Add netstandard2.0 target for .NET Framework/Core compatibility

- TFMs now: netstandard2.0, net8.0, net9.0, net10.0
- netstandard2.0 enables use from .NET Framework 4.8 and .NET Core 3.1+
  (matching the original C++/CLI package's target audience)
- IsAotCompatible only set for net8.0+ (not applicable to netstandard2.0)
- System.Runtime.CompilerServices.Unsafe package added for netstandard2.0
- Updated README, design doc, and presentation slides
- Re-exported PDF

All 16 tests pass.

Author: miniksa

ProjectedFSLib.Managed.CSharp/ProjectedFSLib.Managed.CSharp.csproj

@@ -1,13 +1,13 @@
 <Project Sdk="Microsoft.NET.Sdk">
 
   <PropertyGroup>
-    <TargetFrameworks>net8.0;net9.0;net10.0</TargetFrameworks>
+    <TargetFrameworks>netstandard2.0;net8.0;net9.0;net10.0</TargetFrameworks>
     <RootNamespace>Microsoft.Windows.ProjFS</RootNamespace>
     <AssemblyName>ProjectedFSLib.Managed</AssemblyName>
     <AllowUnsafeBlocks>true</AllowUnsafeBlocks>
     <Nullable>enable</Nullable>
     <LangVersion>latest</LangVersion>
-    <IsAotCompatible>true</IsAotCompatible>
+    <IsAotCompatible Condition="'$(TargetFramework)' != 'netstandard2.0'">true</IsAotCompatible>
 
     <!-- Package metadata -->
     <PackageId>Microsoft.Windows.ProjFS.CSharp</PackageId>
@@ -22,4 +22,8 @@ Requires Windows 10 version 1809+ with the ProjFS optional feature enabled.</Des
     <PackageTags>ProjFS;ProjectedFileSystem;VirtualFileSystem;Windows</PackageTags>
   </PropertyGroup>
 
+  <ItemGroup Condition="'$(TargetFramework)' == 'netstandard2.0'">
+    <PackageReference Include="System.Runtime.CompilerServices.Unsafe" Version="6.0.0" />
+  </ItemGroup>
+
 </Project>

README.md

@@ -46,7 +46,10 @@ This is a complete rewrite of the original C++/CLI wrapper.  Key improvements:
 ### ProjectedFSLib.Managed.CSharp project
 
 This project contains the pure C# P/Invoke implementation of the ProjFS managed wrapper,
-producing `ProjectedFSLib.Managed.dll`.  It targets net8.0 and net10.0.
+producing `ProjectedFSLib.Managed.dll`.  It targets netstandard2.0, net8.0, and net10.0.
+
+The netstandard2.0 target allows use from .NET Framework 4.8 and .NET Core 3.1+ projects,
+providing a migration path from the original C++/CLI package without requiring a TFM upgrade.
 
 ### SimpleProviderManaged project
 

doc/design-pure-csharp.md

@@ -18,7 +18,7 @@ The original ProjFS Managed API is a C++/CLI mixed-mode assembly that wraps the
 | NativeAOT | Not supported (mixed-mode incompatible) | Fully supported |
 | Trimming | Not supported | `IsAotCompatible=true` |
 | Cross-compilation | Requires matching native toolchain | Any machine with .NET SDK |
-| TFMs | net48, netcoreapp3.1 | net8.0, net9.0, net10.0+ |
+| TFMs | net48, netcoreapp3.1 | netstandard2.0, net8.0, net9.0, net10.0+ |
 | Maintenance | Dual C++/C# expertise needed | C# only |
 
 ## Design Goals

doc/projfs-pure-csharp-overview.md

@@ -52,6 +52,7 @@ The current `ProjectedFSLib.Managed.dll` is a **C++/CLI mixed-mode assembly**.
 - `Microsoft.Windows.ProjFS` namespace preserved
 - All types, interfaces, delegates, enums — identical signatures
 - 16/16 existing tests pass (including symlink tests)
+- **netstandard2.0** target for .NET Framework 4.8 / .NET Core 3.1 compatibility
 
 ---
 
@@ -116,7 +117,7 @@ Non-symlink operations work on both NTFS and ReFS.
 | Runtime deps | VC++ Redist + Ijwhost.dll | None |
 | NativeAOT | ❌ | ✅ |
 | Trimming | ❌ | ✅ (`IsAotCompatible=true`) |
-| TFMs | net48, netcoreapp3.1 | net8.0, net9.0, net10.0 |
+| TFMs | net48, netcoreapp3.1 | netstandard2.0, net8.0, net9.0, net10.0 |
 | Tests passing | 16/16 | 16/16 |
 | Lines of code | ~4,000 (C++/CLI) | ~1,700 (C#) |
 | Source files | 30+ (.h, .cpp, .vcxproj) | 5 (.cs, .csproj) |