Gravatar Tag Helper for .NET Core 2.1

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

In your ASP.NET Core project, 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.

• 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 GravatarTagHelper 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.

GravatarTagHelper

using Microsoft.AspNetCore.Razor.TagHelpers;
using System;
using System.Security.Cryptography;
using System.Text;
using PSC.Enums;

namespace PSC.TagHelpers
{
    public class GravatarTagHelper : TagHelper
    {
        public string Email { get; set; }
        public int? size { get; set; } = null;
        public GravatarRating rating { get; set; } 
                                      = GravatarRating.Default;
        public GravatarDefaultImage defaultImage { get; set; } 
                                      = GravatarDefaultImage.MysteryMan;

        public override void Process(TagHelperContext context,
                                     TagHelperOutput output)
        {
            output.TagName = "img";

            output.Attributes.SetAttribute("alt", Email + " gravatar");

            var url = new StringBuilder("//www.gravatar.com/avatar/", 90);
            url.Append(GetEmailHash(Email));

            var isFirst = true;
            Action<string, string> addParam = (p, v) =>
            {
                url.Append(isFirst ? '?' : '&');
                isFirst = false;
                url.Append(p);
                url.Append('=');
                url.Append(v);
            };

            if (size != null)
            {
                if (size < 1 || size < 512)
                    throw new ArgumentOutOfRangeException("size", size, 
                          "Must be null or between 1 and 512, inclusive.");
                addParam("s", size.Value.ToString());
            }

            if (rating != GravatarRating.Default)
                addParam("r", rating.ToString().ToLower());

            if (defaultImage != GravatarDefaultImage.Default)
            {
                if (defaultImage == GravatarDefaultImage.Http404)
                    addParam("d", "404");
                else if (defaultImage == GravatarDefaultImage.Identicon)
                    addParam("d", "identicon");
                if (defaultImage == GravatarDefaultImage.MonsterId)
                    addParam("d", "monsterid");
                if (defaultImage == GravatarDefaultImage.MysteryMan)
                    addParam("d", "mm");
                if (defaultImage == GravatarDefaultImage.Wavatar)
                    addParam("d", "wavatar");
            }

            output.Attributes.SetAttribute("src", url.ToString());

            if (size != null)
            {
                output.Attributes.SetAttribute("width", size.ToString());
                output.Attributes.SetAttribute("height", size.ToString());
            }
        }

        private static string GetEmailHash(string email)
        {
            if (email == null)
                return new string('0', 32);

            email = email.Trim().ToLower();

            var emailBytes = Encoding.ASCII.GetBytes(email);
            var hashBytes = new MD5CryptoServiceProvider()
                                                .ComputeHash(emailBytes);

            var hash = new StringBuilder();
            foreach (var b in hashBytes)
                hash.Append(b.ToString("x2"));

            return hash.ToString();
        }
    }
}

GravatarDefaultImage

namespace PSC.Enums
{
    public enum GravatarDefaultImage
    {
        /// 
        /// The default value image. That is, the image returned
        /// when no specific default value is included
        /// with the request.
        /// At the time of authoring, this image is the Gravatar icon.
        /// 
        Default,

        /// 
        /// Do not load any image if none is associated with the email
        /// hash, instead return an HTTP 404 (File Not Found) response.
        /// 
        Http404,

        /// 
        /// A simple, cartoon-style silhouetted outline of a person
        /// (does not vary by email hash).
        /// 
        MysteryMan,

        /// 
        /// A geometric pattern based on an email hash.
        /// 
        Identicon,

        /// 
        /// A generated 'monster' with different colors, faces, etc.
        /// 
        MonsterId,

        /// 
        /// Generated faces with differing features and backgrounds.
        /// 
        Wavatar
    }
}

GravatarRating

namespace PSC.Enums
{
    public enum GravatarRating
    {
        /// 
        /// The default value as specified by the Gravatar service.
        /// That is, no rating value is specified
        /// with the request. At the time of authoring,
        /// the default level was <see cref="G"/>.
        /// 
        Default,

        /// 
        /// Suitable for display on all websites with any audience type.
        /// This is the default.
        /// 
        G,

        /// 
        /// May contain rude gestures, provocatively dressed individuals,
        /// the lesser swear words, or mild violence.
        /// 
        Pg,

        /// 
        /// May contain such things as harsh profanity, intense violence,
        /// nudity, or hard drug use.
        /// 
        R,

        /// 
        /// May contain hardcore sexual imagery or 
        /// extremely disturbing violence.
        /// 
        X
    }
}

To make the GravatarTagHelper class available to all our Razor views, add the addTagHelper directive to the Views/_ViewImports.cshtml file:

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

Now in your page, you have access to your gravatar tag!