Avoid Unsafe.SkipInit API when it isn't available

README.md

@@ -33,16 +33,20 @@ Install the `Microsoft.Windows.CsWin32` package:
 dotnet add package Microsoft.Windows.CsWin32 --prerelease
 ```
 
-You should also install the `System.Memory` package when targeting .NET Framework 4.5+ or .NET Standard 2.0,
-as that adds APIs that significantly improve much of the code generated by CsWin32:
+You should also install the `System.Memory` and `System.Runtime.CompilerServices.Unsafe` packages when targeting .NET Framework 4.5+ or .NET Standard 2.0,
+as these add APIs that significantly improve much of the code generated by CsWin32:
 
 ```ps1
 dotnet add package System.Memory
+dotnet add package System.Runtime.CompilerServices.Unsafe
 ```
 
-Projects targeting .NET Core 2.1+ or .NET 5+ do *not* need to add the `System.Memory` package reference,
+Projects targeting .NET Core 2.1+ or .NET 5+ do *not* need to add these package references,
 although it is harmless to do so.
 
+Note that while the `System.Memory` package depends on the `System.Runtime.CompilerServices.Unsafe` package,
+referencing the latter directly is still important to get the latest version of the APIs it provides.
+
 Your project must allow unsafe code to support the generated code that will likely use pointers.
 This does *not* automatically make all your code *unsafe*.
 Use of the `unsafe` keyword is required anywhere you use pointers.

src/Microsoft.Windows.CsWin32/Generator.Features.cs

@@ -10,6 +10,7 @@ public partial class Generator
     private readonly bool canUseUnsafeAsRef;
     private readonly bool canUseUnsafeAdd;
     private readonly bool canUseUnsafeNullRef;
+    private readonly bool canUseUnsafeSkipInit;
     private readonly bool canUseUnmanagedCallersOnlyAttribute;
     private readonly bool canUseSetLastPInvokeError;
     private readonly bool unscopedRefAttributePredefined;

src/Microsoft.Windows.CsWin32/Generator.InlineArrays.cs

@@ -605,11 +605,20 @@ InvocationExpressionSyntax SliceAtNullToString(ExpressionSyntax readOnlySpan)
                 ? MemberAccessExpression(SyntaxKind.SimpleMemberAccessExpression, resultLocal, valueFieldName) // result.Value
                 : PrefixUnaryExpression(SyntaxKind.AddressOfExpression, MemberAccessExpression(SyntaxKind.SimpleMemberAccessExpression, resultLocal, firstElementName!)); // &result._0
 
-            // Unsafe.SkipInit(out __char_1 result);
-            implicitSpanToStruct = implicitSpanToStruct.AddBodyStatements(
-                ExpressionStatement(InvocationExpression(
-                    MemberAccessExpression(SyntaxKind.SimpleMemberAccessExpression, IdentifierName(nameof(Unsafe)), IdentifierName("SkipInit")),
-                    ArgumentList().AddArguments(Argument(nameColon: null, Token(SyntaxKind.OutKeyword), DeclarationExpression(fixedLengthStructName.WithTrailingTrivia(Space), SingleVariableDesignation(resultLocal.Identifier)))))));
+            if (this.canUseUnsafeSkipInit)
+            {
+                // Unsafe.SkipInit(out __char_1 result);
+                implicitSpanToStruct = implicitSpanToStruct.AddBodyStatements(
+                    ExpressionStatement(InvocationExpression(
+                        MemberAccessExpression(SyntaxKind.SimpleMemberAccessExpression, IdentifierName(nameof(Unsafe)), IdentifierName("SkipInit")),
+                        ArgumentList().AddArguments(Argument(nameColon: null, Token(SyntaxKind.OutKeyword), DeclarationExpression(fixedLengthStructName.WithTrailingTrivia(Space), SingleVariableDesignation(resultLocal.Identifier)))))));
+            }
+            else
+            {
+                implicitSpanToStruct = implicitSpanToStruct.AddBodyStatements(
+                    LocalDeclarationStatement(VariableDeclaration(fixedLengthStructName)).AddDeclarationVariables(
+                        VariableDeclarator(resultLocal.Identifier).WithInitializer(EqualsValueClause(DefaultExpression(fixedLengthStructName)))));
+            }
 
             // x.Slice(initLength, Length - initLength).Clear();
             StatementSyntax ClearSlice(ExpressionSyntax span) =>

src/Microsoft.Windows.CsWin32/Generator.cs

@@ -90,6 +90,7 @@ public Generator(string metadataLibraryPath, Docs? docs, GeneratorOptions option
         this.canUseUnsafeAsRef = this.compilation?.GetTypeByMetadataName(typeof(Unsafe).FullName)?.GetMembers("Add").Any() is true;
         this.canUseUnsafeAdd = this.compilation?.GetTypeByMetadataName(typeof(Unsafe).FullName)?.GetMembers("AsRef").Any() is true;
         this.canUseUnsafeNullRef = this.compilation?.GetTypeByMetadataName(typeof(Unsafe).FullName)?.GetMembers("NullRef").Any() is true;
+        this.canUseUnsafeSkipInit = this.compilation?.GetTypeByMetadataName(typeof(Unsafe).FullName)?.GetMembers("SkipInit").Any() is true;
         this.canUseUnmanagedCallersOnlyAttribute = this.compilation?.GetTypeByMetadataName("System.Runtime.InteropServices.UnmanagedCallersOnlyAttribute") is not null;
         this.canUseSetLastPInvokeError = this.compilation?.GetTypeByMetadataName("System.Runtime.InteropServices.Marshal")?.GetMembers("GetLastSystemError").IsEmpty is false;
         this.unscopedRefAttributePredefined = this.FindTypeSymbolIfAlreadyAvailable("System.Diagnostics.CodeAnalysis.UnscopedRefAttribute") is not null;

src/Microsoft.Windows.CsWin32/readme.txt

@@ -12,11 +12,15 @@ as that adds APIs that significantly improve much of the code generated by CsWin
 
