Optimize IReadOnlyDictionary check by caching

- Introduced a thread-safe cache to IsIReadOnlyDictionary for improved performance and reduced allocations.
- Added tests for nullable operator with enum-keyed dictionaries to ensure missing keys return empty values.

Author: axunonb

src/SmartFormat.Tests/Extensions/DictionarySourceTests.cs

@@ -2,8 +2,6 @@
 using System.Collections;
 using System.Collections.Generic;
 using System.Dynamic;
-using System.Globalization;
-using System.Linq;
 using NUnit.Framework;
 using SmartFormat.Core.Formatting;
 using SmartFormat.Core.Settings;
@@ -164,7 +162,7 @@ public void Dictionary_Dot_Notation_Nullable()
     [Test]
     public void Dictionary_NullableOperator_MissingKey_ReturnsEmpty()
     {
-        // Test for GitHub issue #522
+        // Test similar to GitHub issue #522, but using string keys
         var sf = Smart.CreateDefaultSmartFormat();
 
         var obj = new {
@@ -193,7 +191,39 @@ public void Dictionary_NullableOperator_MissingKey_ReturnsEmpty()
 
         // Now add the missing key and verify that it is properly resolved
         obj.Changes.Add("Volume", 200);
-        Assert.That(sf.Format("{Changes:{Count};{?.Volume};{Name}}", obj),
+        Assert.That(sf.Format("{Changes.Count};{Changes?.Volume};{Changes.Name}", obj),
+            Is.EqualTo("100;200;ABCD"));
+    }
+
+    private enum ChangeKey { Count, Volume, Name }
+
+    [Test]
+    public void Dictionary_NullableOperator_MissingEnumKey_ReturnsEmpty()
+    {
+        // Test for GitHub issue #522 with non-string dictionary keys
+        var sf = Smart.CreateDefaultSmartFormat();
+        var obj = new {
+            Changes = new Dictionary<ChangeKey, object>
+            {
+                { ChangeKey.Count, 100 },
+                // Volume is intentionally missing
+                // although it is accessed in the format string
+                { ChangeKey.Name, "ABCD" },
+            }
+        };
+        // Use nullable syntax to avoid exceptions for selectors
+        // that cannot be resolved with the DictionarySource:
+        // 'Volume' may be missing, but 'Changes' is expected to be present
+        var result =
+            sf.Format("{Changes.Count};{Changes?.Volume};{Changes.Name}", obj);
+        Assert.Multiple(() =>
+        {
+            Assert.That(result, Is.EqualTo("100;;ABCD"));
+        });
+
+        // Now add the missing key and verify that it is properly resolved
+        obj.Changes.Add(ChangeKey.Volume, 200);
+        Assert.That(sf.Format("{Changes.Count};{Changes?.Volume};{Changes.Name}", obj),
             Is.EqualTo("100;200;ABCD"));
     }
 

src/SmartFormat/Extensions/DictionarySource.cs

@@ -4,6 +4,7 @@
 
 using System;
 using System.Collections;
+using System.Collections.Concurrent;
 using System.Collections.Generic;
 using System.Reflection;
 using SmartFormat.Core.Extensions;
@@ -166,18 +167,28 @@ private bool TryGetDictionaryProperties(Type type, out (PropertyInfo KeyProperty
         return true;
     }
 
+    private static readonly ConcurrentDictionary<Type, bool> RoDictionaryTypeBoolCache = new();
+
     private static bool IsIReadOnlyDictionary(Type type)
     {
+        if (RoDictionaryTypeBoolCache.TryGetValue(type, out var cached))
+            return cached;
+
         // No Linq for less garbage
+        var result = false;
         foreach (var typeInterface in type.GetInterfaces())
         {
             if (typeInterface == typeof(IReadOnlyDictionary<,>) ||
                 (typeInterface.IsGenericType
                  && typeInterface.GetGenericTypeDefinition() == typeof(IReadOnlyDictionary<,>)))
-                return true;
+            {
+                result = true;
+                break;
+            }
         }
 
-        return false;
+        RoDictionaryTypeBoolCache[type] = result;
+        return result;
     }
 
     #endregion