.NET 8 – Mark JsonSerializerOptions as readonly

Starting with .NET 8, you can mark JsonSerializerOptions instances as read-only, preventing further changes to the instance. To freeze the instance, simply call MakeReadOnly on the options instance.

Let’s take an example:

var options = new JsonSerializerOptions
{
    AllowTrailingCommas = true,
    PropertyNamingPolicy = JsonNamingPolicy.SnakeCaseUpper,
    PreferredObjectCreationHandling = JsonObjectCreationHandling.Populate,
};

options.MakeReadOnly();

Furthermore, you can check if an instance was frozen or not by checking the IsReadOnly property.

options.IsReadOnly

Attempting to modify a JsonSerializerOptions instance after it was marked as read only will result in an InvalidOperationException:

Unhandled exception. System.InvalidOperationException: This JsonSerializerOptions instance is read-only or has already been used in serialization or deserialization.
   at System.Text.Json.ThrowHelper.ThrowInvalidOperationException_SerializerOptionsReadOnly(JsonSerializerContext context)
   at System.Text.Json.JsonSerializerOptions.VerifyMutable()

MakeReadOnly(bool populateMissingResolver) overload

When populateMissingResolver is passed as true, the method will go ahead and add the default reflection resolver to your JsonSerializerOptions when missing. Careful when using this method in trimmed / Native AOT applications as it will root the reflection-related assemblies and include them in your build.