How to Customize JSON Serialization in .NET 7 with IJsonTypeInfoResolver?

How to Implement IContractResolver in System.Text.Json

Introduction

The System.Text.Json namespace is a modern alternative to Newtonsoft.Json, providing high-performance JSON serialization and deserialization in .NET. One feature that was previously unavailable in System.Text.Json was a way to customize the contract resolution process, similar to the IContractResolver interface in Newtonsoft.Json. However, this capability is now available in .NET 7 through the implementation of IJsonTypeInfoResolver.

Answer

New .NET 7 Feature: IJsonTypeInfoResolver

From .NET 7, the System.Text.Json namespace introduces the IJsonTypeInfoResolver interface, which enables contract customization. Using this interface, developers can create custom contract resolvers that modify the metadata for a specific type during serialization and deserialization.

DefaultJsonTypeInfoResolver

The System.Text.Json team has provided a default contract resolver, DefaultJsonTypeInfoResolver, which implements the IJsonTypeInfoResolver interface. This resolver offers similar functionality to the DefaultContractResolver in Newtonsoft.Json. You can subclass DefaultJsonTypeInfoResolver or add modifiers to it to customize the contract metadata according to your requirements.

Usage of DefaultJsonTypeInfoResolver with Modifiers

To adapt your SelectiveSerializer class to work with System.Text.Json, you can use the SerializeSelectedFields extension method on the DefaultJsonTypeInfoResolver:

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

This will selectively serialize only the specified properties during serialization.

Additional Notes

• For conditional serialization of properties, you can use the JsonPropertyInfo.ShouldSerialize method.
• JsonPropertyInfo.AttributeProvider returns the underlying PropertyInfo or FieldInfo when created by the reflection or source-gen resolvers.
• Serialization metadata should be constructed using locale-invariant string logic.

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