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Author Tag Helpers in ASP.NET Core
rick-anderson
Learn how to author Tag Helpers in ASP.NET Core.
riande
H1Hack27Feb2017
08/20/2018
mvc/views/tag-helpers/authoring

Author Tag Helpers in ASP.NET Core

By Rick Anderson

View or download sample code (how to download)

Get started with Tag Helpers

This tutorial provides an introduction to programming Tag Helpers. Introduction to Tag Helpers describes the benefits that Tag Helpers provide.

A tag helper is any class that implements the ITagHelper interface. However, when you author a tag helper, you generally derive from TagHelper, doing so gives you access to the Process method.

  1. Create a new ASP.NET Core project called AuthoringTagHelpers. You won't need authentication for this project.

  2. Create a folder to hold the Tag Helpers called TagHelpers. The TagHelpers folder is not required, but it's a reasonable convention. Now let's get started writing some simple tag helpers.

A minimal Tag Helper

In this section, you write a tag helper that updates an email tag. For example:

<email>Support</email>

The server will use our email tag helper to convert that markup into the following:

<a href="mailto:Support@contoso.com">Support@contoso.com</a>

That is, an anchor tag that makes this an email link. You might want to do this if you are writing a blog engine and need it to send email for marketing, support, and other contacts, all to the same domain.

  1. Add the following EmailTagHelper class to the TagHelpers folder.

    [!code-csharp]

    Notes:

    • Tag helpers use a naming convention that targets elements of the root class name (minus the TagHelper portion of the class name). In this example, the root name of EmailTagHelper is email, so the <email> tag will be targeted. This naming convention should work for most tag helpers, later on I'll show how to override it.

    • The EmailTagHelper class derives from TagHelper. The TagHelper class provides methods and properties for writing Tag Helpers.

    • The overridden Process method controls what the tag helper does when executed. The TagHelper class also provides an asynchronous version (ProcessAsync) with the same parameters.

    • The context parameter to Process (and ProcessAsync) contains information associated with the execution of the current HTML tag.

    • The output parameter to Process (and ProcessAsync) contains a stateful HTML element representative of the original source used to generate an HTML tag and content.

    • Our class name has a suffix of TagHelper, which is not required, but it's considered a best practice convention. You could declare the class as:

    public class Email : TagHelper
  2. To make the EmailTagHelper class available to all our Razor views, add the addTagHelper directive to the Views/_ViewImports.cshtml file: [!code-html]

    The code above uses the wildcard syntax to specify all the tag helpers in our assembly will be available. The first string after @addTagHelper specifies the tag helper to load (Use "*" for all tag helpers), and the second string "AuthoringTagHelpers" specifies the assembly the tag helper is in. Also, note that the second line brings in the ASP.NET Core MVC tag helpers using the wildcard syntax (those helpers are discussed in Introduction to Tag Helpers.) It's the @addTagHelper directive that makes the tag helper available to the Razor view. Alternatively, you can provide the fully qualified name (FQN) of a tag helper as shown below:

@using AuthoringTagHelpers
@addTagHelper *, Microsoft.AspNetCore.Mvc.TagHelpers
@addTagHelper AuthoringTagHelpers.TagHelpers.EmailTagHelper, AuthoringTagHelpers

To add a tag helper to a view using a FQN, you first add the FQN (AuthoringTagHelpers.TagHelpers.EmailTagHelper), and then the assembly name (AuthoringTagHelpers). Most developers will prefer to use the wildcard syntax. Introduction to Tag Helpers goes into detail on tag helper adding, removing, hierarchy, and wildcard syntax.

  1. Update the markup in the Views/Home/Contact.cshtml file with these changes:

    [!code-html]

  2. Run the app and use your favorite browser to view the HTML source so you can verify that the email tags are replaced with anchor markup (For example, <a>Support</a>). Support and Marketing are rendered as a links, but they don't have an href attribute to make them functional. We'll fix that in the next section.

SetAttribute and SetContent

In this section, we'll update the EmailTagHelper so that it will create a valid anchor tag for email. We'll update it to take information from a Razor view (in the form of a mail-to attribute) and use that in generating the anchor.

Update the EmailTagHelper class with the following:

[!code-csharp]

Notes:

  • Pascal-cased class and property names for tag helpers are translated into their lower kebab case. Therefore, to use the MailTo attribute, you'll use <email mail-to="value"/> equivalent.

  • The last line sets the completed content for our minimally functional tag helper.

