<PackageReference Include="System.Reactive" Version="6.0.1-preview.1" />

System.Diagnostics.CodeAnalysis.MaybeNullAttribute

sealed class MaybeNullAttribute : Attribute
Specifies that an output may be null even if the corresponding type disallows it.
namespace System.Diagnostics.CodeAnalysis { internal sealed class MaybeNullAttribute : Attribute { public MaybeNullAttribute(); } }