System.Text.Json ignore only when null API enhancement

[CodeBlanch]

Rationale

Right now null handling is an all or nothing thing. We have an option to ignore all null values (JsonSerializerOptions.IgnoreNullValues) and we have an option to ignore properties completely (JsonIgnoreAttribute), but you can't specify per-property handling. Per-property handling is useful when you want to hide something from JSON unless it is populated, perhaps because it was not requested or is only applicable to certain audiences.

This gives us parity with something like Json.NET's NullValueHandling.

Proposed API

Via expansion of JsonIgnoreAttribute...

namespace System.Text.Json.Serialization
{
  /// <summary>
  /// Controls how the <see cref="JsonIgnoreAttribute"/> will decide to ignore properties.
  /// </summary>
  public enum JsonIgnoreCondition
  {
      /// <summary>
      /// Property will always be ignored.
      /// </summary>
      Always,
      /// <summary>
      /// Property will only be ignored if it is null.
      /// </summary>
      WhenNull,
      /// <summary>
      /// Property will always be serialized/deserialized regardless of <see cref="JsonSerializerOptions"/> configuration.
      /// </summary>
      Never
  }

  /// <summary>
  /// Prevents a property from being serialized or deserialized.
  /// </summary>
  [AttributeUsage(AttributeTargets.Property, AllowMultiple = false)]
  public sealed class JsonIgnoreAttribute : JsonAttribute
  {
      /// <summary>
      /// Specifies the condition that must be met before a property will be ignored. Default value: <see cref="JsonIgnoreCondition.Always"/>.
      /// </summary>
      public JsonIgnoreCondition Condition { get; set; } = JsonIgnoreCondition.Always;

      /// <summary>
      /// Initializes a new instance of <see cref="JsonIgnoreAttribute"/>.
      /// </summary>
      public JsonIgnoreAttribute() { }
  }
}

Usage

Always

This is equivalent to the current [JsonIgnore] semantics where properties that have this value are always ignored.

WhenNull

public class ClassUsingIgnoreWhenNullAttribute
{
    [JsonIgnore(Condition = JsonIgnoreCondition.WhenNull)]
    public SimpleTestClass Class { get; set; }

    [JsonIgnore(Condition = JsonIgnoreCondition.WhenNull)]
    public Dictionary<string, string> Dictionary { get; set; } = new Dictionary<string, string> { ["Key"] = "Value" };
}

[Fact]
public static void SerializerSupportsClassUsingIgnoreWhenNullAttribute()
{
    string json = @"{""Class"":{""MyInt16"":18}, ""Dictionary"":null}";

    ClassUsingIgnoreWhenNullAttribute obj = JsonSerializer.Deserialize<ClassUsingIgnoreWhenNullAttribute>(json);

    // Class is deserialized because it is not null in json...
    Assert.NotNull(obj.Class);
    Assert.Equal(18, obj.Class.MyInt16);

    // Dictionary is left alone because it is null in json...
    Assert.NotNull(obj.Dictionary);
    Assert.Equal(1, obj.Dictionary.Count);
    Assert.Equal("Value", obj.Dictionary["Key"]);

    obj = new ClassUsingIgnoreWhenNullAttribute();

    json = JsonSerializer.Serialize(obj);

    // Class is not included in json because it was null, Dictionary is included because it is not null...
    Assert.Equal(@"{""Dictionary"":{""Key"":""Value""}}", json);
}

Never

public class ClassUsingIgnoreWhenNullAttribute
{
    public SimpleTestClass Class { get; set; } new MySimpleTestClass { MyInt16 = 18 };

    [JsonIgnore(Condition = JsonIgnoreCondition.Never)]
    public Dictionary<string, string> Dictionary { get; set; } = new Dictionary<string, string> { ["Key"] = "Value" };
}

[Fact]
public static void SerializerSupportsClassUsingIgnoreNeverAttribute()
{
    string json = @"{""Class"":null, ""Dictionary"":null}";
    var options = new JsonSerializerOptions { IgnoreNullValues = true };