 ```ps1
 dotnet add package System.Memory
+dotnet add package System.Runtime.CompilerServices.Unsafe
 ```
 
-Projects targeting .NET Core 2.1+ or .NET 5+ do *not* need to add the `System.Memory` package reference,
+Projects targeting .NET Core 2.1+ or .NET 5+ do *not* need to add these package references,
 although it is harmless to do so.
 
+Note that while the `System.Memory` package depends on the `System.Runtime.CompilerServices.Unsafe` package,
+referencing the latter directly is still important to get the latest version of the APIs it provides.
+
 Your project must allow unsafe code to support the generated code that will likely use pointers.
 
 Learn more from our README on GitHub: https://github.com/microsoft/CsWin32#readme

test/Microsoft.Windows.CsWin32.Tests/InlineArrayTests.cs

@@ -31,6 +31,33 @@ public void FixedLengthInlineArray(
         this.AssertNoDiagnostics();
     }
 
+    [Theory, CombinatorialData]
+    public async Task UnsafeApiReferencesOnlyWhenAvailable(
+        bool referenceUnsafe,
+        bool referenceMemory,
+        [CombinatorialValues("net35", "net472", "netstandard2.0")] string tfm)
+    {
+        ReferenceAssemblies referenceAssemblies = tfm switch
+        {
+            "net35" => ReferenceAssemblies.NetFramework.Net35.WindowsForms,
+            "net472" => ReferenceAssemblies.NetFramework.Net472.WindowsForms,
+            "netstandard2.0" => ReferenceAssemblies.NetStandard.NetStandard20,
+            _ => throw new ArgumentOutOfRangeException(nameof(tfm)),
+        };
+        if (referenceUnsafe)
+        {
+            referenceAssemblies = referenceAssemblies.AddPackages([MyReferenceAssemblies.ExtraPackages.Unsafe]);
+        }
+
+        if (referenceMemory)
+        {
+            referenceAssemblies = referenceAssemblies.AddPackages([MyReferenceAssemblies.ExtraPackages.Memory]);
+        }
+
+        this.compilation = await this.CreateCompilationAsync(referenceAssemblies, Platform.AnyCpu);
+        this.GenerateApi("THUMBBUTTON");
+    }
+
     [Theory, PairwiseData]
     public void FixedLengthInlineArray_Pointers(
         bool allowMarshaling,

test/Microsoft.Windows.CsWin32.Tests/MyReferenceAssemblies.cs

@@ -8,9 +8,9 @@ internal static class MyReferenceAssemblies
         new PackageIdentity("Microsoft.Windows.SDK.Contracts", "10.0.22621.2428"));
 
     private static readonly ImmutableArray<PackageIdentity> AdditionalModernPackages = AdditionalLegacyPackages.AddRange(ImmutableArray.Create(
-        new PackageIdentity("System.Runtime.CompilerServices.Unsafe", "6.0.0"),
-        new PackageIdentity("System.Memory", "4.5.5"),
-        new PackageIdentity("Microsoft.Win32.Registry", "5.0.0")));
+        ExtraPackages.Unsafe,
+        ExtraPackages.Memory,
+        ExtraPackages.Registry));
 
     internal static readonly ReferenceAssemblies NetStandard20 = ReferenceAssemblies.NetStandard.NetStandard20.AddPackages(AdditionalModernPackages);
 #pragma warning restore SA1202 // Elements should be ordered by access
@@ -26,4 +26,11 @@ internal static class Net
         internal static readonly ReferenceAssemblies Net60 = ReferenceAssemblies.Net.Net60.AddPackages(AdditionalModernPackages);
         internal static readonly ReferenceAssemblies Net70 = ReferenceAssemblies.Net.Net70.AddPackages(AdditionalModernPackages);
     }
+
+    internal static class ExtraPackages
+    {
+        internal static readonly PackageIdentity Unsafe = new PackageIdentity("System.Runtime.CompilerServices.Unsafe", "6.0.0");
+        internal static readonly PackageIdentity Memory = new PackageIdentity("System.Memory", "4.5.5");
+        internal static readonly PackageIdentity Registry = new PackageIdentity("Microsoft.Win32.Registry", "5.0.0");
+    }
 }