How Can I Preserve Default System.Text.Json Serialization Behavior When Implementing a Custom JsonConverter?

Customize System.Text.Json serialization while keeping default behavior

Question:

When implementing a custom System.Text.Json.JsonConverter for data model upgrades, how can I maintain the default serialization behavior in the Write() method without affecting other serialization options?

Answer:

To preserve the default serialization behavior in the Write() method of a custom System.Text.Json.JsonConverter, you can use the following strategy:

Option 1: Use [JsonConverter] on the attribute

• Apply the [JsonConverter] attribute to a specific property.
• Calling JsonSerializer.Serialize() without providing any custom options will generate default serialization.

Option 2: Modify the converter collection

• Inside the Write() method, create a copy of the passed options and remove the custom converter from the Converters collection.
• Pass modified options into nested JsonSerializer calls.

Option 3: Implement DefaultConverterFactory

• Define a custom converter factory (e.g., DefaultConverterFactory) that wraps a default converter.
• The factory's CanConvert() method determines whether the converter is applicable.
• The factory's CreateConverter() method creates a converter using modified options (excluding custom converters).

Limitations:

• Option 3 does not work with custom value types or POCOs that have a [JsonConverter] attribute on their type itself.
• Recursive types may cause issues as modified options are not applied consistently.

Example:

<code class="language-csharp">public sealed class PersonConverter : DefaultConverterFactory<Person>
{
    public override Person Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions modifiedOptions)
    {
        // 自定义读取实现
    }
}

public abstract class DefaultConverterFactory<T> : JsonConverterFactory
{
    public override bool CanConvert(Type typeToConvert) => typeof(T) == typeToConvert;

    public override JsonConverter CreateConverter(Type typeToConvert, JsonSerializerOptions options)
    {
        return new DefaultConverter(options, this);
    }
}

public sealed class DefaultConverter : JsonConverter<Person>
{
    public override void Write(Utf8JsonWriter writer, Person value, JsonSerializerOptions modifiedOptions)
    {
        // 调用默认的 Write 实现
        JsonSerializer.Serialize(writer, value, modifiedOptions);
    }
}</code>

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