Improve MemberExpression performance (#788)

* use array without IReadOnlyList dispatch cost

Author: lahma

Fluid/Ast/MemberExpression.cs

@@ -4,28 +4,33 @@ namespace Fluid.Ast
 {
     public sealed class MemberExpression : Expression
     {
-        public MemberExpression(MemberSegment segment)
+        private readonly MemberSegment[] _segments;
+
+        public MemberExpression(MemberSegment segment) : this([segment])
+        {
+        }
+
+        public MemberExpression(IReadOnlyList<MemberSegment> segments) : this(segments as MemberSegment[] ?? segments.ToArray())
         {
-            Segments = [segment];
         }
 
-        public MemberExpression(IReadOnlyList<MemberSegment> segments)
+        internal MemberExpression(MemberSegment[] segments)
         {
-            Segments = segments ?? [];
+            _segments = segments ?? [];
 
-            if (Segments.Count == 0)
+            if (_segments.Length == 0)
             {
-                throw new ArgumentException("At least one segment is required in a MemberExpression");
+                ExceptionHelper.ThrowArgumentNullException(nameof(segments), "At least one segment is required in a MemberExpression");
             }
         }
 
-        public IReadOnlyList<MemberSegment> Segments { get; }
+        public IReadOnlyList<MemberSegment> Segments => _segments;
 
         public override ValueTask<FluidValue> EvaluateAsync(TemplateContext context)
         {
             // The first segment can only be an IdentifierSegment
 
-            var initial = Segments[0] as IdentifierSegment;
+            var initial = _segments[0] as IdentifierSegment;
 
             // Search the initial segment in the local scope first
 
@@ -46,14 +51,14 @@ public override ValueTask<FluidValue> EvaluateAsync(TemplateContext context)
                 value = context.Model;
             }
 
-            for (var i = start; i < Segments.Count; i++)
+            for (var i = start; i < _segments.Length; i++)
             {
-                var s = Segments[i];
+                var s = _segments[i];
                 var task = s.ResolveAsync(value, context);
 
                 if (!task.IsCompletedSuccessfully)
                 {
-                    return Awaited(task, context, Segments, i + 1);
+                    return Awaited(task, context, _segments, i + 1);
                 }
 
                 value = task.Result;
@@ -71,11 +76,11 @@ public override ValueTask<FluidValue> EvaluateAsync(TemplateContext context)
         private static async ValueTask<FluidValue> Awaited(
             ValueTask<FluidValue> task,
             TemplateContext context,
-            IReadOnlyList<MemberSegment> segments,
+            MemberSegment[] segments,
             int startIndex)
         {
             var value = await task;
-            for (var i = startIndex; i < segments.Count; i++)
+            for (var i = startIndex; i < segments.Length; i++)
             {
                 var s = segments[i];
                 value = await s.ResolveAsync(value, context);

Fluid/ExceptionHelper.cs

@@ -29,6 +29,13 @@ public static void ThrowArgumentOutOfRangeException(string paramName, string mes
             throw new ArgumentOutOfRangeException(paramName, message);
         }
 
+        [DoesNotReturn]
+        [MethodImpl(MethodImplOptions.NoInlining)]
+        public static void ThrowArgumentException(string paramName, string message)
+        {
+            throw new ArgumentException(paramName, message);
+        }
+
         [DoesNotReturn]
         [MethodImpl(MethodImplOptions.NoInlining)]
         public static void ThrowParseException<T>(string message)
@@ -50,4 +57,4 @@ public static void ThrowMaximumStatementsException()
             throw new InvalidOperationException("The maximum number of statements has been reached. Your script took too long to run.");
         }
     }
-}
+}