Use old-school DllImport to be netstandard2.0 compatible

.github/workflows/build.yml

@@ -212,12 +212,17 @@ jobs:
         include: 
           - runs-on: ubuntu-latest
             architecture: x64
+            framework: net6.0
+          - runs-on: macos-latest
+            architecture: x64
+            framework: net6.0
           - runs-on: windows-latest
             architecture: x64
+            framework: net6.0
           - runs-on: windows-latest
-            architecture: x86
-          - runs-on: macos-latest
             architecture: x64
+            framework: net48
+
     runs-on: ${{ matrix.runs-on }}
     needs: [dotnet]
     steps:
@@ -226,7 +231,7 @@ jobs:
       with:
         name: nupkgs
         path: dist/.organize/nupkgs
-    - run: ./build.cmd DotNetTest --architecture ${{ matrix.architecture }}
+    - run: ./build.cmd DotNetTest --architecture ${{ matrix.architecture }} --framework ${{ matrix.framework }}
     - uses: actions/upload-artifact@v3
       with:
         name: DotNet Test Results

build/Build.DotNet.cs

@@ -95,14 +95,18 @@ void DotNetWriteVersionInfoProps()
         (RootDirectory / "lib" / "dotnet" / "Version.props").WriteAllText(props);
     }
 
+    [Parameter] string Framework;
+
     public Target DotNetTest => _ => _
         .DependsOn(PrepareGitHubArtifacts)
+        .Requires(() => Framework)
         .Executes(() =>
         {
             DotNetTasks.DotNetRun(_ => _
                 .SetProcessWorkingDirectory(RootDirectory / "test" / "dotnet" / "AlphaSkia.Test")
                 .SetRuntime(TargetOperatingSystem.Current.DotNetRid + "-" +
                             (Architecture ?? Architecture.Current))
+                .SetFramework(Framework)
                 .AddProcessEnvironmentVariable("NUGET_PACKAGES", TemporaryDirectory / "packages")
             );
         });

lib/dotnet/AlphaSkia/AlphaSkia.csproj

@@ -2,14 +2,15 @@
 
     <PropertyGroup>
         <IsPackable>true</IsPackable>
-        <TargetFramework>net7.0</TargetFramework>
+        <TargetFramework>netstandard2.0</TargetFramework>
         <ImplicitUsings>enable</ImplicitUsings>
         <Nullable>enable</Nullable>
         <PackageReadmeFile>README.md</PackageReadmeFile>
         <AllowUnsafeBlocks>true</AllowUnsafeBlocks>
         <LangVersion>10</LangVersion>
 		<Description>$(ProductDescription)</Description>
         <GenerateDocumentationFile>true</GenerateDocumentationFile>
+        <EmitCompilerGeneratedFiles>true</EmitCompilerGeneratedFiles>
     </PropertyGroup>
 
     <ItemGroup>

lib/dotnet/AlphaSkia/AlphaSkiaPlatform.cs

@@ -1,4 +1,5 @@
-using System.Runtime.InteropServices;
+using System.Reflection;
+using System.Runtime.InteropServices;
 
 namespace AlphaSkia;
 
@@ -40,7 +41,7 @@ private static bool TryLoadManually()
                 NativeMethods.AlphaSkiaNativeLibName + libExtension);
             if (File.Exists(alphaSkiaPath))
             {
-                if (NativeLibrary.TryLoad(alphaSkiaPath, out _) && CheckNativeLib())
+                if (TryLoadLibrary(alphaSkiaPath) && CheckNativeLib())
                 {
                     return true;
                 }
@@ -54,12 +55,42 @@ private static bool TryLoadManually()
         return false;
     }
 
+    private static bool TryLoadLibrary(string path)
+    {
+        // on .net FX or some .netstandard environments the NativeLibrary might not be available
+        // try to use it if available, otherwise fallback.
+        var nativeLibraryTryLoadMethod = typeof(DllImportAttribute).Assembly
+            .GetType("System.Runtime.InteropServices.NativeLibrary")
+            ?.GetMethod("TryLoad", BindingFlags.Static | BindingFlags.Public);
+        if (nativeLibraryTryLoadMethod != null)
+        {
+            return (bool)nativeLibraryTryLoadMethod.Invoke(null, new object[]
+            {
+                path, IntPtr.Zero
+            })!;
+        }
+
+        var handle = LoadLibraryWindows(path);
+        return handle != IntPtr.Zero;
+    }
+
+    [DllImport("kernel32.dll", SetLastError = true, CharSet = CharSet.Unicode, EntryPoint = "LoadLibraryW",
+        ExactSpelling = true)]
+    private static extern nint LoadLibraryWindows(string fileName);
+
+    private static string FallbackRuntimeIdentifier =>
+#if NETSTANDARD2_0
+        "windows-" + RuntimeInformation.OSArchitecture.ToString().ToLowerInvariant();
+#else
+        RuntimeInformation.RuntimeIdentifier;
+#endif
+
     private static string BuildKnownRid(out string libExtension)
     {
-        var officialRid = RuntimeInformation.RuntimeIdentifier;
+        var officialRid = FallbackRuntimeIdentifier;
         var separator = officialRid.IndexOf('-');
-        var officialOs = officialRid[..separator];
-        var officialArch = officialRid[(separator + 1)..];
+        var officialOs = officialRid.Substring(0, separator);
+        var officialArch = officialRid.Substring(separator + 1);
 
         libExtension = Environment.OSVersion.Platform switch
         {
@@ -68,7 +99,7 @@ private static string BuildKnownRid(out string libExtension)
             PlatformID.MacOSX => ".dylib",
             _ => ""
         };
-        
+
         var os = Environment.OSVersion.Platform switch
         {
             PlatformID.Unix => "linux",
@@ -78,7 +109,7 @@ private static string BuildKnownRid(out string libExtension)
         };
         if (string.IsNullOrEmpty(os))
         {
-            return RuntimeInformation.RuntimeIdentifier;
+            return FallbackRuntimeIdentifier;
         }
 
         var arch = RuntimeInformation.OSArchitecture switch