Added a flag to JsonIgnoreAttribute so we can toggle ignoring always or only when null.

[CodeBlanch]

A new feature for consideration by the team. 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.

To illustrate how it works here's the test snippet from the PR:

        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);
        }

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

[ahsonkhan]

@CodeBlanch - thanks for the PR and contribution. Since this is adding new public surface area, it should go through an API review. I have not yet reviewed this PR. Can you file the appropriate issue for the new API to be reviewed with the rationale and sample usage, similar to what you have done in the description here, and we can move forward from there. Thanks!

https://github.com/dotnet/corefx/blob/master/Documentation/project-docs/api-review-process.md

[CodeBlanch]

@ahsonkhan I created https://github.com/dotnet/corefx/issues/40600 for the API review.

[maryamariyan]

Thank you for your contribution. As announced in dotnet/coreclr#27549 this repository will be moving to dotnet/runtime on November 13. If you would like to continue working on this PR after this date, the easiest way to move the change to dotnet/runtime is:

1. In your corefx repository clone, create patch by running git format-patch origin
2. In your runtime repository clone, apply the patch by running git apply --directory src/libraries <path to the patch created in step 1>

[ericstj]

As @ahsonkhan mentioned we need to first follow the API review process. Once that's done we can open a PR (likely against dotnet/runtime) with this change. Thank you for your interest and contribution!

[lfr]

This suggestion does not give us parity with Json.NET's NullValueHandling as suggested by @CodeBlanch. Json.NET allows overriding the default behavior at the property level both when the default is ignoring nulls as well as when the behavior is to include nulls.

Please look at https://github.com/dotnet/corefx/issues/42034 for more context. This suggestion is about ignoring nulls for a specific property, but when the default behavior is ignoring nulls, we may want to force including them for a specific property.