[Instrumentation.AspNet] Improved route template normalization performance (#3241)

Co-authored-by: Martin Costello <[email protected]>

Author: matt-hensley

src/OpenTelemetry.Instrumentation.AspNet/CHANGELOG.md

@@ -2,6 +2,10 @@
 
 ## Unreleased
 
+* Improved performance of replacing static tokens with actual values
+  in the route template.
+  ([#3241](https://github.com/open-telemetry/opentelemetry-dotnet-contrib/pull/3241))
+
 ## 1.13.0-beta.1
 
 Released 2025-Oct-15

src/OpenTelemetry.Instrumentation.AspNet/Implementation/HttpRequestRouteHelper.cs

@@ -1,6 +1,8 @@
 // Copyright The OpenTelemetry Authors
 // SPDX-License-Identifier: Apache-2.0
 
+using System.Runtime.CompilerServices;
+using System.Text;
 using System.Web;
 using System.Web.Routing;
 
@@ -48,6 +50,20 @@ internal sealed class HttpRequestRouteHelper
         return template;
     }
 
+    [MethodImpl(MethodImplOptions.AggressiveInlining)]
+    private static bool CompareStringToSubstring(string example, string target, int start)
+    {
+        for (int i = 0; i < example.Length; i++)
+        {
+            if (target[start + 1 + i] != example[i])
+            {
+                return false;
+            }
+        }
+
+        return true;
+    }
+
     private static string PrepareRouteTemplate(Route route, RouteData routeData)
     {
         const string controllerToken = "controller";
@@ -56,32 +72,58 @@ private static string PrepareRouteTemplate(Route route, RouteData routeData)
         var template = route.Url;
         var controller = (string)routeData.Values[controllerToken];
         var action = (string)routeData.Values[actionToken];
+        var hasController = !string.IsNullOrWhiteSpace(controller);
+        var hasAction = !string.IsNullOrWhiteSpace(action);
+        var sb = new StringBuilder(template.Length);
 
-        if (!string.IsNullOrWhiteSpace(controller))
+        int i = 0;
+        while (i < template.Length)
         {
-            template = template.Replace($"{{{controllerToken}}}", controller);
-        }
+            if (template[i] == '{')
+            {
+                int end = template.IndexOf('}', i + 1);
+                if (end != -1)
+                {
+                    if (hasController && CompareStringToSubstring(controllerToken, template, i))
+                    {
+                        sb.Append(controller);
+                    }
+                    else if (hasAction && CompareStringToSubstring(actionToken, template, i))
+                    {
+                        sb.Append(action);
+                    }
+                    else
+                    {
+                        var defaults = route.Defaults;
+                        var values = routeData.Values;
+                        var token = template.Substring(i + 1, end - i - 1);
+
+                        if (defaults.ContainsKey(token) && !values.ContainsKey(token))
+                        {
+                            // Ignore defaults with no values.
+                        }
+                        else
+                        {
+                            sb.Append('{').Append(token).Append('}');
+                        }
+                    }
+
+                    i = end + 1;
+                    continue;
+                }
+            }
 
-        if (!string.IsNullOrWhiteSpace(action))
-        {
-            template = template.Replace($"{{{actionToken}}}", action);
+            sb.Append(template[i]);
+            i++;
         }
 
-        // Remove defaults with no values.
-        var defaultKeys = route.Defaults.Keys;
-        var valueKeys = routeData.Values.Keys;
-
-        foreach (var token in defaultKeys)
+        // Normalizes endings by removing trailing slashes.
+        int len = sb.Length;
+        while (len > 0 && sb[len - 1] == '/')
         {
-            if (valueKeys.Contains(token))
-            {
-                continue;
-            }
-
-            template = template.Replace($"{{{token}}}", string.Empty);
+            len--;
         }
 
-        return template
-            .TrimEnd('/'); // Normalizes endings
+        return sb.ToString(0, len);
     }
 }