Polymorphic serialization for every child class in .NET 7 using System.Text.Json

Question

In .NET 7, we can use JsonDerivedType to enable polymorphic serialization for a type:

[JsonDerivedType(typeof(ChildClass))]
public class BaseClass
{
    public string FirstName { get; set; }
}

public class ChildClass : BaseClass
{
    public string LastName { get; set; }
}

Suppose I want every class that derives from BaseClass to be polymorphically serializable. Is there a succinct way of doing this instead of having to type an annotation per class? Suppose I have dozens of derived types.

Answer 1

You can configure polymorphism with the contract model.

From the documentation

For use cases where attribute annotations are impractical or impossible

In summary, you define a custom IJsonTypeInfoResolver and apply some reflection to define the polymorphism rules.

Below example shows 2 hard coded types, but you can take it a step further by discovering the derived types via reflection (as long as you can also get the matching discriminator).

public class CustomJsonTypeInfoResolver : DefaultJsonTypeInfoResolver
{
    public override JsonTypeInfo GetTypeInfo(Type type, JsonSerializerOptions options)
    {
        JsonTypeInfo jsonTypeInfo = base.GetTypeInfo(type, options);

        Type baseType = typeof(YourBaseClass);
        if (jsonTypeInfo.Type == baseType)
        {
            jsonTypeInfo.PolymorphismOptions = new JsonPolymorphismOptions
            {
                TypeDiscriminatorPropertyName = "$type" // Or any other of your choosing.
                // Other code removed for brevity.
                DerivedTypes =
                {
                    new JsonDerivedType(typeof(DerivedType1), "discriminator1"),
                    new JsonDerivedType(typeof(DerivedType2), "discriminator2")
                }
                // Or use reflection to retrieve the derived types.
            };        
        }
        
         return jsonTypeInfo;
}

You need to set the TypeInfoResolver on the JsonSerializerOptions.

Below code shows how to do so for an ASP.NET Core web application.

builder.Services
    .AddControllers()
    .AddJsonOptions(options => options.JsonSerializerOptions.TypeInfoResolver = new CustomJsonTypeInfoResolver());