    ClassUsingIgnoreWhenNullAttribute obj = JsonSerializer.Deserialize<ClassUsingIgnoreWhenNullAttribute>(json, options);

    // Class is not deserialized because it is null in json...
    Assert.NotNull(obj.Class);
    Assert.Equal(18, obj.Class.MyInt16);

    // Dictionary is deserialized regardless of being null in json...
    Assert.Null(obj.Dictionary);

    obj = new ClassUsingIgnoreWhenNullAttribute();
    obj.Class = null;

    json = JsonSerializer.Serialize(obj, options);

    // Class is not included in json because it was null, Dictionary is included regardless of being null...
    Assert.Equal(@"{""Dictionary"":null}", json);
}

Considerations

• The serializer will throw on deserialization when a null token is applied to a value type, even if IgnoreNullValues is applied. There is an open issue to relax this behavior: IgnoreNullValues does not work with non-nullable value types #30795, after which those semantics can be applied per property.
• An upcoming feature is being able to globally ignore "default values" on (de)serialization: System.Text.Json option to ignore default values during serialization #779. It would make sense to add a WhenDefault enum value to JsonIgnoreCondition when this is implemented.

Implementation

See dotnet/corefx#40522

[SidShetye]

Requesting to prioritize as we're blocked on this given the all-or-nothing choice. We have many vendor APIs (including our own) that are fine but have one vendor API (an Azure one interestingly) that requires us to not send out a property but only when it's null. This is also enforced in the certification process, so it's hard to avoid.

[steveharter]

Moving to 5.0 for consideration as this is blocking adoption.

Consider if the global option JsonSerializerOptions.IgnoreNullValues==true but we don't want to ignore certain properties: then adding a "Never" value to the enum make sense.

Also, since the current options apply to both serialization and deserialization, are there valid cases for controlling serialization and deserialization separately with the proposed enum?

[ericstj]

/cc @layomia @ahsonkhan

[SidShetye]

Also, since the current options apply to both serialization and deserialization, are there valid cases for controlling serialization and deserialization separately with the proposed enum?

For us, we do not need them to be controlled separately.

[pharring]

Value types were mentioned in "Considerations". I would vote to include value types and Enums. Json.Net's equivalent is DefaultValueHandling.Ignore. In my applications I use that on Guid, TimeSpan, DateTime, Boolean and some Enums.

[pharring]

Also, for completeness, there's precedent in System.Runtime.Serialization.DataMemberAttribute's EmitDefaultValue param which works on value types too.

[lfr]

If implemented as suggested here this enhancement is incomplete. It needs a third option:

    /// <summary>
    /// Controls how the <see cref="JsonIgnoreAttribute"/> will decide to ignore properties.
    /// </summary>
    public enum JsonIgnoreCondition
    {
        /// <summary>
        /// Property will always be ignored.
        /// </summary>
        Always,
        /// <summary>
        /// Property will only be ignored if it is null.
        /// </summary>
        WhenNull,
        /// <summary>
        /// Property will always be serialized/deserialized regardless of Options settings
        /// </summary>
        Never
    }

Please look at https://github.com/dotnet/corefx/issues/42034 for more context, Json.Net does it correctly via NullValueHandling, this suggestion covers the case when we want to for a specific property override the default general options that dictate that null values should be included, but we also need to be able to do the opposite, meaning overriding general options that dictate that null values should be ignored for a specific property.

[layomia]

Also, since the current options apply to both serialization and deserialization, are there valid cases for controlling serialization and deserialization separately with the proposed enum?

