JsonIgnoreCondition.WhenWritingDefault and double

[steveoh]

Description

// startup
.AddJsonOptions(options => {
  options.JsonSerializerOptions.PropertyNamingPolicy = JsonNamingPolicy.CamelCase;
  options.JsonSerializerOptions.DefaultIgnoreCondition = JsonIgnoreCondition.WhenWritingNull & JsonIgnoreCondition.WhenWritingDefault;
  options.JsonSerializerOptions.Converters.Add(new JsonStringEnumConverter(JsonNamingPolicy.CamelCase));
});

// ...

[DefaultValue(typeof(decimal), "-1")]
[DefaultValue(-1.0)]
[DefaultValue(-1D)]
public double Prop { 
	get => _prop;
    set => _prop = Math.Round(value, 2);
}

When the prop is set to -1, which is the default value, the property is serialized when JsonIgnoreCondition.WhenWritingDefault is set. I would expect that property to not be serialized.

Configuration

osx catalina x64
5.0.100-preview.8.20417.9

[pinkfloydx33]

The serializer only looks at the actual default value of the type, meaning default(T). It doesn't look at the DefaultValueAttribute

runtime/src/libraries/System.Text.Json/src/System/Text/Json/Serialization/JsonPropertyInfoOfT.cs

Lines 137 to 138 in 756addd

	if (IgnoreDefaultValuesOnWrite && (
	default(T) == null ? value == null : EqualityComparer<T>.Default.Equals(default, value)))

Aside: your use of & in JsonIgnoreCondition.WhenWritingNull & JsonIgnoreCondition.WhenWritingDefault is a little weird. That equates to 3&2==2 which equals WhenWritingDefault. Assuming you meant to use | I wanted to point out that it's not a Flags enum; it can only ever be one or the other.

[steveoh]

Ok thank you for clearing up that misconception. I will need to use more nullable types then I suspect to get a similar behavior.

How do I get both behaviors of ignoring nulls and defaults? If I set ignorenullvalues I am greeted with System.InvalidOperationException: 'IgnoreNullValues' and 'DefaultIgnoreCondition' cannot both be set to non-default values. Do I need to write my own converters?

[pinkfloydx33]

I believe you want JsonIgnoreCondition.Always

[layomia]

@steveoh @pinkfloydx33 , the setting you want is JsonIgnoreCondition.WhenWritingDefault. This will ignore both null for nullable types and default(T) for non-nullable types. JsonIgnoreCondition.WhenWritingNull will ignore only null properties. Please let us know if you still face issues with this.

wrt. using DefaultValueAttribute or a similar mechanism to specify a custom default value, that is being tracked by #36236. Will close this issue as a duplicate.

[pinkfloydx33]

@layomia what is the behavior for Always? I'd expect that to behave how you describe WhenWritingDefault. Or is that a Serialize vs. Deserialize thing?

[layomia]

@pinkfloydx33 - JsonIgnoreCondition.Always means always ignore when serializing or deserializing. This is the default value of JsonIgnoreAttribute.Condition, but it is not a valid value for JsonSerializerOptions.DefaultIgnoreCondition - it doesn't make sense to ignore all properties for both deserialization and serialization across the entire object graph.

We'll add documentation for all of this behavior - https://docs.microsoft.com/dotnet/standard/serialization/system-text-json-how-to.