Set out parameters to default instead of using SkipInit in Interop generators (#101076)

* Set `out` parameters to default instead of using SkipInit

* Assert value after P/Invoke

src/libraries/System.Runtime.InteropServices/gen/Microsoft.Interop.SourceGeneration/Marshalling/MarshallerHelpers.cs

@@ -263,6 +263,14 @@ namespace Microsoft.Interop
             }
         }
 
+        public static StatementSyntax DefaultInit(TypePositionInfo info, StubCodeContext context)
+        {
+            // Assign out params to default
+            return AssignmentStatement(
+                IdentifierName(info.InstanceIdentifier),
+                LiteralExpression(SyntaxKind.DefaultLiteralExpression, Token(SyntaxKind.DefaultKeyword)));
+        }
+
         /// <summary>
         /// Get the marshalling direction for a given <see cref="TypePositionInfo"/> in a given <see cref="StubCodeContext"/>.
         /// For example, an out parameter is marshalled in the <see cref="MarshalDirection.UnmanagedToManaged"/> direction in a <see cref="MarshalDirection.ManagedToUnmanaged"/> stub,

src/libraries/System.Runtime.InteropServices/gen/Microsoft.Interop.SourceGeneration/VariableDeclarations.cs

@@ -26,7 +26,7 @@ namespace Microsoft.Interop
 
                 if (info.RefKind == RefKind.Out)
                 {
-                    initializations.Add(MarshallerHelpers.SkipInitOrDefaultInit(info, context));
+                    initializations.Add(MarshallerHelpers.DefaultInit(info, context));
                 }
 
                 // Declare variables for parameters

src/libraries/System.Runtime.InteropServices/tests/ComInterfaceGenerator.Tests/UnmanagedToManagedCustomMarshallingTests.cs

@@ -7,6 +7,7 @@ using System.Runtime.CompilerServices;
 using System.Runtime.InteropServices;
 using System.Runtime.InteropServices.Marshalling;
 using System.Threading;
+using Microsoft.Diagnostics.Runtime;
 using SharedTypes;
 using Xunit;
 using static ComInterfaceGenerator.Tests.UnmanagedToManagedCustomMarshallingTests;
@@ -156,11 +157,11 @@ namespace ComInterfaceGenerator.Tests
             try
             {
                 var values = new int[] { 1, 32, 63, 124, 255 };
-
                 int freeCalls = IntWrapperMarshallerToIntWithFreeCounts.NumCallsToFree;
-
+                int oldValue = startingValue + 100;
-                NativeExportsNE.UnmanagedToManagedCustomMarshalling.SumAndSetNativeObjectData(wrapper, values, values.Length, out int _);
+                NativeExportsNE.UnmanagedToManagedCustomMarshalling.SumAndSetNativeObjectData(wrapper, values, values.Length, out oldValue);
-
+                Assert.Equal(values.Sum(), impl.GetData().i);
+                Assert.Equal(startingValue, oldValue);
                 Assert.Equal(freeCalls, IntWrapperMarshallerToIntWithFreeCounts.NumCallsToFree);
             }
             finally
@@ -181,11 +182,11 @@ namespace ComInterfaceGenerator.Tests
             try
             {
                 var values = new int[] { 1, 32, 63, 124, 255 };
-
                 int freeCalls = IntWrapperMarshallerToIntWithFreeCounts.NumCallsToFree;
-
+                int oldValue = startingValue + 100;
-                NativeExportsNE.UnmanagedToManagedCustomMarshalling.SumAndSetNativeObjectData(wrapper, ref values, values.Length, out int _);
+                NativeExportsNE.UnmanagedToManagedCustomMarshalling.SumAndSetNativeObjectData(wrapper, ref values, values.Length, out oldValue);
-
+                Assert.Equal(values.Sum(), impl.GetData().i);
+                Assert.Equal(startingValue, oldValue);
                 Assert.Equal(freeCalls + values.Length, IntWrapperMarshallerToIntWithFreeCounts.NumCallsToFree);
             }
             finally

src/libraries/System.Runtime.InteropServices/tests/LibraryImportGenerator.Tests/RefParameters.cs

@@ -0,0 +1,31 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+using System.Diagnostics;
+using System.Runtime.InteropServices;
+using SharedTypes;
+using Xunit;
+
+namespace LibraryImportGenerator.IntegrationTests
+{
+    partial class NativeExportsNE
+    {
+        public partial class RefParameters
+        {
+            [LibraryImport(NativeExportsNE_Binary, EntryPoint = "out_params")]
+            public static partial int OutParameters(out int values, out IntFields numValues);
+        }
+    }
+    public class RefParameters
+    {
+        [Fact]
+        public void OutParametersAreDefaultInitialized()
+        {
+            int myInt = 12;
+            IntFields myIntFields = new IntFields() { a = 12, b = 12, c = 12 };
+            NativeExportsNE.RefParameters.OutParameters(out myInt, out myIntFields);
+            Debug.Assert(myInt == default);
+            Debug.Assert(myIntFields.a == default && myIntFields.b == default && myIntFields.c == default);
+        }
+    }
+}

src/libraries/System.Runtime.InteropServices/tests/TestAssets/NativeExports/RefParameters.cs

@@ -0,0 +1,25 @@
+// 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.Diagnostics;
+using System.Linq;
+using System.Runtime.InteropServices;
+using System.Text;
+using System.Threading.Tasks;
+using SharedTypes;
+
+namespace NativeExports
+{
+    public static unsafe class RefParameters
+    {
+        [UnmanagedCallersOnly(EntryPoint = "out_params")]
+        public static void EnsureOutParamIsDefault(int* ri, [DNNE.C99Type("struct int_fields*")]IntFields* rif)
+        {
+            int i = *ri;
+            Debug.Assert(i == default);
+            IntFields _if = *rif;
+            Debug.Assert(_if.a == default && _if.b == default && _if.c == default);
+        }
+    }
+}

src/libraries/System.Runtime.InteropServices/tests/TestAssets/NativeExports/VirtualMethodTables.cs

@@ -198,6 +198,7 @@ namespace NativeExports
         [DNNE.C99DeclCode("struct INativeObject;")]
         public static void SumAndSetData([DNNE.C99Type("struct INativeObject*")] NativeObjectInterface* obj, int* values, int numValues, int* oldValue)
         {
+            Debug.Assert(*oldValue == default);
             obj->VTable->sumAndSetData(obj, values, numValues, oldValue);
         }
 
@@ -205,6 +206,7 @@ namespace NativeExports
         [DNNE.C99DeclCode("struct INativeObject;")]
         public static void SumAndSetDataWithRef([DNNE.C99Type("struct INativeObject*")] NativeObjectInterface* obj, int** values, int numValues, int* oldValue)
         {
+            Debug.Assert(*oldValue == default);
             obj->VTable->sumAndSetDataWithRef(obj, values, numValues, oldValue);
         }