Support ability to DeepClone a JsonNode in order to recursively create a copy of all children nodes

[husains]

Background and Motivation

Writable JSON DOM API's are available as part of the .NET 6 private preview. However, the current API's do not support an ability to perform a deep clone (i.e. create a recursive copy of all children nodes).

Including support for a DeepClone API enables the ability to take an input JsonNode, apply transforms or filters, and copy over subsets of the source object into a new JsonNode (which can then be used for further manipulation or for serialization).

JSON.NET has an analogous API with the same name (DeepClone). An example using existing JSON.NET API's is here.

A workaround, in the absence of a DeepClone API is to serialize the JsonNode into a string, and re-convert the same into a new JsonNode. However, a built-in DeepClone API can likely be made to perform better, and avoid the interim conversion into a string (and associated allocations).

Proposed API

namespace System.Text.Json.Nodes
{
      public class JsonNode {
+        public JsonNode DeepClone();
      }
}

Usage Examples

Below is an example of how the new DeepClone API can be used

// some lines of code here

        public string FilterAndSerialize(JsonObject data)
        {
            JsonObject newData = new JsonObject();
            newData["Product"] = new JsonObject();
            newData["Product"]["Eligibility"] = new JsonObject();

            var predicates = new JsonArray();
            newData["Product"]["Eligibility"]["Predicates"] = predicates;

            foreach (var item in data["Product"]["Eligibility"]["Predicates"].AsArray())
            {
                if (item["Type"].GetValue<string>().Equals("ClaimsPredicate"))
                {
                    predicates.Add(item.DeepClone());
                }
            }

            return Serialize(newData);
        }

Alternative Designs

Use combination of ToJsonString & Parse APIs. However, performance would likely suffer due to the intermediate string conversion.

[ghost]

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

---

Background and Motivation

Writable JSON DOM API's are available as part of the .NET 6 private preview. However, the current API's do not support an ability to perform a deep clone (i.e. create a recursive copy of all children nodes).

Including support for a DeepClone API enables the ability to take an input JsonNode, apply transforms or filters, and copy over subsets of the source object into a new JsonNode (which can then be used for further manipulation or for serialization).

JSON.NET has an analogous API with the same name (DeepClone). An example using existing JSON.NET API's is here.

A workaround, in the absence of a DeepClone API is to serialize the JsonNode into a string, and re-convert the same into a new JsonNode. However, a built-in DeepClone API can likely be made to perform better, and avoid the interim conversion into a string (and associated allocations).

Proposed API

namespace System.Text.Json.Nodes
{
      public class JsonNode {
+        public JsonNode DeepClone();
      }
}

Usage Examples

Below is an example of how the new DeepClone API can be used

// some lines of code here

        public string FilterAndSerialize(JsonObject data)
        {
            JsonObject newData = new JsonObject();
            newData["Product"] = new JsonObject();
            newData["Product"]["Eligibility"] = new JsonObject();

            var predicates = new JsonArray();
            newData["Product"]["Eligibility"]["Predicates"] = predicates;

            foreach (var item in data["Product"]["Eligibility"]["Predicates"].AsArray())
            {
                if (item["Type"].GetValue<string>().Equals("ClaimsPredicate"))
                {
                    predicates.Add(item.DeepClone());
                }
            }

            return Serialize(newData);
        }

Alternative Designs

Use combination of ToJsonString & Parse APIs. However, performance would likely suffer due to the intermediate string conversion.

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

[GrabYourPitchforks]

What would happen in the following scenario?

public class Person
{
    public string Name { get; set; }
    public int Age { get; set; }
}

Person p = new Person() { Name = "Andy", Age = 50 };
JsonNode original = JsonValue.Create(p);
Person p2 = original.GetValue<Person>();
JsonNode cloned = original.DeepClone();
Person p3 = cloned.GetValue<Person>();

[husains]

What would happen in the following scenario?

public class Person
{
    public string Name { get; set; }
    public int Age { get; set; }
}

Person p = new Person() { Name = "Andy", Age = 50 };
JsonNode original = JsonValue.Create(p);
Person p2 = original.GetValue<Person>();
JsonNode cloned = original.DeepClone();
Person p3 = cloned.GetValue<Person>();

If we need to convert a .NET object into one of the Json DOM API objects, then I would have expected to use JsonObject.Create(p), or JsonObject.FromObject(p), or new JsonObject(p). Unclear about why JsonValue.Create is being used here. Could you please clarify the same? Not sure of the intention of the current JsonValue.Create API.

I was expecting JsonValue to only be used for primitive Json data types like string, number, and possibly support some conversions into other primitives like dates/timestamps.

[GrabYourPitchforks]

My comment was intended to spark discussion of "what happens if the DOM contains an object which is not cloneable?" JsonNode can contain any arbitrary data, not just primitives. The behavior of any DeepClone method would need to be well-defined when it sees such nodes. Does it fail, make a shallow clone, try calling ICloneable.Clone, serialize + deserialize, or something else? And if a call to GetValue<Person> succeeds before the call to DeepClone, is it expected to succeed on the resulting cloned graph?

I'm not really advocating any particular course of action. If "it throws an exception" is the desired behavior - great! :)

The main point was that I think this proposal needs to define the expected behavior even in the face of all of these edge cases.

[eiriktsarpalis]

cc @steveharter

[steveharter]

As shown above, JsonValue can contain a direct reference to an arbitrary CLR type. This is to avoid an extra serialization and deserialization step.

So if a Person instance already exists, and we required creation of a JsonObject then the steps to serialize would be:

1. Serialize Person to JSON
2. Deserialize that JSON to JsonObject
3. Serialize JsonObject to JSON

instead of just:

1. Serialize Person to JSON

So I think the most obvious DeepClone semantics would be to:

• Serialize node to JSON
• Deserialize that JSON to JsonNode which will lose any arbitrary CLR instances which will thus be represented by a JsonObject (in the Person example), JsonArray or JsonValue

[eiriktsarpalis]

Closing as duplicate of #56592.