System.Text.Json.JsonElement ToObject workaround

Question

I want to know the equivalent of the ToObject<>() method in Json.NET for System.Text.Json.

Using Json.NET you can use any JToken and

Answer 1

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 writing to an intermediate byte buffer rather than to a string, since both JsonDocument and Utf8JsonReader work directly with byte spans rather than strings or char spans. As stated in 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).

public static partial class JsonExtensions
{
    public static T ToObject<T>(this JsonElement element, JsonSerializerOptions options = null)
    {
        var bufferWriter = new ArrayBufferWriter<byte>();
        using (var writer = new Utf8JsonWriter(bufferWriter))
            element.WriteTo(writer);
        return JsonSerializer.Deserialize<T>(bufferWriter.WrittenSpan, options);
    }

    public static T ToObject<T>(this JsonDocument document, JsonSerializerOptions options = null)
    {
        if (document == null)
            throw new ArgumentNullException(nameof(document));
        return document.RootElement.ToObject<T>(options);
    }       
}

Demo fiddle here.

Answer 2

I came across the same issue, so I wrote some extension methods which work fine for now. It would be nice if they provided this as built in to avoid the additional allocation to a string.

public static T ToObject<T>(this JsonElement element)
{
    var json = element.GetRawText();
    return JsonSerializer.Deserialize<T>(json);
}
public static T ToObject<T>(this JsonDocument document)
{
    var json = document.RootElement.GetRawText();
    return JsonSerializer.Deserialize<T>(json);
}

Then use as follows:

jDoc.RootElement.GetProperty("SomeProperty").ToObject<SomeClass>();

Answer 3

Same as dbc's answer, just including the methods which allow you to specify a return type via Type returnType.

public static partial class JsonExtensions
{
    public static T ToObject<T>(this JsonElement element, JsonSerializerOptions options = null)
    {
        var bufferWriter = new ArrayBufferWriter();
        using (var writer = new Utf8JsonWriter(bufferWriter))
        {
            element.WriteTo(writer);
        }

        return JsonSerializer.Deserialize<T>(bufferWriter.WrittenSpan, options);
    }

    public static T ToObject<T>(this JsonDocument document, JsonSerializerOptions options = null)
    {
        if (document == null)
        {
            throw new ArgumentNullException(nameof(document));
        }

        return document.RootElement.ToObject<T>(options);
    }       

    public static object ToObject(this JsonElement element, Type returnType, JsonSerializerOptions options = null)
    {
        var bufferWriter = new ArrayBufferWriter();
        using (var writer = new Utf8JsonWriter(bufferWriter))
        {
            element.WriteTo(writer);
        }

        return JsonSerializer.Deserialize(bufferWriter.WrittenSpan, returnType, options);
    }

    public static object ToObject(this JsonDocument document, Type returnType, JsonSerializerOptions options = null)
    {
        if (document == null)
        {
            throw new ArgumentNullException(nameof(document));
        }

        return document.RootElement.ToObject(returnType, options);
    }       
}