Avoid some redundant list allocations

TUnit.Analyzers.CodeFixers/Base/AssertionRewriter.cs

@@ -70,7 +70,7 @@ protected AssertionRewriter(SemanticModel semanticModel)
                 // Conversion added trivia (TODO comments). Structure should be:
                 // [original whitespace] [TODO comment] [newline] [original whitespace] [await expression]
                 var whitespaceTrivia = originalTrivia.Where(t => t.IsKind(SyntaxKind.WhitespaceTrivia)).ToList();
-                var nonWhitespaceTrivia = originalTrivia.Where(t => !t.IsKind(SyntaxKind.WhitespaceTrivia)).ToList();
+                var nonWhitespaceTrivia = originalTrivia.Where(t => !t.IsKind(SyntaxKind.WhitespaceTrivia));
 
                 var builder = new List<SyntaxTrivia>();
                 builder.AddRange(nonWhitespaceTrivia); // Add any non-whitespace (e.g., leading newlines)

TUnit.Analyzers.CodeFixers/Base/BaseMigrationCodeFixProvider.cs

@@ -208,7 +208,7 @@
         compilationUnit = CleanupEndOfFileTrivia(compilationUnit);
 
         // Normalize line endings to match original document (fixes cross-platform issues)
         compilationUnit = NormalizeLineEndings(compilationUnit, root);
 
         // Add TODO comments for any failures so users know what needs manual attention
         if (context.HasFailures)
@@ -545,12 +545,11 @@
                     !t.IsKind(SyntaxKind.WhitespaceTrivia));
 
                 var newTrivia = trailingTrivia
-                    .Where(t => !t.IsKind(SyntaxKind.EndOfLineTrivia))
-                    .ToList();
+                    .Where(t => !t.IsKind(SyntaxKind.EndOfLineTrivia));
 
                 if (hasContentAfterLastMember)
                 {
                     newTrivia.Add(SyntaxFactory.EndOfLine("\n"));
                 }
 
                 var newLastMember = lastMember.WithTrailingTrivia(newTrivia);

TUnit.Analyzers.CodeFixers/Base/TestAttributeEnsurer.cs

@@ -121,8 +121,7 @@ private static MethodDeclarationSyntax AddTestAttribute(MethodDeclarationSyntax
         // Strip newlines from first attribute's leading trivia, keep only indentation
         // This prevents double newlines when we insert [Test] before it
         var strippedTrivia = firstAttributeList.GetLeadingTrivia()
-            .Where(t => !t.IsKind(SyntaxKind.EndOfLineTrivia))
-            .ToList();
+            .Where(t => !t.IsKind(SyntaxKind.EndOfLineTrivia));
         var updatedFirstAttr = firstAttributeList.WithLeadingTrivia(strippedTrivia);
 
         // Build new attribute list: [Test], then updated first attr (with stripped trivia), then rest

TUnit.Analyzers.CodeFixers/Base/TwoPhase/MigrationAnalyzer.cs

@@ -144,7 +144,7 @@ protected bool IsInterfaceImplementation(TextSpan methodSpan)
     protected virtual CompilationUnitSyntax AnalyzeAssertions(CompilationUnitSyntax root)
     {
         // Find assertions on the ORIGINAL tree (for semantic analysis)
-        var assertionNodes = FindAssertionNodes(_originalRoot).ToList();
+        var assertionNodes = FindAssertionNodes(_originalRoot);
         var currentRoot = root;
 
         foreach (var originalNode in assertionNodes)
@@ -238,7 +238,7 @@ private static bool ContainingMethodHasRefOrOutParameters(SyntaxNode node)
     /// </summary>
     protected virtual CompilationUnitSyntax AnalyzeAttributes(CompilationUnitSyntax root)
     {
-        var attributeNodes = _originalRoot.DescendantNodes().OfType<AttributeSyntax>().ToList();
+        var attributeNodes = _originalRoot.DescendantNodes().OfType<AttributeSyntax>();
         var currentRoot = root;
 
