fix(object-creation-mutator): skip empty initializer for required members

integrationtest/Validation/ValidationProject/ValidateStrykerResults.cs

@@ -81,7 +81,7 @@ public async Task CSharp_NetCore_SingleTestProject()
 
         var report = await strykerRunOutput.DeserializeJsonReportAsync();
 
-        CheckReportMutants(report, total: 660, ignored: 271, survived: 4, killed: 9, timeout: 2, nocoverage: 340);
+        CheckReportMutants(report, total: 660, ignored: 271, survived: 4, killed: 9, timeout: 2, nocoverage: 344);
         CheckReportTestCounts(report, total: 11);
     }
 
@@ -101,7 +101,7 @@ public async Task CSharp_NetCore_WithTwoTestProjects()
 
         var report = await strykerRunOutput.DeserializeJsonReportAsync();
 
-        CheckReportMutants(report, total: 660, ignored: 117, survived: 5, killed: 11, timeout: 2, nocoverage: 491);
+        CheckReportMutants(report, total: 660, ignored: 117, survived: 5, killed: 11, timeout: 2, nocoverage: 495);
         CheckReportTestCounts(report, total: 21);
     }
 
@@ -121,7 +121,7 @@ public async Task CSharp_NetCore_MSTestMTP()
 
         var report = await strykerRunOutput.DeserializeJsonReportAsync();
 
-        CheckReportMutants(report, total: 660, ignored: 271, survived: 2, killed: 1, timeout: 2, nocoverage: 350);
+        CheckReportMutants(report, total: 660, ignored: 271, survived: 2, killed: 1, timeout: 2, nocoverage: 354);
     }
 
     [Fact]
@@ -140,7 +140,7 @@ public async Task CSharp_NetCore_XUnitMTP()
 
         var report = await strykerRunOutput.DeserializeJsonReportAsync();
 
-        CheckReportMutants(report, total: 660, ignored: 271, survived: 1, killed: 1, timeout: 0, nocoverage: 353);
+        CheckReportMutants(report, total: 660, ignored: 271, survived: 1, killed: 1, timeout: 0, nocoverage: 357);
     }
 
     [Fact]
@@ -159,7 +159,7 @@ public async Task CSharp_NetCore_NUnitMTP()
 
         var report = await strykerRunOutput.DeserializeJsonReportAsync();
 
-        CheckReportMutants(report, total: 660, ignored: 271, survived: 1, killed: 1, timeout: 0, nocoverage: 353);
+        CheckReportMutants(report, total: 660, ignored: 271, survived: 1, killed: 1, timeout: 0, nocoverage: 357);
     }
 
     [Fact]
@@ -178,7 +178,7 @@ public async Task CSharp_NetCore_TUnit()
 
         var report = await strykerRunOutput.DeserializeJsonReportAsync();
 
-        CheckReportMutants(report, total: 660, ignored: 271, survived: 1, killed: 1, timeout: 0, nocoverage: 353);
+        CheckReportMutants(report, total: 660, ignored: 271, survived: 1, killed: 1, timeout: 0, nocoverage: 357);
     }
 
     [Fact]
@@ -197,7 +197,7 @@ public async Task CSharp_NetCore_MTPSolution()
 
         var report = await strykerRunOutput.DeserializeJsonReportAsync();
 
-        CheckReportMutants(report, total: 670, ignored: 274, survived: 1, killed: 1, timeout: 0, nocoverage: 360);
+        CheckReportMutants(report, total: 670, ignored: 274, survived: 1, killed: 1, timeout: 0, nocoverage: 364);
         CheckReportTestCounts(report, total: 0); // MTP doesn't report tests yet
     }
 
@@ -237,7 +237,7 @@ public async Task CSharp_NetCore_SolutionRun()
 
         var report = await strykerRunOutput.DeserializeJsonReportAsync();
 
-        CheckReportMutants(report, total: 660, ignored: 271, survived: 4, killed: 9, timeout: 2, nocoverage: 340);
+        CheckReportMutants(report, total: 660, ignored: 271, survived: 4, killed: 9, timeout: 2, nocoverage: 344);
         CheckReportTestCounts(report, total: 23);
     }
 

src/Stryker.Core/Stryker.Core.UnitTest/Mutators/ObjectCreationMutatorTests.cs

@@ -1,3 +1,5 @@
+using System.Linq;
+using Microsoft.CodeAnalysis;
 using Microsoft.CodeAnalysis.CSharp;
 using Microsoft.CodeAnalysis.CSharp.Syntax;
 using Microsoft.VisualStudio.TestTools.UnitTesting;
@@ -69,4 +71,67 @@ public void ShouldNotMutateEmptyInitializer(string initializer)
 
