What is a tag helper in ASP.NET Core and how does it work?

A tag helper is a feature of ASP.NET Core that allows you to create custom HTML tags and attributes that can be used in Razor views. Tag helpers are a way to encapsulate complex HTML markup and logic into reusable components that can be easily integrated into your application.

Tag helpers are defined as C# classes that derive from the TagHelper base class and are decorated with the HtmlTargetElement attribute to specify the HTML elements that the tag helper applies to. Tag helpers can then implement the ProcessAsync method to modify the HTML markup and attributes of the element.

Here's an example of a simple tag helper that adds a custom attribute to a div element:

[HtmlTargetElement("div", Attributes = "custom-attribute")]
public class CustomTagHelper : TagHelper
{
    public string CustomAttribute { get; set; }

    public override async Task ProcessAsync(TagHelperContext context, TagHelperOutput output)
    {
        output.Attributes.SetAttribute("data-custom-attribute", CustomAttribute);
        await base.ProcessAsync(context, output);
    }
}

In this example, the CustomTagHelper class is defined to target div elements with a custom-attribute attribute. The CustomAttribute property is used to set the value of the data-custom-attribute attribute in the generated HTML markup. The ProcessAsync method is used to modify the TagHelperOutput object that represents the generated HTML markup.

To use the tag helper in a Razor view, you can use the tag helper syntax:

 <div custom-attribute="Hello, world!" />

In this example, the div element is decorated with the custom-attribute attribute, which is processed by the CustomTagHelper class to generate the HTML markup with the data-custom-attribute attribute set to the value "Hello, world!".

Tag helpers can also be used to generate complex HTML markup, handle user input, and perform server-side processing. Tag helpers are a powerful way to encapsulate HTML markup and logic into reusable components that can be easily integrated into your application.