A quick Tag Helper for adding an external link icon to external links.

ExternalLinkTagHelper.cs

using System;
using System.Linq;
using System.Threading.Tasks;
using Microsoft.AspNet.Razor.TagHelpers;
using Microsoft.AspNet.Http;

namespace McPeak.TagHelpers {
    
    [HtmlTargetElement("a", Attributes = "href")]
    public class ExternalLinkTagHelper : TagHelper {
        private readonly string _hostName;
        private readonly string[] _schemesToIgnore = new[] {
            "mailto",
            "javascript"
        };
        
        public ExternalLinkTagHelper(IHttpContextAccessor httpContext) {
            _hostName = httpContext.HttpContext.Request.Host.ToString();
        }
        
        public async override Task ProcessAsync(TagHelperContext context, TagHelperOutput output) {
            var href = context.AllAttributes["href"].Value.ToString();
                     
            Uri uri;
            
            if (NotAbsoluteHref(href, out uri) || 
                ShouldFilterScheme(uri) || 
                IsInternalLink(uri)) {
                return;
            }
            
            var existingContent = await output.GetChildContentAsync();
            
            output.Content.Append(existingContent);
            
            // assuming font awesome
            output.Content.AppendHtml(@" <i class=""fa fa-external-link"" aria-hidden=""true""></i>");
        }
        
        private bool NotAbsoluteHref(string href, out Uri uri) {
            return !Uri.TryCreate(href, UriKind.Absolute, out uri);
        }
        
        private bool ShouldFilterScheme(Uri uri) {
            return _schemesToIgnore.Contains(uri.Scheme.ToLower());
        }
        
        private bool IsInternalLink(Uri uri) {
            return uri.Host == _hostName;
        }
    }
}