Why is the TagHelper HtmlAttributeName not resolved in ASP.NET Core 2.2

Question

I'm working on a custom TagHelper, for which I have used the example from Microsoft.

public class EmailTagHelper : TagHelper
{
    private const string EmailDomain = "contoso.com";

    // Can be passed via <email mail-to="..." />. 
    // PascalCase gets translated into kebab-case.
    [HtmlAttributeName("mail-to")]
    public string MailTo { get; set; }

    public override async Task ProcessAsync(TagHelperContext context, TagHelperOutput output)
    {
        output.TagName = "a";                                 // Replaces <email> with <a> tag
        var content = await output.GetChildContentAsync();
        var target = content.GetContent() + "@" + EmailDomain;
        output.Attributes.SetAttribute("href", "mailto:" + target);

        output.Content.SetContent(target);
    }
}

I have used it with these examples:

<email>Support</email>
<email mail-to="Support"></email>

The first example works as expected and gives the following output:

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

But the second example, where the mail-to HtmlAttributeName is used, it is not resolved, as shown below.

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

This took me all afternoon by now, and I'm probably going to bang my head afterwards, but could someone tell me why this is not working?

Answer 1

You've declared MailTo and bound it to the mail-to attribute, but you don't use MailTo in ProcessAsync. Instead, you're always using the GetChildContentAsync() method to get the content.

You'll need to make a decision about which takes precedence or whether you want to generate some kind of exception if both mail-to and content are provided, etc. Here's a rough example that prefers the attribute, for completeness:

public override async Task ProcessAsync(TagHelperContext context, TagHelperOutput output)
{
    output.TagName = "a";

    var mailTo = MailTo;

    if (string.IsNullOrWhiteSpace(mailTo))
    {
        var content = await output.GetChildContentAsync();
        mailTo = content.GetContent();
    }

    var target = mailTo + "@" + EmailDomain;
    output.Attributes.SetAttribute("href", "mailto:" + target);

    output.Content.SetContent(target);
}