[DllImportGenerator] Initialize by-value out array to default

docs/design/libraries/DllImportGenerator/Compatibility.md

@@ -71,6 +71,8 @@ Only single-dimensional arrays are supported for source generated marshalling.
 
 In the source-generated marshalling, arrays will be allocated on the stack (instead of through [`AllocCoTaskMem`](https://docs.microsoft.com/dotnet/api/system.runtime.interopservices.marshal.alloccotaskmem)) if they are passed by value or by read-only reference if they contain at most 256 bytes of data. The built-in system does not support this optimization for arrays.
 
+In the built-in system, marshalling a `char` array by value with `CharSet.Unicode` would default to also marshalling data out. In the source-generated marshalling, the `char` array must be marked with the `[Out]` attribute for data to be marshalled out by value.
+
 ### `in` keyword
 
 For some types - blittable or Unicode `char` - passed by read-only reference via the [`in` keyword](https://docs.microsoft.com/dotnet/csharp/language-reference/keywords/in-parameter-modifier), the built-in system simply pins the parameter. The generated marshalling code does the same, such that there is no behavioural difference. A consequence of this behaviour is that any modifications made by the invoked function will be reflected in the caller. It is left to the user to avoid the situation in which `in` is used for a parameter that will actually be modified by the invoked function.

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

@@ -977,6 +977,18 @@ public IEnumerable<StatementSyntax> GenerateMarshalStatements(TypePositionInfo i
             if (!info.IsByRef && info.ByValueContentsMarshalKind == ByValueContentsMarshalKind.Out)
             {
                 // If the parameter is marshalled by-value [Out], then we don't marshal the contents of the collection.
+                // We do clear the span, so that if the invoke target doesn't fill it, we aren't left with undefined content.
+                // <nativeIdentifier>.NativeValueStorage.Clear();
+                string nativeIdentifier = context.GetIdentifiers(info).native;
+                yield return ExpressionStatement(
+                    InvocationExpression(
+                        MemberAccessExpression(
+                            SyntaxKind.SimpleMemberAccessExpression,
+                            MemberAccessExpression(
+                                SyntaxKind.SimpleMemberAccessExpression,
+                                IdentifierName(nativeIdentifier),
+                                IdentifierName(ManualTypeMarshallingHelper.NativeValueStoragePropertyName)),
+                            IdentifierName("Clear"))));
                 yield break;
             }
 

src/libraries/System.Runtime.InteropServices/tests/DllImportGenerator.Tests/ArrayTests.cs

@@ -38,6 +38,9 @@ public partial class Arrays
             [GeneratedDllImport(NativeExportsNE_Binary, EntryPoint = "sum_char_array", CharSet = CharSet.Unicode)]
             public static partial int SumChars(char[] chars, int numElements);
 
+            [GeneratedDllImport(NativeExportsNE_Binary, EntryPoint = "fill_char_array", CharSet = CharSet.Unicode)]
+            public static partial void FillChars([Out] char[] chars, int length, ushort start);
+
             [GeneratedDllImport(NativeExportsNE_Binary, EntryPoint = "reverse_char_array", CharSet = CharSet.Unicode)]
             public static partial void ReverseChars([MarshalAs(UnmanagedType.LPArray, SizeParamIndex = 1)] ref char[] chars, int numElements);
 
@@ -253,12 +256,34 @@ public void DynamicSizedArrayWithConstantComponent()
         [Fact]
         public void ArrayByValueOutParameter()
         {
-            var testArray = new IntStructWrapper[10];
-            int start = 5;
-
-            NativeExportsNE.Arrays.FillRangeArray(testArray, testArray.Length, start);
-
-            Assert.Equal(Enumerable.Range(start, 10), testArray.Select(wrapper => wrapper.Value));
+            {
+                var testArray = new IntStructWrapper[10];
+                int start = 5;
+
+                NativeExportsNE.Arrays.FillRangeArray(testArray, testArray.Length, start);
+                Assert.Equal(Enumerable.Range(start, testArray.Length), testArray.Select(wrapper => wrapper.Value));
+
+                // Any items not populated by the invoke target should be initialized to default
+                testArray = new IntStructWrapper[10];
+                int lengthToFill = testArray.Length / 2;
+                NativeExportsNE.Arrays.FillRangeArray(testArray, lengthToFill, start);
+                Assert.Equal(Enumerable.Range(start, lengthToFill), testArray[..lengthToFill].Select(wrapper => wrapper.Value));
+                Assert.All(testArray[lengthToFill..], wrapper => Assert.Equal(0, wrapper.Value));
+            }
+            {
+                var testArray = new char[10];
+                ushort start = 65;
+
+                NativeExportsNE.Arrays.FillChars(testArray, testArray.Length, start);
+                Assert.Equal(Enumerable.Range(start, testArray.Length), testArray.Select(c => (int)c));
+
+                // Any items not populated by the invoke target should be initialized to default
+                testArray = new char[10];
+                int lengthToFill = testArray.Length / 2;
+                NativeExportsNE.Arrays.FillChars(testArray, lengthToFill, start);
+                Assert.Equal(Enumerable.Range(start, lengthToFill), testArray[..lengthToFill].Select(c => (int)c));
+                Assert.All(testArray[lengthToFill..], c => Assert.Equal(0, c));
+            }
         }
 
         [Fact]

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

@@ -106,6 +106,21 @@ public static int SumChars(ushort* values, int numValues)
             return sum;
         }
 
+        [UnmanagedCallersOnly(EntryPoint = "fill_char_array")]
+        public static void FillChars(ushort* values, int length, ushort start)
+        {
+            if (values == null)
+            {
+                return;
+            }
+
+            ushort val = start;
+            for (int i = 0; i < length; i++)
+            {
+                values[i] = val++;
+            }
+        }
+
         [UnmanagedCallersOnly(EntryPoint = "reverse_char_array")]
         public static void ReverseChars(ushort** values, int numValues)
         {