can be used to render label for a model property. It replaces method Html.LabelFor in previous versions of MVC.

Let's say you have a model:

public class FormViewModel
{
    public string Name { get; set; }
}

In the view you can use label HTML element and asp-for tag helper:

<form>
    <label asp-for="Name"></label>
    <input asp-for="Name" type="text" />
</form>

This is equivalent to the following code in earlier versions of MVC:

<form>
    @Html.LabelFor(x => x.Name)
    @Html.TextBoxFor(x => x.Name)
</form>

Both code snippets above render the same HTML:

<form>
    <label for="Name">Name</label>
    <input name="Name" id="Name" type="text" value="">
</form>

Anchor tag helper

Anchor tag helper is used generate href attributes to link to a particular controller action or MVC route. Basic example

<a asp-controller="Products" asp-action="Index">Login</a>

Sometimes, we need to specify additional parameters for the controller action that you are binding to. We can specify values for these parameters by adding attributes with the asp-route- prefix.

<a asp-controller="Products" asp-action="Details" asp-route-id="@Model.ProductId">
   View Details
</a>