SystemTextJson.FluentApi 1.0.0-preview.1.0.1

SystemTextJson.FluentApi

SystemTextJson.FluentApi is a fluent configuration library for System.Text.Json that allows developers to configure serialization uses strongly typed fluent interface and lambda expression.

Documentation

All api usually repeats attributes from System.Text.Json.Serialization and set corresponding property in JsonPropertyInfo or JsonTypeInfo. Configuration based on IJsonTypeInfoResolver so developers can configure reflection based TypeInfoResolver and source generator JsonSerializerContext.

Quick start

To use FluentApi need to configure JsonSerializerOptions instance via JsonModelBuilder and pass it to serializer.

var options = new JsonSerializerOptions() { WriteIndented = true };
options.ConfigureDefaultTypeResolver(p =>
p.Entity<Person>()
.Property(p => p.LastName).HasName("surname")
.Property(p => p.FirstName).IsIgnored()
.Property(p => p.Age).HasHumberHandling(JsonNumberHandling.WriteAsString));

var person = new Person() { FirstName = "First name", LastName = "Last name", Age = 12 };
var json = JsonSerializer.Serialize(person, options);

Console.WriteLine(json);

This example produce this JSON

{
  "surname": "Last name",
  "Age": "12"
}

Polymorphism serialization

STJ has build in support polymorphic serialization, but user have to annotate base class with JsonDerivedTypeAttribute with all derived types. In fluent API you can configure each derived type manually or find all derived types in runtime.

builder.Entity<Root>()
.HasDerivedType<Derived1>(nameof(Derived1))
.HasDerivedType<Derived2>(nameof(Derived2))
.HasDerivedType<Root>(nameof(Root))
// or
builder.Entity<Root>().HasDerivedTypesFromAssembly(Assembly.GetExecutingAssembly(), t => t.Name)

var testObject = new Root[] 
{
    new Derived1() { Derived1Property = "derived" },
    new Derived2() { Derived2Property = "derived2" },
    new Root(){ RootProperty = "root"}
};


public class Root
{
    public string? RootProperty { get; set; }
}

public class Derived1 : Root
{
    public string? Derived1Property { get; set; }
}

public class Derived2 : Root
{
    public string? Derived2Property { get; set; }
}

Serialization of testObject collection will produce:

[
   {
      "$type":"Derived1",
      "Derived1Property":"derived",
      "RootProperty":null
   },
   {
      "$type":"Derived2",
      "Derived2Property":"derived2",
      "RootProperty":null
   },
   {
      "$type":"Root",
      "RootProperty":"root"
   }
]

With $type discriminator serializer are able to deserialize this collection. Another approach to serialization is use actual type from object instance, instead of property type. To achieve this behavior serializer can threat specific type as object. Fluent Api has two way for this - configure behavior for specific property with PropertyBuilder.SerializeAsObject or for specific type with JsonSerializerOptionsExtensions.SerializeAsObject.

builder.Entity<AsObjectTestClass>().Property(p => p.Data).SerializeAsObject();
// or 
options.SerializeAsObject<Root>();

var testObject = new AsObjectTestClass { Data = new Derived() { Property = "Prop" } };

public class AsObjectTestClass
{
    public Root? Data { get; set; }
}

public class Root { }

public class Derived : Root
{
    public string? Property { get; set; }
}

Serialization of testObject will produce:

{
   "Data":{
      "Property":"Prop"
   }
}

But in this case only serialization is available because JSON does not contain type discriminator and JsonException will be thrown on deserialization.

Nullable reference type support

STJ has build in support of required properties, but it just check, that value exists in JSON on deserialization and does not prevent setting null to none nullable properties. Fluent Api can configure JsonSerializerOptions to respect NRT annotations on fields and properties. Internally it uses JsonPropertyInfo.Set property and reduces deserialization performance.

builder.RespectNullableReferenceType();

JsonSerializer.Deserialize<TestClass>("""{"Property": null}""", _options); // this throws JsonException
JsonAsserts.AssertObject(new TestClass(), "{}", _options); // BUT this is not because Property is not requred.

public class TestClass
{
    public string Property { get; set; }
}