perf: optimize GetEligibleEventObjects with single-allocation approach

TUnit.Engine/Extensions/TestContextExtensions.cs

@@ -4,44 +4,119 @@ namespace TUnit.Engine.Extensions;
 
 internal static class TestContextExtensions
 {
-    private static object?[] GetInternal(TestContext testContext)
+    public static IEnumerable<object> GetEligibleEventObjects(this TestContext testContext)
     {
-        var testClassArgs = testContext.Metadata.TestDetails.TestClassArguments;
-        var attributes = testContext.Metadata.TestDetails.GetAllAttributes();
-        var testMethodArgs = testContext.Metadata.TestDetails.TestMethodArguments;
-        var injectedProps = testContext.Metadata.TestDetails.TestClassInjectedPropertyArguments;
-
-        // Pre-calculate capacity to avoid reallocations
-        var capacity = 3 + testClassArgs.Length + attributes.Count + testMethodArgs.Length + injectedProps.Count;
-        var result = new List<object?>(capacity);
-
-        result.Add(testContext.ClassConstructor);
-        result.Add(testContext.Events);
-        result.AddRange(testClassArgs);
-        result.Add(testContext.Metadata.TestDetails.ClassInstance);
-        result.AddRange(attributes);
-        result.AddRange(testMethodArgs);
-
-        // Manual loop instead of .Select() to avoid LINQ allocation
-        foreach (var prop in injectedProps)
+        // Return cached result if available
+        if (testContext.CachedEligibleEventObjects != null)
         {
-            result.Add(prop.Value);
+            return testContext.CachedEligibleEventObjects;
         }
 
-        return result.ToArray();
+        // Build result directly with single allocation
+        var result = BuildEligibleEventObjects(testContext);
+        testContext.CachedEligibleEventObjects = result;
+        return result;
     }
 
-    public static IEnumerable<object> GetEligibleEventObjects(this TestContext testContext)
+    private static object[] BuildEligibleEventObjects(TestContext testContext)
     {
-        // Return cached result if available
-        if (testContext.CachedEligibleEventObjects != null)
+        var details = testContext.Metadata.TestDetails;
+        var testClassArgs = details.TestClassArguments;
+        var attributes = details.GetAllAttributes();
+        var testMethodArgs = details.TestMethodArguments;
+        var injectedProps = details.TestClassInjectedPropertyArguments;
+
+        // Count non-null items first to allocate exact size
+        var count = CountNonNull(testContext.ClassConstructor)
+                  + CountNonNull(testContext.Events)
+                  + CountNonNullInArray(testClassArgs)
+                  + CountNonNull(details.ClassInstance)
+                  + attributes.Count  // Attributes are never null
+                  + CountNonNullInArray(testMethodArgs)
+                  + CountNonNullValues(injectedProps);
+
+        if (count == 0)
         {
-            return testContext.CachedEligibleEventObjects;
+            return [];
+        }
+
+        // Single allocation with exact size
+        var result = new object[count];
+        var index = 0;
+
+        // Add items, skipping nulls
+        if (testContext.ClassConstructor is { } constructor)
+        {
+            result[index++] = constructor;
+        }
+
+        if (testContext.Events is { } events)
+        {
+            result[index++] = events;
+        }
+
+        foreach (var arg in testClassArgs)
+        {
+            if (arg is { } nonNullArg)
+            {
+                result[index++] = nonNullArg;
+            }
+        }
+
+        if (details.ClassInstance is { } classInstance)
+        {
+            result[index++] = classInstance;
+        }
+
+        foreach (var attr in attributes)
+        {
+            result[index++] = attr;
+        }
+
+        foreach (var arg in testMethodArgs)
+        {
+            if (arg is { } nonNullArg)
+            {
+                result[index++] = nonNullArg;
+            }
+        }
+
+        foreach (var prop in injectedProps)
+        {
+            if (prop.Value is { } value)
+            {
+                result[index++] = value;
+            }
         }
 
-        // Materialize and cache the result
-        var result = GetInternal(testContext).OfType<object>().ToArray();
-        testContext.CachedEligibleEventObjects = result;
         return result;
     }
+
+    private static int CountNonNull(object? obj) => obj != null ? 1 : 0;
+
+    private static int CountNonNullInArray(object?[] array)
+    {
+        var count = 0;
+        foreach (var item in array)
+        {
+            if (item != null)
+            {
+                count++;
+            }
+        }
+        return count;
+    }
+
+    private static int CountNonNullValues(IDictionary<string, object?> props)
+    {
+        var count = 0;
+        foreach (var prop in props)
+        {
+            if (prop.Value != null)
+            {
+                count++;
+            }
+        }
+        return count;
+    }
 }