Add more MemoryCache test coverage

Cover more concurrency and edge cases.

Author: DustinCampbell

src/Razor/src/Microsoft.CodeAnalysis.Razor.Workspaces/Utilities/MemoryCache`2.TestAccessor.cs

@@ -11,7 +11,7 @@ internal partial class MemoryCache<TKey, TValue>
 {
     internal TestAccessor GetTestAccessor() => new(this);
 
-    internal ref struct TestAccessor(MemoryCache<TKey, TValue> instance)
+    internal struct TestAccessor(MemoryCache<TKey, TValue> instance)
     {
         public event Action Compacted
         {

src/Razor/test/Microsoft.CodeAnalysis.Razor.Workspaces.Test/Utilities/MemoryCachePerfTest.cs

@@ -0,0 +1,47 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+using System;
+using System.Collections.Generic;
+using System.Diagnostics;
+using System.Linq;
+using System.Threading.Tasks;
+using Microsoft.AspNetCore.Razor.Test.Common;
+using Microsoft.CodeAnalysis.Razor.Utilities;
+using Xunit;
+using Xunit.Abstractions;
+
+namespace Microsoft.CodeAnalysis.Razor.Workspaces.Test.Utilities;
+
+[CollectionDefinition(nameof(MemoryCachePerfTest), DisableParallelization = false)]
+[Collection(nameof(MemoryCachePerfTest))]
+public class MemoryCachePerfTest(ITestOutputHelper testOutput) : ToolingTestBase(testOutput)
+{
+    [Fact]
+    public async Task HighFrequencyAccess_MaintainsPerformance()
+    {
+        var cache = new MemoryCache<string, IReadOnlyList<int>>();
+        const string Key = "hot-key";
+        cache.Set(Key, [1, 2, 3]);
+
+        const int AccessCount = 10_000;
+        var stopwatch = Stopwatch.StartNew();
+
+        var tasks = Enumerable.Range(0, Environment.ProcessorCount)
+            .Select(x => Task.Run(() =>
+            {
+                for (var i = 0; i < AccessCount / Environment.ProcessorCount; i++)
+                {
+                    _ = cache.TryGetValue(Key, out _);
+                }
+            }))
+            .ToArray();
+
+        await Task.WhenAll(tasks);
+        stopwatch.Stop();
+
+        // Should complete reasonably quickly (adjust threshold as needed)
+        Assert.True(stopwatch.ElapsedMilliseconds < 1000,
+            $"High-frequency access took too long: {stopwatch.ElapsedMilliseconds}ms");
+    }
+}