  • The highlighted line shows the syntax for adding attributes:

[!code-csharp]

That approach works for the attribute "href" as long as it doesn't currently exist in the attributes collection. You can also use the output.Attributes.Add method to add a tag helper attribute to the end of the collection of tag attributes.

  1. Update the markup in the Views/Home/Contact.cshtml file with these changes: [!code-html]

  2. Run the app and verify that it generates the correct links.

[!NOTE] If you were to write the email tag self-closing (<email mail-to="Rick" />), the final output would also be self-closing. To enable the ability to write the tag with only a start tag (<email mail-to="Rick">) you must decorate the class with the following:

[!code-csharp]

With a self-closing email tag helper, the output would be <a href="mailto:Rick@contoso.com" />. Self-closing anchor tags are not valid HTML, so you wouldn't want to create one, but you might want to create a tag helper that's self-closing. Tag helpers set the type of the TagMode property after reading a tag.

ProcessAsync

In this section, we'll write an asynchronous email helper.

  1. Replace the EmailTagHelper class with the following code:

    [!code-csharp]

    Notes:

    • This version uses the asynchronous ProcessAsync method. The asynchronous GetChildContentAsync returns a Task containing the TagHelperContent.

    • Use the output parameter to get contents of the HTML element.

  2. Make the following change to the Views/Home/Contact.cshtml file so the tag helper can get the target email.

    [!code-html]

  3. Run the app and verify that it generates valid email links.

RemoveAll, PreContent.SetHtmlContent and PostContent.SetHtmlContent

  1. Add the following BoldTagHelper class to the TagHelpers folder.

    [!code-csharp]

    Notes:

    • The [HtmlTargetElement] attribute passes an attribute parameter that specifies that any HTML element that contains an HTML attribute named "bold" will match, and the Process override method in the class will run. In our sample, the Process method removes the "bold" attribute and surrounds the containing markup with <strong></strong>.

    • Because you don't want to replace the existing tag content, you must write the opening <strong> tag with the PreContent.SetHtmlContent method and the closing </strong> tag with the PostContent.SetHtmlContent method.

  2. Modify the About.cshtml view to contain a bold attribute value. The completed code is shown below.

    [!code-html]

  3. Run the app. You can use your favorite browser to inspect the source and verify the markup.

    The [HtmlTargetElement] attribute above only targets HTML markup that provides an attribute name of "bold". The <bold> element wasn't modified by the tag helper.

  4. Comment out the [HtmlTargetElement] attribute line and it will default to targeting <bold> tags, that is, HTML markup of the form <bold>. Remember, the default naming convention will match the class name BoldTagHelper to <bold> tags.

  5. Run the app and verify that the <bold> tag is processed by the tag helper.

Decorating a class with multiple [HtmlTargetElement] attributes results in a logical-OR of the targets. For example, using the code below, a bold tag or a bold attribute will match.

[!code-csharp]

When multiple attributes are added to the same statement, the runtime treats them as a logical-AND. For example, in the code below, an HTML element must be named "bold" with an attribute named "bold" (<bold bold />) to match.

[HtmlTargetElement("bold", Attributes = "bold")]

You can also use the [HtmlTargetElement] to change the name of the targeted element. For example if you wanted the BoldTagHelper to target <MyBold> tags, you would use the following attribute:

[HtmlTargetElement("MyBold")]

Pass a model to a Tag Helper

  1. Add a Models folder.

  2. Add the following WebsiteContext class to the Models folder:

    [!code-csharp]

  3. Add the following WebsiteInformationTagHelper class to the TagHelpers folder.

    [!code-csharp]

    Notes:

    • As mentioned previously, tag helpers translates Pascal-cased C# class names and properties for tag helpers into lower kebab case. Therefore, to use the WebsiteInformationTagHelper in Razor, you'll write <website-information />.

    • You are not explicitly identifying the target element with the [HtmlTargetElement] attribute, so the default of website-information will be targeted. If you applied the following attribute (note it's not kebab case but matches the class name):

    [HtmlTargetElement("WebsiteInformation")]

    The lower kebab case tag <website-information /> wouldn't match. If you want use the [HtmlTargetElement] attribute, you would use kebab case as shown below:

    [HtmlTargetElement("Website-Information")]
    • Elements that are self-closing have no content. For this example, the Razor markup will use a self-closing tag, but the tag helper will be creating a section element (which isn't self-closing and you are writing content inside the section element). Therefore, you need to set TagMode to StartTagAndEndTag to write output. Alternatively, you can comment out the line setting TagMode and write markup with a closing tag. (Example markup is provided later in this tutorial.)

