System.Text.Json: Providing default value for missing properties

[florianbader]

I'm trying to implement handling of changes in my JSON schema. This includes for now adding a new property to the .NET type that is missing in an existing JSON by providing a default value.

While this would work fine with mutable types it's difficult to do with immutable records. Maybe I'm missing something but currently the only way I found is through reflection:

using System.Text.Json;
using System.Text.Json.Serialization;
using System.Text.Json.Serialization.Metadata;

JsonSerializerOptions jsonSerializerOptions = new()
{
    PropertyNameCaseInsensitive = true,
    PropertyNamingPolicy = JsonNamingPolicy.CamelCase,
    DefaultIgnoreCondition = JsonIgnoreCondition.WhenWritingNull,
    WriteIndented = true,
    DictionaryKeyPolicy = JsonNamingPolicy.CamelCase,
    TypeInfoResolver = new DefaultJsonTypeInfoResolver
    {
        Modifiers = { Modify }
    }
};

static void Modify(JsonTypeInfo info)
{
    if (info.Kind != JsonTypeInfoKind.Object)
    {
        return;
    }

    if (info.Type != typeof(Document))
    {
        return;
    }

    info.OnDeserialized = (obj) =>
    {
        Document document = (Document)obj;
        if (document.Options is null)
        {
            typeof(Document).GetProperty("Options", System.Reflection.BindingFlags.Public | System.Reflection.BindingFlags.Instance)
                .SetValue(document, new Options());
        }
    };
}

string json = """{"name": "test"}""";

Console.WriteLine("Deserializing...");
Document? obj = JsonSerializer.Deserialize<Document>(json, jsonSerializerOptions);
Console.WriteLine("Options is " + (obj?.Options is null ? "null" : "not null"));

Console.WriteLine();
Console.WriteLine("Serializing...");
string result = JsonSerializer.Serialize(obj, jsonSerializerOptions);
Console.WriteLine(result);

public record Document(string Name, Options Options);
public record Options(bool IsEnabled = true);

A JsonConverter doesn't work as the Read method is not called for properties that are missing in the JSON. There is also no way to use a TypeInfoResolver on my Document object for the Options property as the Set delegate is also not called for missing properties in JSON.

Would be nice to have some way to actually provide a default value for missing properties through a JSON contract though something like

static void Modify(JsonTypeInfo info)
{
    if (info.Kind != JsonTypeInfoKind.Object)
    {
        return;
    }

    if (info.Type != typeof(Document))
    {
        return;
    }

    var optionsProperty = info.Properties.Single(property => property.Name == "options");
    optionsProperty.DefaultValue = new Options();
}

Any thoughts?

[huoyaoyuan]

Is the type owned by you? Can you set default values in its constructor?

[florianbader]

It's owned by me, so I could do something like this:

using System.Text.Json;
using System.Text.Json.Serialization;

JsonSerializerOptions jsonSerializerOptions = new()
{
    PropertyNameCaseInsensitive = true,
    PropertyNamingPolicy = JsonNamingPolicy.CamelCase,
    DefaultIgnoreCondition = JsonIgnoreCondition.WhenWritingNull,
    WriteIndented = true,
    DictionaryKeyPolicy = JsonNamingPolicy.CamelCase
};

string json = """{"name": "test"}""";

Console.WriteLine("Deserializing...");
Document? obj = JsonSerializer.Deserialize<Document>(json, jsonSerializerOptions);
Console.WriteLine("Options is " + (obj?.Options is null ? "null" : "not null"));

Console.WriteLine();
Console.WriteLine("Serializing...");
string result = JsonSerializer.Serialize(obj, jsonSerializerOptions);
Console.WriteLine(result);

public record Document(string Name, Options? Options)
{
    public Options Options { get; init; } = Options ?? new Options();
}
public record Options(bool IsEnabled = true);

I just dislike that I have to blow up my record type and define the property and also if I make Options a required property it will fail even though I have a default value for it.

[BrianVallelunga]

We're a couple of years past and there's still no good way to provide default values for missing JSON fields that I'm aware of. This is especially true with immutable F# records. Does .NET 10 or 11 have any new functionality that might help?