System.Text.Json - Support property flattening - [JsonPropertyFlatten]

[bugproof]

I propose [JsonPropertyFlatten] attribute that can be used to flatten property. Similar to #[serde(flatten)] in Rust's Serde.

https://stackoverflow.com/questions/65323277/flatten-an-object-with-system-text-json

Imagine I have a class like this:

public class ResponseBase
{
    [JsonPropertyName("status")]
    [JsonConverter(typeof(StatusConverter))]
    public bool IsSuccessStatus { get; set; }
    
    [JsonPropertyName("error_message")] 
    public string? ErrorMessage { get; set; }
    
    [JsonPropertyName("error_code")] 
    public string? ErrorCode { get; set; }
}

If flattening was supported I could refactor it to something like this:

public class Error
{
    [JsonPropertyName("error_code")] 
    public string Code { get; set; }

	[JsonPropertyName("error_message")] 
    public string Message { get; set; }
}

public class ResponseBase
{
    [JsonPropertyName("status")]
    [JsonConverter(typeof(StatusConverter))]
    public bool IsSuccessStatus { get; set; }
    
    [JsonPropertyFlatten] 
    public Error? Error { get; set; }
}

But it would still serialize and deserialize like Error properties were part of the ResponseBase class instead of "error": {...} because I'm writing a client for third party api...

Possibly blocking? #31257

[ghost]

Tagging subscribers to this area: @eiriktsarpalis, @layomia
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Issue Details

---

I propose [JsonPropertyFlatten] attribute that can be used to flatten property. Similar to #[serde(flatten)] in Rust's Serde.

https://stackoverflow.com/questions/65323277/flatten-an-object-with-system-text-json

Imagine I have a class like this:

public class ResponseBase
{
    [JsonPropertyName("status")]
    [JsonConverter(typeof(StatusConverter))]
    public bool IsSuccessStatus { get; set; }
    
    [JsonPropertyName("error_message")] 
    public string? ErrorMessage { get; set; }
    
    [JsonPropertyName("error_code")] 
    public string? ErrorCode { get; set; }
}

If flattening was supported I could refactor it to something like this:

public class Error
{
    [JsonPropertyName("error_code")] 
    public string Code { get; set; }

	[JsonPropertyName("error_message")] 
    public string Message { get; set; }
}

public class ResponseBase
{
    [JsonPropertyName("status")]
    [JsonConverter(typeof(StatusConverter))]
    public bool IsSuccessStatus { get; set; }
    
    [JsonPropertyFlatten] 
    public Error Error { get; set; }
}

But it would still serialize and deserialize like Error properties were part of the ResponseBase class.

Author:	bugproof
Assignees:	-
Labels:	api-suggestion, area-System.Text.Json, untriaged
Milestone:	-

[Fydar]

This could be implemented using source generators.

A source generator would have to:

• Add a [JsonIgnore] attribute to the member with the [JsonPropertyFlatten] attribute.
• Add private properties that get and set the member properties on the object.

Using a source generator, the following code:

public class ResponseBase
{
    [JsonPropertyName("status")]
    [JsonConverter(typeof(StatusConverter))]
    public bool IsSuccessStatus { get; set; }
    
    [JsonPropertyFlatten] 
    public Error? Error { get; set; }
}

Could be converted to:

public class ResponseBase
{
    [JsonPropertyName("status")]
    [JsonConverter(typeof(StatusConverter))]
    public bool IsSuccessStatus { get; set; }
    
    [JsonPropertyFlatten] 
    [JsonIgnore]
    public Error? Error { get; set; }
    
    [JsonPropertyName("error_code")] 
    private string Code
    {
        get => Error.ErrorCode;
        set => Error.ErrorCode = value;
    }
    
    [JsonPropertyName("error_message")] 
    private string Message 
    {
        get => Error.Message;
        set => Error.Message = value;
    }
}

[eiriktsarpalis]

How would such a mechanism deal with potentially conflicting property names (same key occurring in both parent and child objects)? How does serde deal with flattened types whose serialization is a number or array?

It's unlikely we would consider such a feature for 7.0.0. My recommendation is to use a custom converter (should be easier to achieve using the new JsonNode to construct an intermediate representation). We can revisit for future releases if sufficient demand for this feature arises.

[ghost]

This issue has been marked with the api-needs-work label. This may suggest that the proposal requires further refinement before it can be considered for API review. Please refer to our API review guidelines for a detailed description of the process.

When ready to submit an amended proposal, please ensure that the original post in this issue has been updated, following the API proposal template and examples as provided in the guidelines.

[bugproof]

How would such a mechanism deal with potentially conflicting property names (same key occurring in both parent and child objects)?

The same way as it deals with this. It would just treat all the properties of the flattened property as they were part of the same object. I think it should just throw an exception.

[JsonPropertyName("name")]
public string Name { get; set; }

[JsonPropertyName("name")]
public string Name2 { get; set; }

How does serde deal with flattened types whose serialization is a number or array?

Not sure about that. But it should just support class/struct as I see no reason to flatten number or array. If you try to flatten number or array it should ignore it or throw an exception. I see no reason why would anyone do this.

[DjArt]

Not sure about that. But it should just support class/struct as I see no reason to flatten number or array. If you try to flatten number or array it should ignore it or throw an exception. I see no reason why would anyone do this.

ofc we should able to flat array or any class with indexer. Just introduce new attribute (or constructor overload for JsonPropertyName), where one of parameter will be a text representation of index value, second for original property name and third for JSON name.

And we get something like that:

public class ResponseBase
{
    [JsonPropertyName("status")]
    [JsonConverter(typeof(StatusConverter))]
    public bool IsSuccessStatus { get; set; }
    
    [JsonPropertyFlatten]
    [JsonPropertyName(nameof(Error.Status), "status")]
    [JsonPropertyName(nameof(Error.Message), "message")]
    [JsonPropertyName(nameof(Error.Lines), "0", "first_line")]
    public Error? Error { get; set; }

    [JsonPropertyFlatten]
    [JsonPropertyName("", "0", "C0")]
    [JsonPropertyName("", "1", "C1")]
    [JsonPropertyName("", "2", "C2")]
    public int[] Codes { get; set; }
}