[API Proposal]: Expose a JsonSerializerOptions.Default property

[eiriktsarpalis]

Background and motivation

JsonSerializerOptions currently exposes an internal s_defaultOptions instance for use by System.Text.Json. This is useful since JsonSerializerOptions holds the reflection-based serialization metadata cache, as such creating a default new JsonSerializerOptions() instance can be very expensive since it forces the regeneration of that cache.

I propose we make this singleton a public property so that users can read default configuration cheaply (e.g. the default JsonConverter for a requested type).

API Proposal

namespace System.Text.Json
{
    public partial class JsonSerializerOptions
    {
        public static JsonSerializerOptions Default { get; }
    }
}

API Usage

Here's an (artificial) example of how the property can be used to cheaply recover the default converter used by STJ for integers:

public class MyCustomConverter : JsonConverter<int>
{
    private readonly static JsonConverter<int> s_defaultConverter = (JsonConverter<int>)JsonSerializerOptions.Default.GetConverter(typeof(int));

    // custom serialization logic
    public override void Write(Utf8JsonWriter writer, int value, JsonSerializerOptions options)
    {
        return writer.WriteStringValue(value.ToString());
    }

    // fall back to default deserialization logic
    public override int Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
    {
        return s_defaultConverter.Read(ref reader, typeToConvert, options);
    }
}

Alternative Designs

No response

Risks

No response

[ghost]

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Issue Details

---

Background and motivation

JsonSerializerOptions currently exposes an internal s_defaultOptions instance for use by System.Text.Json. This is useful since JsonSerializerOptions holds the reflection-based serialization metadata cache, as such creating a default new JsonSerializerOptions() instance can be very expensive since it forces the regeneration of that cache.

I propose we make this singleton a public property so that users can read default configuration cheaply (e.g. the default JsonConverter for a requested type).

API Proposal

namespace System.Text.Json
{
    public partial class JsonSerializerOptions
    {
        public static JsonSerializerOptions Default { get; }
    }
}

API Usage

Here's an (artificial) example of how the property can be used to cheaply recover the default converter used by STJ for integers:

public class MyCustomConverter : JsonConverter<int>
{
    private readonly static JsonConverter<int> s_defaultConverter = (JsonConverter<int>)JsonSerializerOptions.Default.GetConverter(typeof(int));

    // custom serialization logic
    public override void Write(Utf8JsonWriter writer, int value, JsonSerializerOptions options)
    {
        return writer.WriteStringValue(value.ToString());
    }

    // fall back to default deserialization logic
    public override int Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
    {
        return s_defaultConverter.Read(ref reader, typeToConvert, options);
    }
}

Alternative Designs

No response

Risks

No response

Author:	eiriktsarpalis
Assignees:	eiriktsarpalis
Labels:	api-suggestion, area-System.Text.Json
Milestone:	7.0.0

[TonyValenti]

Would the defaults be mutable? My recommendation is "No".

[eiriktsarpalis]

It would not be. Moreover the implementation of this property would need to make sure that the instance is locked for modification before being returned.

[terrajobst]

Video

• We need to make sure the instance is marked as "you can't touch this anymore"

namespace System.Text.Json
{
    public partial class JsonSerializerOptions
    {
        public static JsonSerializerOptions Default { get; }
    }
}