System.Text.Json improvements in .NET 7

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Microsoft has added high impact reliability, library extensibility, consistency, and performance-orientated features to improve the System.Text.Json library. Some of the improvements to the .NET library include:

  • Contract customization
  • Type hierarchies
  • Required members
  • JsonSerializerOptions.Default

System.Text.Json Updates

Let’s tackle each of the updates to System.Text.Json one-by-one.

Read: Best Online Courses to Learn C#

Contract Customization in .NET

Contract customization gives developers more control on type serialization and deserialization, user-defined type hierarchies, and support for required members. The System.Text.Json library figures out how a .NET type should be serialized and deserialized by establishing a JSON contract which is derived from the .NET type’s shape. This shape includes the properties and fields, the constructor, and checks the implementation of IEnumerable or IDictionary.

The metadata has always been JsonTypeInfo , but now, in .NET 7, all of its metadata has been exposed to be modifiable, thus allowing programmers to write their own resolution logic for the JSON contracts.

Here is a small code example showing how to define a custom contract resolver in .NET:

var JSONserOptions = new JsonSerializerOptions
    TypeInfoResolver = new propertyContractResolver()

JsonSerializer.Serialize(new { value = 44 }, JSONserOptions); 

public class propertyContractResolver : DefaultJsonTypeInfoResolver
    public override JsonTypeInfo  GetTypeInfo(Type t, JsonSerializerOptions o)
        JsonTypeInfo type = base.GetTypeInfo(t, o);

        if (type.Kind == JsonTypeInfoKind.Object)
            foreach (JsonPropertyInfo prop in type.Properties)
                prop.Name = prop.Name.ToUpperInvariant();

        return type;

Type Hierarchies in .NET

Polymorphic serialization and deserialization of user-defined type hierarchies is now supported by System.Text.Json. This can be done with the JsonDerivedTypeAttribute library. A code example follows demonstrating polymorphic serialization and deserialization in .NET:

public class Temp
    public int i { get; set; }

public class DerivedClass : Temp
    public int j { get; set; }

The above code example enables polymorphic serialization for the Base class:

Base val = new DerivedClass();

The run-time type above is DerivedClass.

Required Members in .NET

With C# 11, support has been added for required members. This lets class authors specify properties and fields and the reflection serialization that must be populated when being instantiated. A quick C# code example follows:

using System.Text.Json;
JsonSerializer.Deserialize("""{"StudentCourse": 7}"""); 

public class Student
    public required string StudentName { get; set; }
    public int NumberOfSubjects { get; set; }

With this, a JSON exception will be thrown because the required parameter (StudentName) was not supplied.


A read-only instance of JsonSerializerOptions can now be passed and accessed by developers through the use of the JsonSerializerOptions.Default static property. Here is some example code demonstrating this:

public class Convert : JsonConverter
    private readonly static JsonConverter s_default = 

    public override void Write(Utf8JsonWriter w, int val, JsonSerializerOptions o)
        return w.WriteStringValue(val.ToString());

    public override int Read(ref Utf8JsonReader r, Type type, JsonSerializerOptions o)
        return s_default.Read(ref r, t, o);

Read more .NET programming tutorials and software development tips.

Hannes DuPreez
Hannes DuPreez
Ockert J. du Preez is a passionate coder and always willing to learn. He has written hundreds of developer articles over the years detailing his programming quests and adventures. He has written the following books: Visual Studio 2019 In-Depth (BpB Publications) JavaScript for Gurus (BpB Publications) He was the Technical Editor for Professional C++, 5th Edition (Wiley) He was a Microsoft Most Valuable Professional for .NET (2008–2017).

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