Adding a reCAPTCHA V3 tag. This can be used to automatically bind the challenge to a button.

Author: jooni91

src/ReCaptcha/Localization/Resources.Designer.cs

@@ -117,6 +117,9 @@
   <resheader name="writer">
     <value>System.Resources.ResXResourceWriter, System.Windows.Forms, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089</value>
   </resheader>
+  <data name="ActionPropertyNullErrorMessage" xml:space="preserve">
+    <value>The action is a madatory property, but was not set. </value>
+  </data>
   <data name="CallbackPropertyNullErrorMessage" xml:space="preserve">
     <value>A callback function name must be specified. Invisible reCAPTCHA does not work without it.</value>
   </data>

src/ReCaptcha/Localization/Resources.resx

@@ -0,0 +1,109 @@
+using System;
+using System.Text.Encodings.Web;
+using Griesoft.AspNetCore.ReCaptcha.Configuration;
+using Griesoft.AspNetCore.ReCaptcha.Extensions;
+using Griesoft.AspNetCore.ReCaptcha.Localization;
+using Microsoft.AspNetCore.Mvc.Razor.TagHelpers;
+using Microsoft.AspNetCore.Razor.TagHelpers;
+using Microsoft.Extensions.Options;
+
+namespace Griesoft.AspNetCore.ReCaptcha.TagHelpers
+{
+    /// <summary>
+    /// A reCAPTCHA V3 tag helper, which can be used to automatically bind the challenge to a button.
+    /// </summary>
+    /// <remarks>
+    /// The <see cref="FormId"/> is required. With the exception that you set a <see cref="Callback"/> instead.
+    /// When setting both the value set to <c>Callback</c> always wins and the <c>FormId</c> value is basically irrelevant.
+    /// 
+    /// For easiest use of this tag helper set only the <c>FormId</c>. This will add a default callback function to the body. That function does
+    /// submit the form after a successful reCAPTCHA challenge.
+    /// 
+    /// If the tag is not inside the form that is going to be submitted, you should use a custom callback function. The default callback function
+    /// does not add the reCAPTCHA token to the form, which will result in response verification failure.
+    /// </remarks>
+    /// <example>
+    /// The simplest use of the tag would be:
+    /// <code>
+    /// <recaptchav3 formid="myForm" action="submit">Submit</recaptchav3>
+    /// </code>
+    /// 
+    /// Which will translate into the following HTML:
+    /// <code>
+    /// <button class="g-recaptcha" data-sitekey="your_site_key" data-callback='submitmyForm' data-action='submit'>Submit</button>
+    /// </code>
+    /// </example>
+    [HtmlTargetElement(Attributes = "callback,action")]
+    [HtmlTargetElement(Attributes = "formid,action")]
+    public class RecaptchaV3TagHelper : TagHelper
+    {
+        private readonly ITagHelperComponentManager _tagHelperComponentManager;
+        private readonly RecaptchaSettings _settings;
+
+        /// <summary>
+        /// 
+        /// </summary>
+        /// <param name="settings"></param>
+        /// <param name="tagHelperComponentManager"></param>
+        /// <exception cref="ArgumentNullException"></exception>
+        public RecaptchaV3TagHelper(IOptionsMonitor<RecaptchaSettings> settings, ITagHelperComponentManager tagHelperComponentManager)
+        {
+            _ = settings ?? throw new ArgumentNullException(nameof(settings));
+            _ = tagHelperComponentManager ?? throw new ArgumentNullException(nameof(tagHelperComponentManager));
+
+            _settings = settings.CurrentValue;
+            _tagHelperComponentManager = tagHelperComponentManager;
+        }
+
+        /// <summary>
+        /// The id of the form that will be submitted after a successful reCAPTCHA challenge. 
+        /// </summary>
+        /// <remarks>This does only apply when not specifying a <see cref="Callback"/>.</remarks>
+        public string? FormId { get; set; }
+
+        /// <summary>
+        /// Set the name of your callback function, which is called when the reCAPTCHA challenge was successful. 
+        /// A "g-recaptcha-response" token is added to your callback function parameters for server-side verification.
+        /// </summary>
+        public string Callback { get; set; } = string.Empty;
+
+        /// <summary>
+        /// The name of the action that was triggered.
+        /// </summary>
+        /// <remarks>You should verify that the server-side verification response returns the same action.</remarks>
+        public string Action { get; set; } = string.Empty;
+
+        /// <inheritdoc />
+        /// <exception cref="ArgumentNullException"></exception>
+        /// <exception cref="NullReferenceException">Thrown when both <see cref="Callback"/> and <see cref="FormId"/> or <see cref="Action"/> are/is null or empty.</exception>
+        public override void Process(TagHelperContext context, TagHelperOutput output)
+        {
+            _ = output ?? throw new ArgumentNullException(nameof(output));
+
+            if (string.IsNullOrEmpty(Callback) && string.IsNullOrEmpty(FormId))
+            {
+                throw new NullReferenceException(Resources.CallbackPropertyNullErrorMessage);
+            }
+
+            if (string.IsNullOrEmpty(Action))
+            {
+                throw new NullReferenceException(Resources.ActionPropertyNullErrorMessage);
+            }
+
+            if (string.IsNullOrEmpty(Callback))
+            {
+                Callback = $"submit{FormId}";
+                _tagHelperComponentManager.Components.Add(new CallbackScriptTagHelperComponent(FormId!));
+            }
+
+            output.TagMode = TagMode.StartTagAndEndTag;
+
+            output.TagName = "button";
+            output.AddClass("g-recaptcha", HtmlEncoder.Default);
+
+            output.Attributes.SetAttribute("data-sitekey", _settings.SiteKey);
+            output.Attributes.SetAttribute("data-callback", Callback);
+            output.Attributes.SetAttribute("data-action", Action);
+        }
+    }
+}