.NET core 3: Order of serialization for JsonPropertyName (System.Text.Json.Serialization)

While this feature is not implemented in .NET Core, we can apply desired ordering by creating a custom JsonConverter. There are a few ways how that can be achievable. Below is the implementation I've came up with.

Explanation - the JsonPropertyOrderConverter handles the types having at least one property with a custom order value applied. For each of those types, it creates and caches a sorter function that converts an original object into an ExpandoObject with the properties set in a specific order. ExpandoObject maintains the order of properties, so it can be passed back to JsonSerializer for further serialization. The converter also respects JsonPropertyNameAttribute and JsonPropertyOrderAttribute attributes applied to serializing properties.

Please note that Sorter functions deal with PropertyInfo objects that can add some extra latency. If the performance is critical in your scenario, consider implementing Function<object, object> sorter based on Expression trees.

class Program{    static void Main(string[] args)    {        var test = new Test { Bar = 1, Baz = 2, Foo = 3 };        // Add JsonPropertyOrderConverter to enable ordering        var opts = new JsonSerializerOptions();        opts.Converters.Add(new JsonPropertyOrderConverter());        var serialized = JsonSerializer.Serialize(test, opts);        // Outputs: {"Bar":1,"Baz":2,"Foo":3}        Console.WriteLine(serialized);    }}class Test{    [JsonPropertyOrder(1)]    public int Foo { get; set; }    [JsonPropertyOrder(-1)]    public int Bar { get; set; }    // Default order is 0    public int Baz { get; set; }}/// <summary>/// Sets a custom serialization order for a property./// The default value is 0./// </summary>[AttributeUsage(AttributeTargets.Property, Inherited = true, AllowMultiple = false)]sealed class JsonPropertyOrderAttribute : Attribute{    public int Order { get; }    public JsonPropertyOrderAttribute(int order)    {        Order = order;    }}/// <summary>/// For Serialization only./// Emits properties in the specified order./// </summary>class JsonPropertyOrderConverter : JsonConverter<object>{    delegate ExpandoObject SorterFunc(object value, bool ignoreNullValues);    private static readonly ConcurrentDictionary<Type, SorterFunc> _sorters        = new ConcurrentDictionary<Type, SorterFunc>();    public override bool CanConvert(Type typeToConvert)    {        // Converter will not run if there is no custom order applied        var sorter = _sorters.GetOrAdd(typeToConvert, CreateSorter);        return sorter != null;    }    public override object Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)    {        throw new NotSupportedException();    }    public override void Write(Utf8JsonWriter writer, object value, JsonSerializerOptions options)    {        // Resolve the sorter.        // It must exist here (see CanConvert).        var sorter = _sorters.GetOrAdd(value.GetType(), CreateSorter);        // Convert value to an ExpandoObject        // with a certain property order        var sortedValue = sorter(value, options.IgnoreNullValues);        // Serialize the ExpandoObject        JsonSerializer.Serialize(writer, (IDictionary<string, object>)sortedValue, options);    }    private SorterFunc CreateSorter(Type type)    {        // Get type properties ordered according to JsonPropertyOrder value        var sortedProperties = type            .GetProperties(BindingFlags.Instance | BindingFlags.Public)            .Where(x => x.GetCustomAttribute<JsonIgnoreAttribute>(true) == null)            .Select(x => new            {                Info = x,                Name = x.GetCustomAttribute<JsonPropertyNameAttribute>(true)?.Name ?? x.Name,                Order = x.GetCustomAttribute<JsonPropertyOrderAttribute>(true)?.Order ?? 0,                IsExtensionData = x.GetCustomAttribute<JsonExtensionDataAttribute>(true) != null            })            .OrderBy(x => x.Order)            .ToList();        // If all properties have the same order,        // there is no sense in explicit sorting        if (!sortedProperties.Any(x => x.Order != 0))        {            return null;        }                // Return a function assigning property values        // to an ExpandoObject in a specified order        return new SorterFunc((src, ignoreNullValues) =>        {            IDictionary<string, object> dst = new ExpandoObject();                       var isExtensionDataProcessed = false;            foreach (var prop in sortedProperties)            {                var propValue = prop.Info.GetValue(src);                if (prop.IsExtensionData)                {                    if (propValue is IDictionary extensionData)                    {                        if (isExtensionDataProcessed)                        {                            throw new InvalidOperationException($"The type '{src.GetType().FullName}' cannot have more than one property that has the attribute '{typeof(JsonExtensionDataAttribute).FullName}'.");                        }                        foreach (DictionaryEntry entry in extensionData)                        {                            dst.Add((string)entry.Key, entry.Value);                        }                    }                                        isExtensionDataProcessed = true;                }                else if (!ignoreNullValues || !(propValue is null))                {                    dst.Add(prop.Name, propValue);                }            }            return (ExpandoObject)dst;        });    }}

I ended up having a 2-pass approach. First pass is my normal json serializer with all converters, pocos, etc. 2nd pass is a "normalizer" to deal with whitespace/indenting/property order/etc.

There are so many corner cases trying to do this with a converter in a single pass. Properties aren't just via reflection, they can be hidden in:

1. Dictionaries
2. [JsonExtensionData] attributes
3. JSonElement
4. other converters!

It's very challenging to write a converter that deals with all of these. So I went with the 2-pass approach. The 2nd pass just operates on JsonElement and a json writer, and so avoids all the corner cases.

