Use System.Text.Json in Netfx build of MSBuildSdkResolver (#39573)

### Summary

Switch the .NET Framework version of `MSBuildSdkResolver` to use `System.Text.Json` instead of `Newtonsoft.Json` to align it with MSBuild and make it possible to load the resolver with `Assembly.Load`.

### Notes

We cannot build against the SDK version of S.T.J because that's generally not available in MSBuild. I have set the version to 8.0.0 and I believe it can stay like this for as long as we don't need any newer functionality from the library. When the need to upgrade arises, we should just check that MSBuild in VS where this is inserted to has the new (or higher) version bundled with it.

Author: ladipro

src/Resolvers/Microsoft.DotNet.MSBuildSdkResolver/Microsoft.DotNet.MSBuildSdkResolver.csproj

@@ -13,9 +13,12 @@
 
     <Nullable>Enable</Nullable>
 
-    <UseSystemTextJson Condition="'$(TargetFramework)'!='netstandard2.0' And '$(TargetFramework)'!='net472'">True</UseSystemTextJson>
+    <UseSystemTextJson Condition="'$(TargetFramework)'!='netstandard2.0'">True</UseSystemTextJson>
     <DefineConstants Condition="'$(UseSystemTextJson)'=='True'">$(DefineConstants);USE_SYSTEM_TEXT_JSON</DefineConstants>
 
+    <!-- Netfx version of the resolver builds against the lowest version of System.Text.Json that's guaranteed to be shipped with MSBuild in VS -->
+    <SystemTextJsonVersionOverride>8.0.0</SystemTextJsonVersionOverride>
+
     <!-- Create FileDefinitions items for ResolveHostfxrCopyLocalContent target -->
     <EmitLegacyAssetsFileItems>true</EmitLegacyAssetsFileItems>
 
@@ -100,7 +103,8 @@
   </ItemGroup>
 
   <ItemGroup>
-    <!-- No PackageReference to System.Text.Json necessary, because it's included in .NET 5.0 and higher -->
+    <!-- No PackageReference to System.Text.Json necessary when targeting .NET, because it's included in .NET 5.0 and higher -->
+    <PackageReference Include="System.Text.Json" VersionOverride="$(SystemTextJsonVersionOverride)" Condition="'$(UseSystemTextJson)'=='True' and '$(TargetFramework)'=='net472'"/>
     <PackageReference Include="Newtonsoft.Json" Condition="'$(UseSystemTextJson)'!='True'"/>
 
     <!-- Reference this package to avoid package downgrade errors.  See https://github.com/dotnet/sdk/issues/3044 for details -->
@@ -139,13 +143,16 @@
 
     <ItemGroup>
       <ExpectedDependencies Include="Microsoft.Deployment.DotNet.Releases, Version=2.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35" />
-      <ExpectedDependencies Include="Newtonsoft.Json, Version=13.0.0.0, Culture=neutral, PublicKeyToken=30ad4fe6b2a6aeed" />
+      <ExpectedDependencies Include="System.Text.Json, Version=8.0.0.0, Culture=neutral, PublicKeyToken=cc7b13ffcd2ddd51" />
+      <ExpectedDependencies Include="System.Text.Encodings.Web, Version=8.0.0.0, Culture=neutral, PublicKeyToken=cc7b13ffcd2ddd51" />
       <ExpectedDependencies Include="Microsoft.Build.Framework, Version=15.1.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a" />
       <ExpectedDependencies Include="System.Collections.Immutable, Version=8.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a" />
       <ExpectedDependencies Include="System.Memory, Version=4.0.1.2, Culture=neutral, PublicKeyToken=cc7b13ffcd2ddd51" />
       <ExpectedDependencies Include="System.Runtime.CompilerServices.Unsafe, Version=6.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a" />
       <ExpectedDependencies Include="System.Numerics.Vectors, Version=4.1.4.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a" />
       <ExpectedDependencies Include="System.Buffers, Version=4.0.3.0, Culture=neutral, PublicKeyToken=cc7b13ffcd2ddd51" />
+      <ExpectedDependencies Include="System.ValueTuple, Version=4.0.3.0, Culture=neutral, PublicKeyToken=cc7b13ffcd2ddd51" />
+      <ExpectedDependencies Include="System.Threading.Tasks.Extensions, Version=4.2.0.1, Culture=neutral, PublicKeyToken=cc7b13ffcd2ddd51" />
     </ItemGroup>
 
     <!-- Check that the dependencies of the output assembly match our expectations -->

src/Resolvers/Microsoft.NET.Sdk.WorkloadManifestReader/WorkloadManifestReader.SystemTextJson.cs

@@ -3,10 +3,7 @@
 
 #if USE_SYSTEM_TEXT_JSON
 
-using System;
 using System.Buffers;
-using System.Collections.Generic;
-using System.IO;
 using System.Text.Json;
 
 namespace Microsoft.NET.Sdk.WorkloadManifestReader
@@ -47,17 +44,17 @@ internal ref struct Utf8JsonStreamReader
             Utf8JsonReader reader;
             readonly Stream stream;
 
-            IMemoryOwner<byte> buffer;
+            byte[]? buffer;
 
             Span<byte> span;
 
             public Utf8JsonStreamReader(Stream stream, JsonReaderOptions readerOptions)
             {
                 this.stream = stream;
 
-                buffer = MemoryPool<byte>.Shared.Rent(segmentSize);
-                var readCount = stream.Read(buffer.Memory.Span);
-                span = buffer.Memory.Slice(0, readCount).Span;
+                buffer = ArrayPool<byte>.Shared.Rent(segmentSize);
+                var readCount = stream.Read(buffer, 0, buffer.Length);
+                span = buffer.AsSpan().Slice(0, readCount);
 
                 if (span.StartsWith(utf8Bom))
                 {
@@ -73,6 +70,11 @@ public bool Read()
                 {
                     if (reader.IsFinalBlock)
                     {
+                        if (buffer != null)
+                        {
+                            ArrayPool<byte>.Shared.Return(buffer);
+                            buffer = null;
+                        }
                         return false;
                     }
 
@@ -86,18 +88,21 @@ public bool Read()
 
                     int remaining = (int)(span.Length - reader.BytesConsumed);
 
-                    var newBuffer = MemoryPool<byte>.Shared.Rent(newSegmentSize);
+                    var newBuffer = ArrayPool<byte>.Shared.Rent(newSegmentSize);
 
                     if (remaining > 0)
                     {
-                        span.Slice((int)reader.BytesConsumed).CopyTo(newBuffer.Memory.Span);
+                        span.Slice((int)reader.BytesConsumed).CopyTo(newBuffer);
                     }
 
-                    var readCount = stream.Read(newBuffer.Memory.Span.Slice(remaining));
+                    var readCount = stream.Read(newBuffer, remaining, newBuffer.Length - remaining);
 
-                    buffer.Dispose();
+                    if (buffer != null)
+                    {
+                        ArrayPool<byte>.Shared.Return(buffer);
+                    }
                     buffer = newBuffer;
-                    span = newBuffer.Memory.Slice(0, remaining + readCount).Span;
+                    span = newBuffer.AsSpan().Slice(0, remaining + readCount);
 
                     reader = new Utf8JsonReader(span, stream.Position >= stream.Length, reader.CurrentState);
                 }