Improve performance of action selector

Author: Chris Martinez

src/Microsoft.AspNetCore.Mvc.Versioning/Abstractions/ActionDescriptorExtensions.cs

@@ -13,48 +13,30 @@
     [CLSCompliant( false )]
     public static class ActionDescriptorExtensions
     {
-        const string VersionsAggregated = "MS_" + nameof( HasAggregatedVersions );
+        const string VersionPolicyIsAppliedKey = "MS_" + nameof( VersionPolicyIsApplied );
 
-        static bool HasAggregatedVersions( this ActionDescriptor action ) => action.Properties.GetOrDefault( VersionsAggregated, false );
+        static void VersionPolicyIsApplied( this ActionDescriptor action, bool value ) => action.Properties[VersionPolicyIsAppliedKey] = value;
 
-        static void HasAggregatedVersions( this ActionDescriptor action, bool value ) => action.Properties[VersionsAggregated] = value;
+        internal static bool VersionPolicyIsApplied( this ActionDescriptor action ) => action.Properties.GetOrDefault( VersionPolicyIsAppliedKey, false );
 
         internal static void AggregateAllVersions( this ActionDescriptor action, IEnumerable<ActionDescriptor> matchingActions )
         {
             Contract.Requires( action != null );
             Contract.Requires( matchingActions != null );
 
-            if ( action.HasAggregatedVersions() )
+            if ( action.VersionPolicyIsApplied() )
             {
                 return;
             }
 
-            action.HasAggregatedVersions( true );
+            action.VersionPolicyIsApplied( true );
 
             var model = action.GetProperty<ApiVersionModel>();
             Contract.Assume( model != null );
 
             action.SetProperty( model.Aggregate( matchingActions.Select( a => a.GetProperty<ApiVersionModel>() ).Where( m => m != null ) ) );
         }
 
-        internal static void AggregateAllVersions( this ActionDescriptor action, ActionSelectionContext context )
-        {
-            Contract.Requires( action != null );
-            Contract.Requires( context != null );
-
-            if ( action.HasAggregatedVersions() )
-            {
-                return;
-            }
-
-            action.HasAggregatedVersions( true );
-
-            var model = action.GetProperty<ApiVersionModel>();
-            Contract.Assume( model != null );
-
-            action.SetProperty( model.Aggregate( context.AllVersions ) );
-        }
-
         /// <summary>
         /// Returns a value indicating whether the provided action implicitly maps to the specified version.
         /// </summary>

src/Microsoft.AspNetCore.Mvc.Versioning/Versioning/ApiVersionActionSelector.cs

@@ -108,7 +108,24 @@ public virtual ActionDescriptor SelectBestCandidate( RouteContext context, IRead
             properties.ApiVersion = selectionContext.RequestedVersion;
             selectionResult.CandidateActions.AddRange( candidates );
 
-            if ( finalMatches?.Count > 0 )
+            if ( finalMatches == null )
+            {
+                return null;
+            }
+
+            if ( finalMatches.Count == 1 )
+            {
+                var selectedAction = finalMatches[0];
+
+                // note: short-circuit if the api version policy has already been applied to the match
+                if ( selectedAction.VersionPolicyIsApplied() )
+                {
+                    httpContext.ApiVersionProperties().ApiVersion = selectionContext.RequestedVersion;
+                    return selectedAction;
+                }
+            }
+
+            if ( finalMatches.Count > 0 )
             {
                 var routeData = new RouteData( context.RouteData );
                 selectionResult.MatchingActions.AddRange( finalMatches.Select( action => new ActionDescriptorMatch( action, routeData ) ) );