Optimize assignment and look up of the best candidate match

Author: Chris Martinez

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

@@ -4,6 +4,7 @@
     using System;
     using System.Collections.Generic;
     using System.Linq;
+    using static System.Threading.Interlocked;
 
     /// <summary>
     /// Represents an API versioning action selection result for which a versioning policy can be applied.
@@ -12,6 +13,7 @@ public class ActionSelectionResult
     {
         readonly Dictionary<int, ICollection<ActionDescriptor>> candidateActions = new Dictionary<int, ICollection<ActionDescriptor>>();
         readonly Dictionary<int, ICollection<ActionDescriptorMatch>> matchingActions = new Dictionary<int, ICollection<ActionDescriptorMatch>>();
+        ActionDescriptorMatch bestMatch;
 
         /// <summary>
         /// Gets the number of action selection iterations that have occurred.
@@ -26,26 +28,7 @@ public class ActionSelectionResult
         /// <remarks>This property returns the first occurrence of a single match in the earliest iteration. If
         /// no matches exist in any iteration or multiple matches exist, this property returns <c>null</c>.</remarks>
         [CLSCompliant( false )]
-        public ActionDescriptorMatch BestMatch
-        {
-            get
-            {
-                foreach ( var iteration in matchingActions )
-                {
-                    switch ( iteration.Value.Count )
-                    {
-                        case 0:
-                            break;
-                        case 1:
-                            return iteration.Value.ElementAt( 0 );
-                        default:
-                            return null;
-                    }
-                }
-
-                return null;
-            }
-        }
+        public ActionDescriptorMatch BestMatch => bestMatch;
 
         /// <summary>
         /// Gets a collection of candidate actions grouped by action selection iteration.
@@ -160,5 +143,23 @@ public void EndIteration()
 
             ++Iterations;
         }
+
+        /// <summary>
+        /// Attempts to update the best match.
+        /// </summary>
+        /// <param name="match">The <see cref="ActionDescriptorMatch">match</see> to attempt to set as the best match.
+        /// This value can be <c>null</c>.</param>
+        /// <returns>True if the match is successfully set; otherwise, false.</returns>
+        [CLSCompliant( false )]
+        public bool TrySetBestMatch( ActionDescriptorMatch match )
+        {
+            if ( match == null )
+            {
+                return false;
+            }
+
+            const ActionDescriptorMatch NoMatch = default( ActionDescriptorMatch );
+            return CompareExchange( ref bestMatch, match, NoMatch ) == NoMatch;
+        }
     }
 }

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

@@ -13,6 +13,7 @@
     using System.Diagnostics.Contracts;
     using System.Linq;
     using static ErrorCodes;
+    using System.Collections;
 
     /// <summary>
     /// Represents the logic for selecting an API-versioned, action method.
@@ -129,7 +130,10 @@ public virtual ActionDescriptor SelectBestCandidate( RouteContext context, IRead
             if ( finalMatches.Count > 0 )
             {
                 var routeData = new RouteData( context.RouteData );
-                selectionResult.AddMatches( finalMatches.Select( action => new ActionDescriptorMatch( action, routeData ) ) );
+                var matchingActions = new MatchingActionSequence( finalMatches, routeData );
+
+                selectionResult.AddMatches( matchingActions );
+                selectionResult.TrySetBestMatch( matchingActions.BestMatch );
             }
 
             // note: even though we may have had a successful match, this method could be called multiple times. the final decision
@@ -360,5 +364,41 @@ IReadOnlyList<ActionSelectorCandidate> EvaluateActionConstraintsCore( RouteConte
                 return EvaluateActionConstraintsCore( context, actionsWithoutConstraint, order );
             }
         }
+
+        sealed class MatchingActionSequence : IEnumerable<ActionDescriptorMatch>
+        {
+            readonly IReadOnlyList<ActionDescriptor> matches;
+            readonly RouteData routeData;
+
+            internal MatchingActionSequence( IReadOnlyList<ActionDescriptor> matches, RouteData routeData )
+            {
+                Contract.Requires( matches != null );
+                Contract.Requires( matches.Count > 0 );
+                Contract.Requires( routeData != null );
+
+                this.matches = matches;
+                this.routeData = routeData;
+            }
+
+            internal ActionDescriptorMatch BestMatch { get; private set; }
+
+            public IEnumerator<ActionDescriptorMatch> GetEnumerator()
+            {
+                if ( matches.Count == 1 )
+                {
+                    BestMatch = new ActionDescriptorMatch( matches[0], routeData );
+                    yield return BestMatch;
+                }
+                else
+                {
+                    for ( var i = 0; i < matches.Count; i++ )
+                    {
+                        yield return new ActionDescriptorMatch( matches[i], routeData );
+                    }
+                }
+            }
+
+            IEnumerator IEnumerable.GetEnumerator() => GetEnumerator();
+        }
     }
 }

test/Microsoft.AspNetCore.Mvc.Versioning.Tests/Versioning/TestApiVersionActionSelector.cs

@@ -21,9 +21,8 @@ public override ActionDescriptor SelectBestCandidate( RouteContext context, IRea
         {
             var bestCandidate = base.SelectBestCandidate( context, candidates );
             var selectionResult = context.HttpContext.ApiVersionProperties().SelectionResult;
-            var matchingActions = selectionResult.MatchingActions.OrderBy( kvp => kvp.Key ).SelectMany( kvp => kvp.Value ).Distinct();
 
-            SelectedCandidate = matchingActions.FirstOrDefault()?.Action;
+            SelectedCandidate = selectionResult.BestMatch?.Action;
 
             return bestCandidate;
         }