Add multiple target frameworks

Author: Havret

src/Protobuf.System.Text.Json/FieldInfo.cs

@@ -5,11 +5,11 @@ namespace Protobuf.System.Text.Json;
 
 internal class FieldInfo
 {
-    public IFieldAccessor Accessor { get; init; }
+    public IFieldAccessor Accessor { get; set; } = null!;
     public InternalConverter? Converter { get; set; }
-    public bool IsRepeated { get; init; }
-    public Type FieldType { get; init; }
-    public string JsonName { get; set; }
+    public bool IsRepeated { get; set; }
+    public Type FieldType { get; set; } = null!;
+    public string JsonName { get; set; } = null!;
     public bool IsOneOf { get; set; }
     public bool IsMap { get; set; }
 }

src/Protobuf.System.Text.Json/Protobuf.System.Text.Json.csproj

@@ -3,7 +3,7 @@
     <PropertyGroup>
         <Summary>System.Text.Json extensions for Protobuf types.</Summary>
         <Description>System.Text.Json extensions for Protobuf types.</Description>
-        <TargetFramework>net6.0</TargetFramework>
+        <TargetFrameworks>net6.0;net5.0;netcoreapp3.1;netstandard2.0;netstandard2.1</TargetFrameworks>
         <LangVersion>10</LangVersion>
         <PackageProjectUrl>https://github.com/Havret/System.Text.Json.Protobuf</PackageProjectUrl>
         <PackageId>Protobuf.System.Text.Json</PackageId>
@@ -22,8 +22,15 @@
     </PropertyGroup>
 
     <ItemGroup>
-        <PackageReference Include="Google.Protobuf" Version="3.19.4" />
-        <PackageReference Include="System.Text.Json" Version="6.0.2" />
+        <PackageReference Include="Google.Protobuf" Version="3.19.0" />
+    </ItemGroup>
+
+    <ItemGroup Condition=" '$(TargetFramework)' == 'netstandard2.0' ">
+        <PackageReference Include="System.Text.Json" Version="4.6.0" />
+    </ItemGroup>
+
+    <ItemGroup Condition=" '$(TargetFramework)' == 'netstandard2.1' ">
+        <PackageReference Include="System.Text.Json" Version="4.6.0" />
     </ItemGroup>
 
 </Project>

src/Protobuf.System.Text.Json/ProtobufJsonConverterFactory.cs

@@ -15,12 +15,12 @@ public ProtobufJsonConverterFactory(JsonProtobufSerializerOptions options)
 
     public override bool CanConvert(Type typeToConvert)
     {
-        return typeToConvert.IsAssignableTo(typeof(IMessage));
+        return typeof(IMessage).IsAssignableFrom(typeToConvert);
     }
 
     public override JsonConverter CreateConverter(Type typeToConvert, JsonSerializerOptions options)
     {
         var converterType = typeof(ProtobufConverter<>).MakeGenericType(typeToConvert);
-        return (JsonConverter) Activator.CreateInstance(converterType, new object[]{options.PropertyNamingPolicy, _options})!;
+        return (JsonConverter) Activator.CreateInstance(converterType, new object[] {options.PropertyNamingPolicy, _options})!;
     }
 }