[9.5] Restore delegate caching in IdResolver (#8973)

src/Elastic.Clients.Elasticsearch/_Shared/Core/Infer/Id/IdResolver.cs

@@ -73,6 +73,9 @@ public class IdResolver
 			return null;
 		}
 
+		// Avoid reflection calls for subsequent invocations.
+		delegateCache.TryAdd(type, getterDelegate);
+
 		return getterDelegate(instance);
 	}
 

tests/Tests/ClientConcepts/HighLevel/Caching/IdResolverCacheTests.cs

@@ -0,0 +1,52 @@
+// Licensed to Elasticsearch B.V under one or more agreements.
+// Elasticsearch B.V licenses this file to you under the Apache 2.0 License.
+// See the LICENSE file in the project root for more information.
+
+using System.Collections;
+using System.Reflection;
+
+namespace Tests.ClientConcepts.HighLevel.Caching
+{
+	public class IdResolverCacheTests
+	{
+		private class MyDocument
+		{
+			public string Id { get; set; }
+		}
+
+		private class MyKeyedDocument
+		{
+			public string Key { get; set; }
+		}
+
+		private static IDictionary GlobalCache =>
+			(IDictionary)typeof(IdResolver)
+				.GetField("GlobalDelegateCache", BindingFlags.NonPublic | BindingFlags.Static)
+				.GetValue(null);
+
+		private static IDictionary LocalCache(IdResolver resolver) =>
+			(IDictionary)typeof(IdResolver)
+				.GetField("_localDelegateCache", BindingFlags.NonPublic | BindingFlags.Instance)
+				.GetValue(resolver);
+
+		[U] public void CachesGetterDelegate()
+		{
+			var resolver = new IdResolver(new ElasticsearchClientSettings());
+
+			resolver.Resolve(new MyDocument { Id = "1" }).Should().Be("1");
+			GlobalCache.Contains(typeof(MyDocument)).Should().BeTrue();
+			resolver.Resolve(new MyDocument { Id = "2" }).Should().Be("2");
+		}
+
+		[U] public void CachesGetterDelegateForCustomIdProperty()
+		{
+			var settings = new ElasticsearchClientSettings()
+				.DefaultMappingFor<MyKeyedDocument>(m => m.IdProperty(p => p.Key));
+			var resolver = new IdResolver(settings);
+
+			resolver.Resolve(new MyKeyedDocument { Key = "1" }).Should().Be("1");
+			LocalCache(resolver).Contains(typeof(MyKeyedDocument)).Should().BeTrue();
+			GlobalCache.Contains(typeof(MyKeyedDocument)).Should().BeFalse();
+		}
+	}
+}