Does the new `System.Text.Json` have a required property attribute?

Question

I've combed through the MS docs but cannot find an attribute equivalent to the NewtonSoft JsonPropertyRequired.

What I'm looking for is this:

public class Videogame
{
    [JsonProperty(Required = Required.Always)]
    public string Name { get; set; }
}

Am I just missing something or does this level of validation not exist in the Microsoft library?

Answer 1

Not as of .NET Core 3.0. The only ones supported are:

JsonConverterAttribute
JsonExtensionDataAttribute
JsonIgnoreAttribute
JsonPropertyNameAttribute

Update: In .NET 5.0 the set is

JsonConstructorAttribute
JsonConverterAttribute
JsonExtensionDataAttribute
JsonIgnoreAttribute
JsonIncludeAttribute
JsonNumberHandlingAttribute
JsonPropertyNameAttribute

---

Unfortunately even a custom converter with HandleNull => true shown in How to write custom converters for JSON serialization (marshalling) in .NET won't work because if the property in not present Read and Write methods are not called (tested in 5.0, and a modified version in 3.0)

public class Radiokiller
{
    [JsonConverter(typeof(MyCustomNotNullConverter))] 
    public string Name { get; set; }  
}

public class MyCustomNotNullConverter : JsonConverter<string>
{
    public override bool HandleNull => true;

    public override string Read(
        ref Utf8JsonReader reader,
        Type typeToConvert,
        JsonSerializerOptions options) =>
        reader.GetString() ?? throw new Exception("Value required.");

    public override void Write(
        Utf8JsonWriter writer,
        string value,
        JsonSerializerOptions options) =>
        writer.WriteStringValue(value);

}

var json = "{}";
var o = JsonSerializer.Deserialize<Radiokiller>(json); // no exception :(

json = "{  \"Name\" : null}";
o = JsonSerializer.Deserialize<Radiokiller>(json); // throws

Answer 2

Please try this library I wrote as an extension to System.Text.Json to offer missing features: https://github.com/dahomey-technologies/Dahomey.Json.

You will find support for JsonRequiredAttribute.

public class Videogame
{
    [JsonRequired(RequirementPolicy.Always)]
    public string Name { get; set; }
}

Setup json extensions by calling on JsonSerializerOptions the extension method SetupExtensions defined in the namespace Dahomey.Json. Then deserialize your class with the regular Sytem.Text.Json API.

JsonSerializerOptions options = new JsonSerializerOptions();
options.SetupExtensions();

const string json = @"{""Name"":""BGE2""}";
Videogame obj = JsonSerializer.Deserialize<Videogame>(json, options);