         foreach (var originalNode in attributeNodes)
@@ -311,7 +311,7 @@ protected virtual CompilationUnitSyntax AnalyzeAttributes(CompilationUnitSyntax
     /// </summary>
     protected virtual CompilationUnitSyntax AnalyzeParameterAttributes(CompilationUnitSyntax root)
     {
-        var parameterNodes = _originalRoot.DescendantNodes().OfType<ParameterSyntax>().ToList();
+        var parameterNodes = _originalRoot.DescendantNodes().OfType<ParameterSyntax>();
         var currentRoot = root;
 
         foreach (var parameter in parameterNodes)
@@ -379,7 +379,7 @@ protected virtual CompilationUnitSyntax AnalyzeMethodsForMissingAttributes(Compi
     /// </summary>
     protected virtual CompilationUnitSyntax AnalyzeBaseTypes(CompilationUnitSyntax root)
     {
-        var classNodes = _originalRoot.DescendantNodes().OfType<ClassDeclarationSyntax>().ToList();
+        var classNodes = _originalRoot.DescendantNodes().OfType<ClassDeclarationSyntax>();
         var currentRoot = root;
 
         foreach (var classNode in classNodes)

TUnit.Analyzers.CodeFixers/NUnitAssertMultipleRewriter.cs

@@ -44,7 +44,7 @@ private SyntaxNode ConvertAssertMultipleLambda(ExpressionStatementSyntax origina
         if (lambda.Body is BlockSyntax block)
         {
             // Visit each statement to convert inner assertions
-            var convertedStatements = block.Statements.Select(s => (StatementSyntax)Visit(s)!).ToArray();
+            var convertedStatements = block.Statements.Select(s => (StatementSyntax)Visit(s)!);
             statements = SyntaxFactory.List(convertedStatements);
         }
         else if (lambda.Body is ExpressionSyntax expr)
@@ -67,7 +67,7 @@ private SyntaxNode ConvertAssertMultipleSimpleLambda(ExpressionStatementSyntax o
         SyntaxList<StatementSyntax> statements;
         if (lambda.Body is BlockSyntax block)
         {
-            var convertedStatements = block.Statements.Select(s => (StatementSyntax)Visit(s)!).ToArray();
+            var convertedStatements = block.Statements.Select(s => (StatementSyntax)Visit(s)!);
             statements = SyntaxFactory.List(convertedStatements);
         }
         else if (lambda.Body is ExpressionSyntax expr)

TUnit.Analyzers.CodeFixers/NUnitMigrationCodeFixProvider.cs

@@ -683,7 +683,7 @@ private SyntaxNode ConvertAssertMultipleLambda(ExpressionStatementSyntax origina
         if (lambda.Body is BlockSyntax block)
         {
             // Visit each statement to convert inner assertions
-            var convertedStatements = block.Statements.Select(s => (StatementSyntax)Visit(s)!).ToArray();
+            var convertedStatements = block.Statements.Select(s => (StatementSyntax)Visit(s)!);
             statements = SyntaxFactory.List(convertedStatements);
         }
         else if (lambda.Body is ExpressionSyntax expr)
@@ -706,7 +706,7 @@ private SyntaxNode ConvertAssertMultipleSimpleLambda(ExpressionStatementSyntax o
         SyntaxList<StatementSyntax> statements;
         if (lambda.Body is BlockSyntax block)
         {
-            var convertedStatements = block.Statements.Select(s => (StatementSyntax)Visit(s)!).ToArray();
+            var convertedStatements = block.Statements.Select(s => (StatementSyntax)Visit(s)!);
             statements = SyntaxFactory.List(convertedStatements);
         }
         else if (lambda.Body is ExpressionSyntax expr)