If there are such cases, instead of using one enum for both, maybe we can have two properties on JsonIgnoreAttribute:

/// <summary>
/// Prevents a property from being serialized or deserialized.
/// </summary>
[AttributeUsage(AttributeTargets.Property, AllowMultiple = false)]
public sealed class JsonIgnoreAttribute : JsonAttribute
{
    /// <summary>
    /// Specifies the condition that must be met before a property will be ignored on deserialization. Default value: <see cref="JsonIgnoreCondition.Always"/>.
    /// </summary>
    public JsonIgnoreCondition ReadCondition { get; set; } = JsonIgnoreCondition.Always;

    /// <summary>
    /// Specifies the condition that must be met before a property will be ignored on serialization. Default value: <see cref="JsonIgnoreCondition.Always"/>.
    /// </summary>
    public JsonIgnoreCondition WriteCondition { get; set; } = JsonIgnoreCondition.Always;

    /// <summary>
    /// Initializes a new instance of <see cref="JsonIgnoreAttribute"/>.
    /// </summary>
    public JsonIgnoreAttribute() { }
}

[CodeBlanch]

@steveharter @lfr I added "Never" to the JsonIgnoreCondition definition in the description.

To mitigate #31786 & #682, here's a curveball:

    /// <summary>
    /// Controls how the <see cref="JsonIgnoreAttribute"/> will decide to ignore properties.
    /// </summary>
    public enum JsonIgnoreCondition
    {
        /// <summary>
        /// Property will always be ignored.
        /// </summary>
        Always,
        /// <summary>
        /// Property will only be ignored if it is null.
        /// </summary>
        WhenNull,
        /// <summary>
        /// Property will always be serialized/deserialized regardless of <see cref="JsonSerializerOptions"/> configuration.
        /// </summary>
        Never
    }

    /// <summary>
    /// Controls how the <see cref="JsonIgnoreAttribute"/> will decide to ignore items contained in IEnumerable and IDictionary properties.
    /// </summary>
    public enum JsonChildItemIgnoreCondition
    {
        /// <summary>
        /// Children will always be serialized/deserialized regardless of value.
        /// </summary>
        Never,
        /// <summary>
        /// Children will only be ignored when they are null.
        /// </summary>
        WhenNull
    }

    /// <summary>
    /// Prevents a property from being serialized or deserialized.
    /// </summary>
    [AttributeUsage(AttributeTargets.Property, AllowMultiple = false)]
    public sealed class JsonIgnoreAttribute : JsonAttribute
    {
        /// <summary>
        /// Specifies the condition that must be met before a property will be ignored. Default value: <see cref="JsonIgnoreCondition.Always"/>.
        /// </summary>
        public JsonIgnoreCondition PropertyCondition { get; set; } = JsonIgnoreCondition.Always;

        /// <summary>
        /// Specifies the condition that must be met before a child item will be ignored. Default value: <see cref="JsonChildItemIgnoreCondition.Never"/>.
        /// </summary>
        /// <remarks>
        /// Applies to IEnumerable and IDictionary properties only.
        /// </remarks>
        public JsonChildItemIgnoreCondition ChildItemCondition { get; set; } = JsonChildItemIgnoreCondition.Never;

        /// <summary>
        /// Initializes a new instance of <see cref="JsonIgnoreAttribute"/>.
        /// </summary>
        public JsonIgnoreAttribute() { }
    }

Introduces JsonChildItemIgnoreCondition enum which can be applied as "ChildItemCondition" on JsonIgnoreAttribute. "Condition" on JsonIgnoreAttribute renamed "PropertyCondition."

The idea being, JsonChildItemIgnoreCondition.WhenNull will cause null items in IEnumerable or IDictionary props to be ignored. Example:

public class TestClass
{
   [JsonIgnore(PropertyCondition=JsonIgnoreCondition.WhenNull, ChildItemCondition=JsonChildItemIgnoreCondition.WhenNull)]
   [JsonExtensionData]
   public IDictionary<string, object> Fields { get; set; } = new Dictionary<string, object>
   {
      ["Prop1"] = "value",
      ["Prop2"] = null
   }
}

[lfr]

Thanks @CodeBlanch, looking forward to testing it!