         result.ShouldBeEmpty();
     }
+
+    [TestMethod]
+    [DataRow("public required int Value { get; set; }")]
+    [DataRow("public required int Value;")]
+    [DataRow("public int Other { get; set; } public required int Value { get; set; }")]
+    public void ShouldPreserveRequiredMembersWhenMutatingObjectInitializer(string members)
+    {
+        var (semanticModel, expression) = CreateSemanticModel(
+            $$"""
+              class Target { {{members}} }
+              class Caller { void M() { var t = new Target { Value = 1 }; } }
+              """);
+
+        var result = new ObjectCreationMutator().ApplyMutations(expression, semanticModel).ToList();
+
+        var mutation = result.ShouldHaveSingleItem();
+        mutation.Type.ShouldBe(Mutator.Initializer);
+        var replacement = mutation.ReplacementNode.ShouldBeOfType<ObjectCreationExpressionSyntax>();
+        replacement.Initializer.Expressions.ShouldHaveSingleItem().ToString().ShouldBe("Value=default!");
+    }
+
+    [TestMethod]
+    public void ShouldPreserveRequiredMembersFromBaseTypeWhenMutatingObjectInitializer()
+    {
+        var (semanticModel, expression) = CreateSemanticModel(
+            """
+            class Base { public required int Value { get; set; } }
+            class Derived : Base { public int Other { get; set; } }
+            class Caller { void M() { var t = new Derived { Value = 1, Other = 2 }; } }
+            """);
+
+        var result = new ObjectCreationMutator().ApplyMutations(expression, semanticModel).ToList();
+
+        var mutation = result.ShouldHaveSingleItem();
+        var replacement = mutation.ReplacementNode.ShouldBeOfType<ObjectCreationExpressionSyntax>();
+        replacement.Initializer.Expressions.ShouldHaveSingleItem().ToString().ShouldBe("Value=default!");
+    }
+
+    [TestMethod]
+    public void ShouldMutateObjectInitializerWhenTypeHasNoRequiredMembers()
+    {
+        var (semanticModel, expression) = CreateSemanticModel(
+            """
+            class Target { public int Value { get; set; } }
+            class Caller { void M() { var t = new Target { Value = 1 }; } }
+            """);
+
+        var result = new ObjectCreationMutator().ApplyMutations(expression, semanticModel).ToList();
+
+        result.ShouldHaveSingleItem().Type.ShouldBe(Mutator.Initializer);
+    }
+
+    private static (SemanticModel semanticModel, ObjectCreationExpressionSyntax expression) CreateSemanticModel(string source)
+    {
+        var syntaxTree = CSharpSyntaxTree.ParseText(source);
+        var compilation = CSharpCompilation.Create("TestAssembly")
+            .WithOptions(new CSharpCompilationOptions(OutputKind.DynamicallyLinkedLibrary))
+            .AddReferences(MetadataReference.CreateFromFile(typeof(object).Assembly.Location))
+            .AddSyntaxTrees(syntaxTree);
+        var semanticModel = compilation.GetSemanticModel(syntaxTree);
+        var expression = syntaxTree.GetRoot().DescendantNodes().OfType<ObjectCreationExpressionSyntax>().Single();
+        return (semanticModel, expression);
+    }
 }

src/Stryker.Core/Stryker.Core/Mutators/ObjectCreationMutator.cs

@@ -1,4 +1,5 @@
 using System.Collections.Generic;
+using System.Linq;
 using Microsoft.CodeAnalysis;
 using Microsoft.CodeAnalysis.CSharp;
 using Microsoft.CodeAnalysis.CSharp.Syntax;
@@ -25,13 +26,59 @@ public override IEnumerable<Mutation> ApplyMutations(ObjectCreationExpressionSyn
         }
         if (node.Initializer?.Kind() == SyntaxKind.ObjectInitializerExpression && node.Initializer.Expressions.Count > 0)
         {
+            // An empty initializer would fail to compile with CS9035 when the type has
+            // required members, so we preserve them by assigning `default!` to each.
+            var requiredMembers = GetRequiredMemberNames(node, semanticModel);
+            var replacementInitializer = requiredMembers.Count == 0
+                ? SyntaxFactory.InitializerExpression(SyntaxKind.ObjectInitializerExpression)
+                : SyntaxFactory.InitializerExpression(
+                    SyntaxKind.ObjectInitializerExpression,
+                    SyntaxFactory.SeparatedList<ExpressionSyntax>(
+                        requiredMembers.Select(name =>
+                            SyntaxFactory.AssignmentExpression(
+                                SyntaxKind.SimpleAssignmentExpression,
+                                SyntaxFactory.IdentifierName(name),
+                                SyntaxFactory.PostfixUnaryExpression(
+                                    SyntaxKind.SuppressNullableWarningExpression,
+                                    SyntaxFactory.LiteralExpression(SyntaxKind.DefaultLiteralExpression))))));
+
             yield return new Mutation()
             {
                 OriginalNode = node,
-                ReplacementNode = node.ReplaceNode(node.Initializer, SyntaxFactory.InitializerExpression(SyntaxKind.ObjectInitializerExpression)).WithCleanTrivia(),
+                ReplacementNode = node.ReplaceNode(node.Initializer, replacementInitializer).WithCleanTrivia(),
                 DisplayName = "Object initializer mutation",
                 Type = Mutator.Initializer,
             };
         }
     }
+
+    private static IReadOnlyList<string> GetRequiredMemberNames(ObjectCreationExpressionSyntax node, SemanticModel semanticModel)
+    {
+        if (semanticModel is null)
+        {
+            return [];
+        }
+
+        if (semanticModel.GetTypeInfo(node).Type is not INamedTypeSymbol typeSymbol)
+        {
+            return [];
+        }
+
+        var names = new List<string>();
+        var seen = new HashSet<string>();
+        for (var current = typeSymbol; current is not null; current = current.BaseType)
+        {
+            foreach (var member in current.GetMembers())
+            {
+                var isRequired = (member is IPropertySymbol prop && prop.IsRequired) ||
+                                 (member is IFieldSymbol field && field.IsRequired);
+                if (isRequired && seen.Add(member.Name))
+                {
+                    names.Add(member.Name);
+                }
+            }
+        }
+
+        return names;
+    }
 }