(we're using this in production at: https://github.com/microsoft/PowerApps-Language-Tooling/blob/master/src/PAModel/Utility/JsonNormalizer.cs )

// Write out Json in a normalized sorted order. // Orders properties, whitespace/indenting, etc. internal class JsonNormalizer{    public static string Normalize(string jsonStr)    {        using (JsonDocument doc = JsonDocument.Parse(jsonStr))        {           return Normalize(doc.RootElement);        } // free up array pool rent    }    public static string Normalize(JsonElement je)    {        var ms = new MemoryStream();        JsonWriterOptions opts = new JsonWriterOptions        {            Indented = true,            Encoder = JavaScriptEncoder.UnsafeRelaxedJsonEscaping        };        using (var writer = new Utf8JsonWriter(ms, opts))        {            Write(je, writer);        }        var bytes = ms.ToArray();        var str = Encoding.UTF8.GetString(bytes);        return str;    }    private static void Write(JsonElement je, Utf8JsonWriter writer)    {        switch(je.ValueKind)        {            case JsonValueKind.Object:                writer.WriteStartObject();                // !!! This is where we can order the properties.                 foreach (JsonProperty x in je.EnumerateObject().OrderBy(prop => prop.Name))                {                    writer.WritePropertyName(x.Name);                    Write(x.Value, writer);                }                writer.WriteEndObject();                break;                // When normalizing... original msapp arrays can be in any order...            case JsonValueKind.Array:                writer.WriteStartArray();                foreach(JsonElement x in je.EnumerateArray())                {                    Write(x, writer);                }                writer.WriteEndArray();                break;            case JsonValueKind.Number:                writer.WriteNumberValue(je.GetDouble());                break;            case JsonValueKind.String:                // Escape the string                 writer.WriteStringValue(je.GetString());                break;            case JsonValueKind.Null:                writer.WriteNullValue();                break;                            case JsonValueKind.True:                writer.WriteBooleanValue(true);                break;            case JsonValueKind.False:                writer.WriteBooleanValue(false);                break;                            default:                throw new NotImplementedException($"Kind: {je.ValueKind}");        }    }}    

I think the answers here all help with the 'issue'... Here's my custom solution that has been working for me.

JsonPropertyOrderAttribute spot in the @AndreyCh answer.
Adding here as well:

/// <summary>/// Orders a property to be in a specific order when serailizing/// </summary>[AttributeUsage(AttributeTargets.Property, AllowMultiple = false)]public sealed class JsonPropertyOrderAttribute : JsonAttribute{   public JsonPropertyOrderAttribute(int order)   {      Order = order;   }       public int Order { get; }}

But this is my converter... handling the 'reads' as well has allowed me to make it a 'global' converter in my JsonSerializerOptions.

public class JsonPropertyOrderConverter : JsonConverter<object>{    public override bool CanConvert(Type typeToConvert) =>         typeToConvert.GetProperties().Any(x => x.GetCustomAttribute<JsonPropertyOrderAttribute>(true) != null);    public override object Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)    {        var newOptions = new JsonSerializerOptions(options);        if (newOptions.Converters.Contains(this))        {            newOptions.Converters.Remove(this);        }        return JsonSerializer.Deserialize(ref reader, typeToConvert, newOptions);    }    public override void Write(Utf8JsonWriter writer, object value, JsonSerializerOptions options)    {        var orderedProperites = value.GetType().GetProperties()            .Where(x => x.GetCustomAttribute<JsonIgnoreAttribute>(true) == null)            .Select(x => new            {                Info = x,                Order = x.GetCustomAttribute<JsonPropertyOrderAttribute>(true)?.Order ?? 0            })            .OrderBy(x => x.Order)            .Select(x => x.Info);        var work = new Dictionary<string, object>();        foreach (var property in orderedProperites)        {            if (property.PropertyType.IsClass)            {                var propValue = property.GetValue(value, null);                if (propValue == null && options.IgnoreNullValues)                {                    //do nothing                }                else                {                    var classObj = JsonSerializer.Deserialize<object>(JsonSerializer.Serialize(propValue, options));                    var jsonPropertyName = property.GetCustomAttribute<JsonPropertyNameAttribute>(true)?.Name;                    if (!string.IsNullOrEmpty(jsonPropertyName))                        work[jsonPropertyName] = classObj;                    else                        work[options.PropertyNamingPolicy?.ConvertName(property.Name) ?? property.Name] = classObj;                }            }            else            {                var propValue = property.GetValue(value, null);                if (propValue == null && options.IgnoreNullValues)                {                    //do nothing                }                else                {                    var jsonPropertyName = property.GetCustomAttribute<JsonPropertyNameAttribute>(true)?.Name;                    if (!string.IsNullOrEmpty(jsonPropertyName))                        work[jsonPropertyName] = propValue;                    else                        work[options.PropertyNamingPolicy?.ConvertName(property.Name) ?? property.Name] = propValue;                }            }        }        var newValue = JsonSerializer.Deserialize<object>(JsonSerializer.Serialize(work));        JsonSerializer.Serialize(writer, newValue, options);    }}