    • The $ (dollar sign) in the following line uses an interpolated string:

    $@"<ul><li><strong>Version:</strong> {Info.Version}</li>
  4. Add the following markup to the About.cshtml view. The highlighted markup displays the web site information.

    [!code-html]

    [!NOTE] In the Razor markup shown below:

    [!code-html]

    Razor knows the info attribute is a class, not a string, and you want to write C# code. Any non-string tag helper attribute should be written without the @ character.

  5. Run the app, and navigate to the About view to see the web site information.

    [!NOTE] You can use the following markup with a closing tag and remove the line with TagMode.StartTagAndEndTag in the tag helper:

    [!code-html]

Condition Tag Helper

The condition tag helper renders output when passed a true value.

  1. Add the following ConditionTagHelper class to the TagHelpers folder.

    [!code-csharp]

  2. Replace the contents of the Views/Home/Index.cshtml file with the following markup:

    [!code-csharp]

  3. Replace the Index method in the Home controller with the following code:

    [!code-csharp]

  4. Run the app and browse to the home page. The markup in the conditional div won't be rendered. Append the query string ?approved=true to the URL (for example, http://localhost:1235/Home/Index?approved=true). approved is set to true and the conditional markup will be displayed.

[!NOTE] Use the nameof operator to specify the attribute to target rather than specifying a string as you did with the bold tag helper:

[!code-csharp]

The nameof operator will protect the code should it ever be refactored (we might want to change the name to RedCondition).

Avoid Tag Helper conflicts

In this section, you write a pair of auto-linking tag helpers. The first will replace markup containing a URL starting with HTTP to an HTML anchor tag containing the same URL (and thus yielding a link to the URL). The second will do the same for a URL starting with WWW.

Because these two helpers are closely related and you may refactor them in the future, we'll keep them in the same file.

  1. Add the following AutoLinkerHttpTagHelper class to the TagHelpers folder.

    [!code-csharp]

    [!NOTE] The AutoLinkerHttpTagHelper class targets p elements and uses Regex to create the anchor.

  2. Add the following markup to the end of the Views/Home/Contact.cshtml file:

    [!code-html]

  3. Run the app and verify that the tag helper renders the anchor correctly.

  4. Update the AutoLinker class to include the AutoLinkerWwwTagHelper which will convert www text to an anchor tag that also contains the original www text. The updated code is highlighted below:

    [!code-csharp]

  5. Run the app. Notice the www text is rendered as a link but the HTTP text isn't. If you put a break point in both classes, you can see that the HTTP tag helper class runs first. The problem is that the tag helper output is cached, and when the WWW tag helper is run, it overwrites the cached output from the HTTP tag helper. Later in the tutorial we'll see how to control the order that tag helpers run in. We'll fix the code with the following:

    [!code-csharp]

    [!NOTE] In the first edition of the auto-linking tag helpers, you got the content of the target with the following code:

    [!code-csharp]

    That is, you call GetChildContentAsync using the TagHelperOutput passed into the ProcessAsync method. As mentioned previously, because the output is cached, the last tag helper to run wins. You fixed that problem with the following code:

    [!code-csharp]

    The code above checks to see if the content has been modified, and if it has, it gets the content from the output buffer.

  6. Run the app and verify that the two links work as expected. While it might appear our auto linker tag helper is correct and complete, it has a subtle problem. If the WWW tag helper runs first, the www links won't be correct. Update the code by adding the Order overload to control the order that the tag runs in. The Order property determines the execution order relative to other tag helpers targeting the same element. The default order value is zero and instances with lower values are executed first.

    [!code-csharp]

    The above code will guarantee that the HTTP tag helper runs before the WWW tag helper. Change Order to MaxValue and verify that the markup generated for the WWW tag is incorrect.

Inspect and retrieve child content

The tag helpers provide several properties to retrieve content.

  • The result of GetChildContentAsync can be appended to output.Content.
  • You can inspect the result of GetChildContentAsync with GetContent.
  • If you modify output.Content, the TagHelper body won't be executed or rendered unless you call GetChildContentAsync as in our auto-linker sample:

[!code-csharp]

  • Multiple calls to GetChildContentAsync returns the same value and doesn't re-execute the TagHelper body unless you pass in a false parameter indicating not to use the cached result.