WA34: WebAPI: ReflectedHttpActionDescriptor: Cache repeated calls to MethodInfo.GetParameters.

In WebAPI scenarios involving paramters, there are repeated calls to extract the same set of paramters on the long lifetime ReflectedHttpActionDescriptor. This changes caches them and will prevent one array allocation per request and should improve throughput by 0.3-0.5% depending on the scenario.

src/System.Web.Http/Controllers/ReflectedHttpActionDescriptor.cs

@@ -26,6 +26,7 @@ namespace System.Web.Http.Controllers
         private static readonly object[] _empty = new object[0];
 
         private readonly Lazy<Collection<HttpParameterDescriptor>> _parameters;
+        private ParameterInfo[] _parameterInfos;
 
         private Lazy<ActionExecutor> _actionExecutor;
         private MethodInfo _methodInfo;
@@ -98,6 +99,18 @@ namespace System.Web.Http.Controllers
             }
         }
 
+        private ParameterInfo[] ParameterInfos
+        {
+            get
+            {
+                if (_parameterInfos == null)
+                {
+                    _parameterInfos = _methodInfo.GetParameters();
+                }
+                return _parameterInfos;
+            }
+        }
+
         /// <inheritdoc/>
         public override Type ReturnType
         {
@@ -154,6 +167,7 @@ namespace System.Web.Http.Controllers
         private void InitializeProperties(MethodInfo methodInfo)
         {
             _methodInfo = methodInfo;
+            _parameterInfos = null;
             _returnType = GetReturnType(methodInfo);
             _actionExecutor = new Lazy<ActionExecutor>(() => InitializeActionExecutor(_methodInfo));
             _declaredOnlyAttributeCache = _methodInfo.GetCustomAttributes(inherit: false);
@@ -180,7 +194,7 @@ namespace System.Web.Http.Controllers
         {
             Contract.Assert(_methodInfo != null);
 
-            List<HttpParameterDescriptor> parameterInfos = _methodInfo.GetParameters().Select(
+            List<HttpParameterDescriptor> parameterInfos = ParameterInfos.Select(
                 (item) => new ReflectedHttpParameterDescriptor(this, item)).ToList<HttpParameterDescriptor>();
             return new Collection<HttpParameterDescriptor>(parameterInfos);
         }
@@ -193,7 +207,7 @@ namespace System.Web.Http.Controllers
                 return _empty;
             }
 
-            ParameterInfo[] parameterInfos = MethodInfo.GetParameters();
+            ParameterInfo[] parameterInfos = ParameterInfos;
             int parameterCount = parameterInfos.Length;
             object[] parameterValues = new object[parameterCount];
             for (int parameterIndex = 0; parameterIndex < parameterCount; parameterIndex++)