Can System.Text.Json's `IJsonTypeInfoResolver` Achieve the Functionality of `IContractResolver`?

Can System.Text.Json Mimic IContractResolver?

In the System.Text.Json namespace, contract customization is eagerly anticipated in .NET 7 and is currently available in Preview 6. Contract metadata for a specific type, represented by JsonTypeInfo, can now be customized by developers.

public interface IJsonTypeInfoResolver
{
    JsonTypeInfo? GetTypeInfo(Type type, JsonSerializerOptions options);
}

Creating a Custom IJsonTypeInfoResolver

Here are a few methods for creating your custom IJsonTypeInfoResolver:

1. Subclass DefaultJsonTypeInfoResolver:

   public class CustomJsonTypeInfoResolver : DefaultJsonTypeInfoResolver
   {
       public override JsonTypeInfo? GetTypeInfo(Type type, JsonSerializerOptions options)
       {
           // Implement your custom logic here
           return base.GetTypeInfo(type, options);
       }
   }

2. Add Action Modifiers:

   var resolver = new DefaultJsonTypeInfoResolver();
   resolver.Modifiers.Add(typeInfo =>
   {
       // Modify the default JsonTypeInfo here
   });

3. Create a New IJsonTypeInfoResolver:

   public class CustomJsonTypeInfoResolver : IJsonTypeInfoResolver
   {
       public JsonTypeInfo? GetTypeInfo(Type type, JsonSerializerOptions options)
       {
           // Implement your custom logic here
           return new JsonTypeInfo(type, JsonTypeInfoKind.Object);
       }
   }

Using Your Custom Resolver

Once you have a custom resolver, set it via JsonSerializerOptions.TypeInfoResolver.

Example

The following example shows how to create a DefaultJsonTypeInfoResolver that serializes only selected fields:

using System.Text.Json.Serialization;

public static class JsonSerializerExtensions
{
    public static DefaultJsonTypeInfoResolver SerializeSelectedFields(this DefaultJsonTypeInfoResolver resolver, IEnumerable<string> membersToSerialize)
    {
        resolver.Modifiers.Add(typeInfo =>
        {
            if (typeInfo.Kind == JsonTypeInfoKind.Object)
            {
                foreach (var property in typeInfo.Properties)
                {
                    if (!membersToSerialize.Contains(property.GetMemberName()))
                        property.ShouldSerialize = static (obj, value) => false;
                }
            }
        });

        return resolver;
    }
}

// Usage
var options = new JsonSerializerOptions
{
    TypeInfoResolver = new DefaultJsonTypeInfoResolver()
        .SerializeSelectedFields("FirstName", "Email", "Id"),
    PropertyNamingPolicy = JsonNamingPolicy.CamelCase,
    WriteIndented = true,
};

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