Add possibility to ignore JsonElement during serializing.

[bjornen77]

I am trying to use the new System.Text.Json to Serialize and Deserialize messages based on the JSONRPC2.0 specification(https://www.jsonrpc.org/specification). In the specification there are JSON properties that may be omitted, for example the params property.

The params property value could be any JSON value, which I think maps good to the JsonElement type.

However, when serializing, and the params property is omitted, an InvalidOperationException occurs. Is this by design? Or would it be a good idea to treat the ValuKind.Undefined the same as a null value with IgnoreNullValues=true? Perhaps a new setting IgnoreUndefinedValues=true?

I created a simple example program to reproduce this issue:

class Program
{
    public static void Main(string[] args)
    {
        var request = new Request();
        var json = JsonSerializer.Serialize(request, new JsonSerializerOptions() {IgnoreNullValues = true});
    }

}
class Request
{
    public string Method { get; set; } = "Test";
    public JsonElement Params { get; set; }
}

Using JsonElement? or object instead works, but I think that using JsonElement will be best if possible. Because, If using object instead, I must unbox it to a JsonElement after deserializing.

[bjornen77]

To summarize: The suggestion is to treat a JsonElement property with no value(JsonValueKind.Undefined) the same as a property with a null value, when JsonSerializerOption IgnoreNullValues=true (or adding a new JsonSerializerOption, for example IgnoreUndefinedValues=true).

[layomia]

A JsonElement is a value type and can't be null even when it has no value, so I don't think the serializer should treat it as such. cc @ahsonkhan

Since you want to ignore Params on serialization when default, an option is to change the type to JsonElement? and use options.IgnoreNullValues.

As you mention in #779 (comment), conditional ignore semantics, global and/or per-property, "ignore when default", would also help here.

[ahsonkhan]

I agree. Is there a reason why you can't use JsonElement? here?

This specific issue isn't actionable and we have existing issues to enable more flexible ignore semantics, so I consider this issue as a duplicate of the others that @layomia linked to.

[bjornen77]

@ahsonkhan There is no reason why I can´t use JsonElement? at the moment and this is what I ended up doing. If there will be other possibilities in the future I could change this(if it will be better in some way).

I would like to thank you all for the great work you do with dotnet, I also think the new S.T.J is a good addition to the platform and it works great!

[ahsonkhan]

There is no reason why I can´t use JsonElement? at the moment and this is what I ended up doing. If there will be other possibilities in the future I could change this(if it will be better in some way).

Great. That sounds good.

I would like to thank you all for the great work you do with dotnet, I also think the new S.T.J is a good addition to the platform and it works great!

Appreciate it. We have a few up-for-grabs issues if you are interested in contributing :)
https://github.com/dotnet/corefx/issues?q=is%3Aopen+is%3Aissue+label%3Aup-for-grabs+label%3Aarea-System.Text.Json