In this post, we’ll see how to create a custom tag helper in ASP.NET Core project.

Custom tag helpers can be created by implementing TagHelper class. The Tag helper class has two methods that can be overridden.

Tag Helper class:

public abstract class TagHelper : Microsoft.AspNetCore.Razor.TagHelpers.ITagHelper { }

Methods of Tag helper class:

Method name Summary Init(Tag Helper Context) Initializes the ITagHelper with the given context. Additions to Items should be done within this method to ensure they’re added prior to executing the children. Process(Tag Helper Context, Tag Helper Output) Synchronously executes the TagHelper with the given context and output. Process Async(Tag Helper Context, Tag Helper Output) Asynchronously executes the TagHelper with the given context and output.

For the purpose of this post, we’ll override the Process method to synchronously execute the tag helper with the context and output our HTML .

Use Case

In this article, we’ll create a label with color by default. So, the user can supply the color by providing color as an attribute to the colorlabel .

Ex:

<color-label color="red">red color</color-label>

The above custom tag should output the label in red color.

Implementing TagHelper class

We’ll create a new ColorLabelTagHelper class and let’s inherit from TagHelper class which is in Microsoft.AspNetCore.Razor.TagHelpers namespace.

Note that TagHelper at the end of the class name is just a convention. We can omit the convention and just have the name directly.

using Microsoft.AspNetCore.Razor.TagHelpers; namespace CustomTagHelpers.TagHelpers { ​ public class ColorLabelTagHelper : TagHelper ​ { ​ public string Color { get; set; } ​ public override void Process(TagHelperContext context, TagHelperOutput output) ​ { ​ output.TagName = "coloredlabel"; ​ var colorStyle = $"color:{Color}"; ​ output.Attributes.SetAttribute("style", colorStyle); ​ } ​ } }

The Color property in the above class will act as an attribute to the coloredlabel . So, you can set color from the HTML.

Observe how we are setting the attributes for the tag using output.Attributes.SetAttribute . We can set all the necessary html attributes that the element can support.

Access custom tag helper in views

Once we’ve done that we need to use this tag helper in our views with the @addTagHelper directive. Now, goto _ViewImports.cshtml file in the views folder and add the following line

@addTagHelper *, CustomTagHelpers

You could specify just the namespace of our custom tag helper but here I’m adding everything in the assembly.

Using the custom tag helper in Views

Let’s go ahead and try to get our custom tag helper working.

For the purpose of this article, I’ll clear all the existing HTML tags in the Index.cshtml file and add our awesome colorlabel custom tag.

Here is how the Index.cshtml looks after adding the custom tag.

@{ ​ ViewData["Title"] = "Index"; } <h2>Index</h2> <colorlabel color="red">the red color</colorlabel>

Here is the output. Yay

Note that we didn’t get the red color that we’ve expected. This is because we have the wrong syntax of custom tag helper in place.

Custom tag helpers properties to Kebab case

The Pascal-cased class and the property names of the tag helpers will be translated into their lower kebab case. So, in order to use our custom helper class and property, we need to use <color-label color="red">the red color</color-label> .

Let’s try that in our Index.cshtml page and see it.

We got our custom tag helpers working now.

Let’s take a look at the HTML generated

<coloredlabel style="color:red">the red color</coloredlabel>

We have the new tag in the HTML i.e., coloredlabel this is because we’ve specified the output tag name to be coloredlabel .

output.TagName = "coloredlabel";

And the style attribute will be generated as any normal HTML tag would generate.

Set Content

The SetContent method will set the content of the HTML tag. Note that this method just sets the content and not the HTML.

Let’s modify our coloredlabel class to facilitate the Set content method.

<color-label color="red">Should output red</color-label>

public override void Process(TagHelperContext context, TagHelperOutput output) { ​ output.TagName = "coloredlabel"; colorStyle = $"color:{Color}"; output.Attributes.SetAttribute("style", colorStyle); if (Color == "red") output.Content.SetContent("Text from custom helper"); }

This will just modify only the contents of the tags with red color. So, our color label will output as Text from custom helper (in red color).

Pre Content and Post Content

Pre content and Post content can modify the head and tail of the custom tag helper. We can modify the contents of the tag.

Let’s see how to append the content at the beginning and at the end.

output.PreContent.Append("Start: "); output.PostContent.Append(" :end");

When the above code is executed it will output like this:

Start: Text from custom helper :end (in red)

Let’s try to wrap our custom tag helper content with a div that has a 2px border and blue color.

We’ve to add HTML content around our custom tag helper.

public override void Process(TagHelperContext context, TagHelperOutput output) { ​ output.TagName = "coloredlabel"; var colorStyle = $"color:{Color}"; output.Attributes.SetAttribute("style", colorStyle); output.PreContent.SetHtmlContent("<div style='border: 2px solid blue; padding: 5px'>"); output.PostContent.SetHtmlContent("</div>"); }

Notice how we are ending the div tag with PostContent .

This is how it looks like

Let’s examine the generated HTML for the above code.

<coloredlabel style="color:red"> <div style="border: 2px solid blue; padding: 5px">Should output red</div></coloredlabel>

So, we got our <div> element inside of coloredlabel tag because we’ve used PreContent and Postcontent. As the word says, it just the content that is going to wrap not the HTML.

How to wrap up the custom helper? (PreElement, PostElement)

The PreElement and the PostElement will wrap the custom helper inside the element that we supply.

Let’s try the same example of wrapping our custom tag helper with a div that has 2px border.

public override void Process(TagHelperContext context, TagHelperOutput output) { ​ output.TagName = "coloredlabel"; var colorStyle = $"color:{Color}"; output.Attributes.SetAttribute("style", colorStyle); output.PreElement.SetHtmlContent("<div style='border: 2px solid blue; padding: 5px'>"); output.PostElement.SetHtmlContent("</div>"); }

we’ve just replaced PreContent and PostContent in the previous example with PreElement and PostElement. Let’s see the output

Now, the output is the same as PreContent and PostContent . But, the HTML should vary. Let’s see the HTML generated.

<div style="border: 2px solid blue; padding: 5px"> <coloredlabel style="color:red">Should output red</coloredlabel> </div>

Did you see the difference here? The <div> tag wrapped around custom tag helper in this case.

So, it is up to the developer whether to use Pre/Post Content or to use Pre/Post Element.

Conclusion

Custom tag helpers allow us to customize the content of the HTML by providing some useful methods to tweak the HTML and render the content dynamically.

Tag helpers were added with an intension to make the UI developers life easy to mitigate with the HTML. If the markup is modified with PostContent, PreContent or with PreElement, PostElement the UI developer will have a hard time figuring out how the element is generated/added

Though the PreContent, PostContent behaved same as PreElement, PostElement. It is up to you to decide what markup you need.

If we gave the wrong styles or wrong HTML, the markup will be rejected and only the plain HTML will be displayed. This is a good thing. Try it yourself.

Thanks for reading.

References

Tag helper class from msdn.