Reduce CPU time when initiating RepositoryCacheKeys (#18267)

* Avoid an unneeded lookups in the Keys dictionary when initiating key cache

* Add further comments and unit tests around updated code.

---------

Co-authored-by: Andy Butland <abutland73@gmail.com>

src/Umbraco.Core/Persistence/Repositories/RepositoryCacheKeys.cs

@@ -1,3 +1,5 @@
+using System.Runtime.InteropServices;
+
 namespace Umbraco.Cms.Core.Persistence.Repositories;
 
 /// <summary>
@@ -5,15 +7,34 @@ namespace Umbraco.Cms.Core.Persistence.Repositories;
 /// </summary>
 public static class RepositoryCacheKeys
 {
-    // used to cache keys so we don't keep allocating strings
+    /// <summary>
+    /// A cache for the keys we don't keep allocating strings.
+    /// </summary>
     private static readonly Dictionary<Type, string> Keys = new();
 
+    /// <summary>
+    /// Gets the repository cache key for the provided type.
+    /// </summary>
     public static string GetKey<T>()
     {
         Type type = typeof(T);
-        return Keys.TryGetValue(type, out var key) ? key : Keys[type] = "uRepo_" + type.Name + "_";
+
+        // The following code is a micro-optimization to avoid an unnecessary lookup in the Keys dictionary, when writing the newly created key.
+        // Previously, the code was:
+        //   return Keys.TryGetValue(type, out var key)
+        //     ? key
+        //     : Keys[type] = "uRepo_" + type.Name + "_";
+
+        // Look up the existing value or get a reference to the newly created default value.
+        ref string? key = ref CollectionsMarshal.GetValueRefOrAddDefault(Keys, type, out _);
+
+        // As we have the reference, we can just assign it if null, without the expensive write back to the dictionary.
+        return key ??= "uRepo_" + type.Name + "_";
     }
 
+    /// <summary>
+    /// Gets the repository cache key for the provided type and Id.
+    /// </summary>
     public static string GetKey<T, TId>(TId? id)
     {
         if (EqualityComparer<TId?>.Default.Equals(id, default))

tests/Umbraco.Tests.UnitTests/Umbraco.Core/Persistence/Repositories/RepositoryCacheKeysTests.cs

@@ -0,0 +1,26 @@
+// Copyright (c) Umbraco.
+// See LICENSE for more details.
+
+using NUnit.Framework;
+using Umbraco.Cms.Core.Models;
+using Umbraco.Cms.Core.Persistence.Repositories;
+
+namespace Umbraco.Cms.Tests.UnitTests.Umbraco.Core.PropertyEditors;
+
+[TestFixture]
+public class RepositoryCacheKeysTests
+{
+    [Test]
+    public void GetKey_Returns_Expected_Key_For_Type()
+    {
+        var key = RepositoryCacheKeys.GetKey<IContent>();
+        Assert.AreEqual("uRepo_IContent_", key);
+    }
+
+    [Test]
+    public void GetKey_Returns_Expected_Key_For_Type_And_Id()
+    {
+        var key = RepositoryCacheKeys.GetKey<IContent, int>(1000);
+        Assert.AreEqual("uRepo_IContent_1000", key);
+    }
+}