TUnit.Analyzers.CodeFixers/TwoPhase/XUnitTwoPhaseAnalyzer.cs

@@ -1112,7 +1112,7 @@ private static string GetAttributeName(AttributeSyntax attribute)
 
     protected override CompilationUnitSyntax AnalyzeBaseTypes(CompilationUnitSyntax root)
     {
-        var classNodes = OriginalRoot.DescendantNodes().OfType<ClassDeclarationSyntax>().ToList();
+        var classNodes = OriginalRoot.DescendantNodes().OfType<ClassDeclarationSyntax>();
         var currentRoot = root;
 
         foreach (var classNode in classNodes)
@@ -1481,8 +1481,7 @@ protected override CompilationUnitSyntax AnalyzeMembers(CompilationUnitSyntax ro
         var members = OriginalRoot.DescendantNodes()
             .Where(n =>
                 (n is PropertyDeclarationSyntax prop && IsTestOutputHelperType(prop.Type)) ||
-                (n is FieldDeclarationSyntax field && IsTestOutputHelperType(field.Declaration.Type)))
-            .ToList();
+                (n is FieldDeclarationSyntax field && IsTestOutputHelperType(field.Declaration.Type)));
 
         foreach (var originalMember in members)
         {
@@ -1553,8 +1552,7 @@ protected override CompilationUnitSyntax AnalyzeConstructorParameters(Compilatio
         // Find parameters on ORIGINAL tree (for semantic analysis)
         var parameters = OriginalRoot.DescendantNodes()
             .OfType<ParameterSyntax>()
-            .Where(p => p.Type != null && IsTestOutputHelperType(p.Type))
-            .ToList();
+            .Where(p => p.Type != null && IsTestOutputHelperType(p.Type));
 
         foreach (var originalParam in parameters)
         {
@@ -1625,8 +1623,7 @@ private CompilationUnitSyntax AnalyzeRecordExceptionCalls(CompilationUnitSyntax
         // Find Record.Exception calls on the ORIGINAL tree
         var recordExceptionCalls = OriginalRoot.DescendantNodes()
             .OfType<InvocationExpressionSyntax>()
-            .Where(IsRecordExceptionCall)
-            .ToList();
+            .Where(IsRecordExceptionCall);
 
         foreach (var originalCall in recordExceptionCalls)
         {
@@ -1737,8 +1734,7 @@ private CompilationUnitSyntax AnalyzeTestOutputHelperCalls(CompilationUnitSyntax
         // Find ITestOutputHelper.WriteLine calls on the ORIGINAL tree
         var testOutputHelperCalls = OriginalRoot.DescendantNodes()
             .OfType<InvocationExpressionSyntax>()
-            .Where(IsTestOutputHelperWriteLineCall)
-            .ToList();
+            .Where(IsTestOutputHelperWriteLineCall);
 
         foreach (var originalCall in testOutputHelperCalls)
         {
@@ -1826,8 +1822,7 @@ protected override CompilationUnitSyntax AnalyzeTheoryData(CompilationUnitSyntax
         // Find all TheoryData<T> types in the original tree (field and property declarations)
         var theoryDataNodes = OriginalRoot.DescendantNodes()
             .OfType<GenericNameSyntax>()
-            .Where(g => g.Identifier.Text == "TheoryData")
-            .ToList();
+            .Where(g => g.Identifier.Text == "TheoryData");
 
         foreach (var originalGeneric in theoryDataNodes)
         {

TUnit.Analyzers/AssemblyTestHooksAnalyzer.cs

@@ -34,8 +34,7 @@ private void AnalyzeSymbol(SymbolAnalysisContext context)
         var attributes = methodSymbol.GetAttributes();
 
         var onlyOnceAttributes = attributes
-            .Where(x => x.IsStandardHook(context.Compilation, out _, out var level, out _) && level == HookLevel.Assembly)
-            .ToList();
+            .Where(x => x.IsStandardHook(context.Compilation, out _, out var level, out _) && level == HookLevel.Assembly);
 
         if (!onlyOnceAttributes.Any())
         {

TUnit.Analyzers/ClassHooksAnalyzer.cs

@@ -33,8 +33,7 @@ private void AnalyzeSymbol(SymbolAnalysisContext context)
         var attributes = methodSymbol.GetAttributes();
 
