Background & Motivation

The JsonSerializerOptions.AddContext<TContext>() method was introduced in .NET 6 as a means to associate JsonSerializerOptions instances with a given JsonSerializerContext type. This method was largely superseded in .NET 7 with the introduction of contract customization and the JsonSerializerOptions.TypeInfoResolver property.

We should obsolete AddContext for the following reasons:

1. Contrary to what the name suggests, it replaces user configuration instead of appending to it.
2. It can be confused with the new JsonSerializerOptions.TypeInfoResolverChain when manipulating resolver chains.
3. The method only works with JsonSerializerContext type parameters, not with arbitrary IJsonTypeInfoResolver instances.
4. The method freezes the JsonSerializerOptions for further modification:

   JsonSerializerOptions options = new();
   options.AddContext<Context1>(); // success
   options.AddContext<Context2>(); // InvalidOperationException

API Proposal

public class JsonSerializerOptions
{
     [Obsolete(/* SYLIB code TBD? */)]
     public void AddContext<TContext>() : where TContext : JsonSerializerContext { }
}

cc @eerhardt