add support for netstandard2.1 / net8 / net9

Author: pchalamet

README.md

@@ -21,7 +21,9 @@ Repository origins are:
 
 ## Supported platforms
 
-This project targets `netstandard2.1` ([compatible runtimes](https://learn.microsoft.com/en-us/dotnet/standard/net-standard?tabs=net-standard-2-1#select-net-standard-version)). 
+This project targets `.net 8`, `.net 9` and  `netstandard2.1` ([compatible runtimes](https://learn.microsoft.com/en-us/dotnet/standard/net-standard?tabs=net-standard-2-1#select-net-standard-version))
+
+:warning: NRT support starts with `.net sdk 9.0.200`. F# compiler in .net sdk 9.0.10x does not set correctly nullable attributes on F# types. NRT are not supported on `netstandard2.1`.
 
 ## Contributing
   * If you have a question about the library, then create an [issue][issues] with the `question` label.
@@ -89,7 +91,9 @@ NRT are serialized as:
 * `null` if `Null`
 * `object` if `NonNull` object
 
-:warning: As of now, NRT can't be considered `null` when deserializing if value is missing.
+On deserialization, missing value is mapped to `null`.
+
+:warning: NRT support starts with .net sdk 9.0.200. F# compiler in .net sdk 9.0.10x does not set correctly nullable attributes on F# types.
 
 # License
 The contents of this library are made available under the [Apache License, Version 2.0][license].

src/FSharp.MongoDB.Bson/FSharp.MongoDB.Bson.fsproj

@@ -1,7 +1,7 @@
 <Project Sdk="Microsoft.NET.Sdk">
 
   <PropertyGroup>
-    <TargetFrameworks>net9.0;net8.0</TargetFrameworks>
+    <TargetFrameworks>net9.0;net8.0;netstandard2.1</TargetFrameworks>
     <GenerateDocumentationFile>true</GenerateDocumentationFile>
     <Nullable>enable</Nullable>
     <TreatWarningsAsErrors>true</TreatWarningsAsErrors>

src/FSharp.MongoDB.Bson/Serialization/FSharpTypeHelpers.fs

@@ -23,7 +23,9 @@ module private Helpers =
 
     let bindingFlags = BindingFlags.Public ||| BindingFlags.NonPublic
 
+#if !NETSTANDARD2_1
     let nrtContext = NullabilityInfoContext()
+#endif
 
     /// <summary>
     /// Returns <c>Some typ</c> when <c>pred typ</c> returns true, and <c>None</c> when
@@ -95,6 +97,10 @@ module private Helpers =
 
     let mkMemberNullable (memberMap: BsonClassMap) (propertyInfo: PropertyInfo) =
         let memberMap = memberMap.MapMember(propertyInfo)
+#if !NETSTANDARD2_1
         let nrtInfo = nrtContext.Create(propertyInfo)
         if nrtInfo.WriteState = NullabilityState.Nullable then
             memberMap.SetDefaultValue(null).SetIsRequired(false) |> ignore
+#else
+        ()
+#endif