         var onlyOnceAttributes = attributes
-                .Where(x => x.IsStandardHook(context.Compilation, out _, out var level, out _) && level == HookLevel.Class)
-                .ToList();
+                .Where(x => x.IsStandardHook(context.Compilation, out _, out var level, out _) && level == HookLevel.Class);
 
         if (!onlyOnceAttributes.Any())
         {

TUnit.Analyzers/CombinedDataSourceAnalyzer.cs

@@ -63,8 +63,7 @@ private void CheckCombinedDataSourceErrors(SymbolAnalysisContext context,
             x.IsCombinedDataSourceAttribute(context.Compilation));
 
         var parametersWithDataSources = parameters
-            .Where(p => p.HasDataSourceAttribute(context.Compilation))
-            .ToList();
+            .Where(p => p.HasDataSourceAttribute(context.Compilation));
 
         // Rule 1: If parameters have data source attributes, CombinedDataSources must be present
         if (parametersWithDataSources.Any() && !hasCombinedDataSource)
@@ -81,8 +80,7 @@ private void CheckCombinedDataSourceErrors(SymbolAnalysisContext context,
             // Filter out CancellationToken parameters as they're handled by the engine
             var nonCancellationTokenParams = parameters
                 .Where(p => p.Type.GloballyQualifiedNonGeneric() !=
-                           "global::System.Threading.CancellationToken")
-                .ToList();
+                           "global::System.Threading.CancellationToken");
 
             foreach (var parameter in nonCancellationTokenParams)
             {

TUnit.Analyzers/DisposableFieldPropertyAnalyzer.cs

@@ -163,7 +163,7 @@ private static void CheckFieldInitializers(SyntaxNodeAnalysisContext context, IN
     private static void CheckSetUps(SyntaxNodeAnalysisContext context, IMethodSymbol methodSymbol, ConcurrentDictionary<ISymbol, HookLevel?> createdObjects)
     {
         var syntaxNodes = methodSymbol.DeclaringSyntaxReferences
-            .SelectMany(x => x.GetSyntax().DescendantNodesAndSelf()).ToArray();
+            .SelectMany(x => x.GetSyntax().DescendantNodesAndSelf());
 
         var isHookMethod = methodSymbol.IsHookMethod(context.Compilation, out _, out var level, out _);
 
@@ -227,7 +227,7 @@ private static void CheckSetUps(SyntaxNodeAnalysisContext context, IMethodSymbol
     private static void CheckTeardowns(SyntaxNodeAnalysisContext context, IMethodSymbol methodSymbol, ConcurrentDictionary<ISymbol, HookLevel?> createdObjects)
     {
         var syntaxNodes = methodSymbol.DeclaringSyntaxReferences
-            .SelectMany(x => x.GetSyntax().DescendantNodesAndSelf()).ToArray();
+            .SelectMany(x => x.GetSyntax().DescendantNodesAndSelf());
 
         foreach (var assignment in syntaxNodes
                      .Where(x => x.IsKind(SyntaxKind.InvocationExpression)))

TUnit.Analyzers/InstanceTestHooksAnalyzer.cs

@@ -27,8 +27,7 @@ private void AnalyzeSymbol(SymbolAnalysisContext context)
         var attributes = methodSymbol.GetAttributes();
 
         var onlyOnceAttributes = attributes
-            .Where(x => x.IsStandardHook(context.Compilation, out _, out var level, out _) && level == HookLevel.Test)
-            .ToList();
+            .Where(x => x.IsStandardHook(context.Compilation, out _, out var level, out _) && level == HookLevel.Test);
 
         if (!onlyOnceAttributes.Any())
         {

TUnit.Analyzers/SingleTUnitAttributeAnalyzer.cs

@@ -23,9 +23,9 @@ private void AnalyzeSymbol(SymbolAnalysisContext context)
 
         var attributes = symbol.GetAttributes();
 
-        var singleAttributes = attributes.Select(ToClassInheritingSingleAttribute).OfType<INamedTypeSymbol>().ToList();
+        var singleAttributes = attributes.Select(ToClassInheritingSingleAttribute).OfType<INamedTypeSymbol>();
 
