[Perf] Using single UrlHelper per HttpContext and one StringBuilder per UrlHelper to reduce allocations

src/Microsoft.AspNetCore.Mvc.Core/Routing/UrlHelper.cs

@@ -15,6 +15,10 @@ namespace Microsoft.AspNetCore.Mvc.Routing
     /// </summary>
     public class UrlHelper : IUrlHelper
     {
+
+        // Perf: Share the StringBuilder object across multiple calls of GenerateURL for this UrlHelper
+        private StringBuilder _stringBuilder;
+
         /// <summary>
         /// Initializes a new instance of the <see cref="UrlHelper"/> class using the specified action context and
         /// action selector.
@@ -201,6 +205,16 @@ namespace Microsoft.AspNetCore.Mvc.Routing
             });
         }
 
+        private StringBuilder GetStringBuilder()
+        {
+            if(_stringBuilder == null)
+            {
+                _stringBuilder = new StringBuilder();
+            }
+
+            return _stringBuilder;
+        }
+
         /// <summary>
         /// Generates the URL using the specified components.
         /// </summary>
@@ -219,29 +233,38 @@ namespace Microsoft.AspNetCore.Mvc.Routing
             // VirtualPathData.VirtualPath returns string.Empty instead of null.
             Debug.Assert(pathData.VirtualPath != null);
 
-            var builder = new StringBuilder();
+            var builder = GetStringBuilder();
-            if (string.IsNullOrEmpty(protocol) && string.IsNullOrEmpty(host))
+            try
             {
-                AppendPathAndFragment(builder, pathData, fragment);
+                if (string.IsNullOrEmpty(protocol) && string.IsNullOrEmpty(host))
-                // We're returning a partial URL (just path + query + fragment), but we still want it to be rooted.
-                if (builder.Length == 0 || builder[0] != '/')
                 {
-                    builder.Insert(0, '/');
+                    AppendPathAndFragment(builder, pathData, fragment);
+                    // We're returning a partial URL (just path + query + fragment), but we still want it to be rooted.
+                    if (builder.Length == 0 || builder[0] != '/')
+                    {
+                        builder.Insert(0, '/');
+                    }
                 }
+                else
+                {
+                    protocol = string.IsNullOrEmpty(protocol) ? "http" : protocol;
+                    builder.Append(protocol);
+
+                    builder.Append("://");
+
+                    host = string.IsNullOrEmpty(host) ? HttpContext.Request.Host.Value : host;
+                    builder.Append(host);
+                    AppendPathAndFragment(builder, pathData, fragment);
+                }
+
+                var path = builder.ToString();
+                return path;
             }
-            else
+            finally
             {
-                protocol = string.IsNullOrEmpty(protocol) ? "http" : protocol;
+                // Clear the StringBuilder so that it can reused for the next call.
-                builder.Append(protocol);
+                builder.Clear();
-
-                builder.Append("://");
-
-                host = string.IsNullOrEmpty(host) ? HttpContext.Request.Host.Value : host;
-                builder.Append(host);
-                AppendPathAndFragment(builder, pathData, fragment);
             }
-
-            return builder.ToString();
         }
     }
 }

src/Microsoft.AspNetCore.Mvc.Core/Routing/UrlHelperFactory.cs

@@ -1,6 +1,10 @@
 // Copyright (c) .NET Foundation. All rights reserved.
 // Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
 
+using System;
+using Microsoft.AspNetCore.Http;
+using Microsoft.AspNetCore.Mvc.Core;
+
 namespace Microsoft.AspNetCore.Mvc.Routing
 {
     /// <summary>
@@ -11,7 +15,31 @@ namespace Microsoft.AspNetCore.Mvc.Routing
         /// <inheritdoc />
         public IUrlHelper GetUrlHelper(ActionContext context)
         {
-            return new UrlHelper(context);
+            var httpContext = context.HttpContext;
+
+            if (httpContext == null)
+            {
+                throw new ArgumentException(Resources.FormatPropertyOfTypeCannotBeNull(
+                    nameof(ActionContext.HttpContext),
+                    nameof(ActionContext)));
+            }
+
+            if (httpContext.Items == null)
+            {
+                throw new ArgumentException(Resources.FormatPropertyOfTypeCannotBeNull(
+                    nameof(HttpContext.Items),
+                    nameof(HttpContext)));
+            }
+
+            // Perf: Create only one UrlHelper per context
+            var urlHelper = httpContext.Items[typeof(IUrlHelper)] as IUrlHelper;
+            if (urlHelper == null)
+            {
+                urlHelper = new UrlHelper(context);
+                httpContext.Items[typeof(IUrlHelper)] = urlHelper;
+            }
+
+            return urlHelper;
         }
     }
 }

test/Microsoft.AspNetCore.Mvc.Core.Test/LocalRedirectResultTest.cs

@@ -3,6 +3,7 @@
 
 using System;
 using Microsoft.AspNetCore.Http;
+using Microsoft.AspNetCore.Http.Internal;
 using Microsoft.AspNetCore.Mvc.Abstractions;
 using Microsoft.AspNetCore.Mvc.Infrastructure;
 using Microsoft.AspNetCore.Mvc.Routing;
@@ -120,6 +121,8 @@ namespace Microsoft.AspNetCore.Mvc
                        .Returns(response);
             httpContext.SetupGet(o => o.RequestServices)
                        .Returns(serviceProvider);
+            httpContext.SetupGet(o => o.Items)
+                       .Returns(new ItemsDictionary());
             httpContext.Setup(o => o.Request.PathBase)
                        .Returns(new PathString(appRoot));
 

test/Microsoft.AspNetCore.Mvc.Core.Test/RedirectResultTest.cs

@@ -3,6 +3,7 @@
 
 using System;
 using Microsoft.AspNetCore.Http;
+using Microsoft.AspNetCore.Http.Internal;
 using Microsoft.AspNetCore.Mvc.Abstractions;
 using Microsoft.AspNetCore.Mvc.Infrastructure;
 using Microsoft.AspNetCore.Mvc.Routing;
@@ -128,6 +129,8 @@ namespace Microsoft.AspNetCore.Mvc
                        .Returns(response);
             httpContext.SetupGet(o => o.RequestServices)
                        .Returns(serviceProvider);
+            httpContext.SetupGet(o => o.Items)
+                       .Returns(new ItemsDictionary());
             httpContext.Setup(o => o.Request.PathBase)
                        .Returns(new PathString(appRoot));