How Can I Customize Serialization Contracts in System.Text.Json?

Customizing Serialization Contracts in System.Text.Json

The new System.Text.Json API provides the ability to customize serialization contracts, offering functionality similar to Newtonsoft's IContractResolver.

Contract Customization in .NET 7

In .NET 7, contract customization is available through the IJsonTypeInfoResolver interface. This interface allows developers to create custom resolvers that return configured JsonTypeInfo instances for specified types and JsonSerializerOptions combinations.

Creating a Custom Resolver

One way to create a custom resolver is to subclass the DefaultJsonTypeInfoResolver and override the GetTypeInfo(Type, JsonSerializerOptions) method. Alternatively, you can add an Action to the DefaultJsonTypeInfoResolver.Modifiers collection to modify the default JsonTypeInfo after creation.

Example: Selective Field Serialization

To replicate the functionality of the SelectiveSerializer class in System.Text.Json, you can use a modifier action similar to the following:

resolver.Modifiers.Add(typeInfo =>
{
    if (typeInfo.Kind == JsonTypeInfoKind.Object)
    {
        foreach (var property in typeInfo.Properties)
        {
            if (property.GetMemberName() is {} name && !membersToSerializeSet.Contains(name))
                property.ShouldSerialize = static (obj, value) => false;
        }
    }
});

This modifier checks for properties that match specified field names and sets their ShouldSerialize property to false to exclude them from serialization.

Setting the Resolver

Once a custom resolver is created, it can be set via the JsonSerializerOptions.TypeInfoResolver property. For example:

var options = new JsonSerializerOptions
{
    TypeInfoResolver = new DefaultJsonTypeInfoResolver()
        .SerializeSelectedFields("FirstName,Email,Id"),
    // Other options as required
};

Additional Notes

• PropertyNamePolicy and WriteIndented can be set to customize property naming and indentation.
• JsonPropertyInfo.ShouldSerialize can be used for conditional serialization of properties.
• System.Text.Json is case-sensitive by default, so it's important to use case-sensitive comparison when filtering selected fields.

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