-        var notDistinctAttributes = singleAttributes.GroupBy(x => x, SymbolEqualityComparer.Default).Where(x => x.Count() > 1).Select(x => x.Key!).ToList();
+        var notDistinctAttributes = singleAttributes.GroupBy(x => x, SymbolEqualityComparer.Default).Where(x => x.Count() > 1).Select(x => x.Key!);
 
         foreach (var notDistinctAttribute in notDistinctAttributes)
         {

TUnit.Assertions.SourceGenerator/Generators/AssertionMethodGenerator.cs

@@ -494,8 +494,7 @@ private static void GenerateAssertionsForSpecificClass(SourceProductionContext c
                 .OfType<IPropertySymbol>()
                 .Where(p => p.Type.SpecialType == SpecialType.System_Boolean &&
                     p is { GetMethod: not null, IsStatic: false } &&
-                    SymbolEqualityComparer.Default.Equals(p.ContainingType, attributeData.TargetType))
-                .ToList();
+                    SymbolEqualityComparer.Default.Equals(p.ContainingType, attributeData.TargetType));
 
             var matchingMethods = methodMembers.ToList();
 

TUnit.Core.SourceGenerator/CodeGenerationHelpers.cs

@@ -103,7 +103,7 @@ public static string GenerateAttributeInstantiation(AttributeData attr, Immutabl
                                 syntaxIndex++;
                                 elementIndex++;
                                 return TypedConstantParser.GetRawTypedConstantValue(v, paramType);
-                            }).ToList();
+                            });
                             argStrings.AddRange(elements);
                         }
                     }
@@ -287,8 +287,7 @@ public static string GenerateTestAttributes(IMethodSymbol methodSymbol)
 
         // Group attributes by type
         var attributesByType = allAttributes
-            .GroupBy(attr => attr.AttributeClass?.ToDisplayString(SymbolDisplayFormat.FullyQualifiedFormat) ?? "System.Attribute")
-            .ToList();
+            .GroupBy(attr => attr.AttributeClass?.ToDisplayString(SymbolDisplayFormat.FullyQualifiedFormat) ?? "System.Attribute");
 
         using var writer = new CodeWriter("", includeHeader: false);
 
@@ -301,12 +300,11 @@ public static string GenerateTestAttributes(IMethodSymbol methodSymbol)
         foreach (var group in attributesByType)
         {
             var typeString = group.Key;
-            var attrs = group.ToList();
 
             writer.Append($"[typeof({typeString})] = new global::System.Attribute[] {{ ");
 
             var attributeStrings = new List<string>();
-            foreach (var attr in attrs)
+            foreach (var attr in group)
             {
                 attributeStrings.Add(GenerateAttributeInstantiation(attr));
             }

TUnit.Core/Extensions/ReflectionExtensions.cs

@@ -142,8 +142,7 @@ private static Attribute[] GetAttributesViaCustomAttributeData(ICustomAttributeP
         var filteredList = customAttributeDataList
             .Where(x => attributeType == typeof(Attribute) || (inherit
                 ? x.AttributeType.IsAssignableTo(attributeType)
-                : x.AttributeType == attributeType))
-            .ToList();
+                : x.AttributeType == attributeType));
 
         foreach (var attributeData in filteredList)
         {

TUnit.Engine/Building/TestBuilder.cs

@@ -1179,8 +1179,7 @@ private AbstractExecutableTest CreateFailedTestForClassDataSourceCircularDepende
     {
         var instanceClassDataSources = metadata.ClassDataSources
             .Where(ds => ds is IAccessesInstanceData)
-            .Select(ds => ds.GetType().Name)
-            .ToList();
+            .Select(ds => ds.GetType().Name);
 
         var dataSourceNames = string.Join(", ", instanceClassDataSources);
         var genericParams = string.Join(", ", metadata.TestClassType.GetGenericArguments().Select(t => t.Name));