c# - 将对象转换为 System.Text.Json.JsonElement

Question

假设我有一个类型的对象：

public class MyClass
{
    public string Data { get; set; }
}

我需要将其转换为 System.Text.Json.JsonElement。我找到的唯一方法是：

var json = JsonSerializer.Serialize(new MyClass { Data = "value" });

using var document = JsonDocument.Parse(json);

var jsonElement = document.RootElement;

我必须先序列化它然后解析它，这似乎很奇怪。有更好的方法吗？

以前我是JObject从 Newtonsoft.Json 使用的，我可以这样做：

var jobject = JObject.FromObject(new MyClass { Data = "value" });

标签： c#jsonsystem.text.json

Answer 1

In .NET 6 methods are being added to JsonSerializer to serialize an object directly to a JsonElement or JsonDocument:

public static partial class JsonSerializer
{
    public static JsonDocument SerializeToDocument<TValue>(TValue value, JsonSerializerOptions? options = null);
    public static JsonDocument SerializeToDocument(object? value, Type inputType, JsonSerializerOptions? options = null);
    public static JsonDocument SerializeToDocument<TValue>(TValue value, JsonTypeInfo<TValue> jsonTypeInfo);
    public static JsonDocument SerializeToDocument(object? value, Type inputType, JsonSerializerContext context);

    public static JsonElement SerializeToElement<TValue>(TValue value, JsonSerializerOptions? options = null);
    public static JsonElement SerializeToElement(object? value, Type inputType, JsonSerializerOptions? options = null);
    public static JsonElement SerializeToElement<TValue>(TValue value, JsonTypeInfo<TValue> jsonTypeInfo);
    public static JsonElement SerializeToElement(object? value, Type inputType, JsonSerializerContext context);
}

Thus in .NET 6 you will be able to do:

using var jsonDocument = JsonSerializer.SerializeToDocument(new MyClass { Data = "value" });

or

var jsonElement = JsonSerializer.SerializeToElement(new MyClass { Data = "value" });

Notes:

• JsonSerializerContext and JsonTypeInfo<T> are newly exposed in .NET 6 and provide metadata about a set of types, or a single type T, that is relevant to JSON serialization. They are used when serializing using metadata and code generated at compile time. See Try the new System.Text.Json source generator for details.

• JsonDocument is IDisposable, and in fact must needs be disposed because, according to the docs:

  JsonDocument builds an in-memory view of the data into a pooled buffer. Therefore, unlike JObject or JArray from Newtonsoft.Json, the JsonDocument type implements IDisposable and needs to be used inside a using block.

  In your sample code you do not dispose of the document returned by JsonDocument.Parse(), but you should.

• The new methods should be present in .NET 6 RC1.

In .NET 5 and earlier a method equivalent to JObject.FromObject() is not currently available out of the box in System.Text.Json. There is an open enhancement about this, currently targeted for Future:

• We should be able serialize and serialize from DOM #31274.

In the interim you may get better performance by serializing to an intermediate byte array rather than to a string, since both JsonDocument and Utf8JsonReader work directly with byte spans rather than strings or char spans, like so:

public static partial class JsonExtensions
{
    public static JsonDocument JsonDocumentFromObject<TValue>(TValue value, JsonSerializerOptions options = default) 
        => JsonDocumentFromObject(value, typeof(TValue), options);

    public static JsonDocument JsonDocumentFromObject(object value, Type type, JsonSerializerOptions options = default)
    {
        var bytes = JsonSerializer.SerializeToUtf8Bytes(value, type, options);
        return JsonDocument.Parse(bytes);
    }

    public static JsonElement JsonElementFromObject<TValue>(TValue value, JsonSerializerOptions options = default)
        => JsonElementFromObject(value, typeof(TValue), options);

    public static JsonElement JsonElementFromObject(object value, Type type, JsonSerializerOptions options = default)
    {
        using var doc = JsonDocumentFromObject(value, type, options);
        return doc.RootElement.Clone();
    }
}

And then call it like:

using var doc = JsonExtensions.JsonDocumentFromObject(new MyClass { Data = "value" });

Or, if you need to use the root element outside the scope of a using statement:

var element = JsonExtensions.JsonElementFromObject(new MyClass { Data = "value" });

Notes:

• As noted above, a JsonDocument needs to be disposed after being created. The above JsonExtensions.JsonElementFromObject() extension methods correctly dispose of their internal document and returns a clone of the root element, as recommended in the documentation.

• Serializing to an intermediate Utf8 byte sequence is likely to be more performant than serializing to a string because, according to the docs:

  Serializing to UTF-8 is about 5-10% faster than using the string-based methods. The difference is because the bytes (as UTF-8) don't need to be converted to strings (UTF-16).

• For the inverse method, see System.Text.Json.JsonElement ToObject workaround.

Demo fiddle here.