[JsonSerializer] Provide option to specify a custom default value for a type, property, or field

[layomia]

#779 discusses options to ignore CLR default values during serialization (API approved).

Rather than using the CLR default, users may want to specify a custom default to be ignored. This issue is a placeholder for an API proposal to make this achievable.

[layomia]

From @steveoh in #41630

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

[walterpg]

Raising the flag

FYI, I am considering returning to a tried-and-true third party to get this functionality. Custom JsonConverters are not sustainable workarounds for my scenarios.

[eiriktsarpalis]

Might need to be considered in conjunction with this proposal: #55781 (comment)

[eiriktsarpalis]

Also related #593 and #40395.

[eiriktsarpalis]

I believe that most of that functionality can now be achieved using a combination of the JsonPropertyInfo.ShouldSerialize/Get/Set delegates in the contract customization layer. There a limits to what values can be specified in the attribute layer, so I'd be inclined to close this in favor of the API surface exposed in #63686.