serialization - .NET Core: Remove null fields from API JSON response

Question

On a global level in .NET Core 1.0 (all API responses), how can I configure Startup.cs so that null fields are removed/ignored in JSON responses?

Using Newtonsoft.Json, you can apply the following attribute to a property, but I'd like to avoid having to add it to every single one:

[JsonProperty(NullValueHandling = NullValueHandling.Ignore)]
public string FieldName { get; set; }
[JsonProperty(NullValueHandling = NullValueHandling.Ignore)]
public string OtherName { get; set; }

Answer 1

[.NET Core 1.0]

In Startup.cs, you can attach JsonOptions to the service collection and set various configurations, including removing null values, there:

public void ConfigureServices(IServiceCollection services)
{
     services.AddMvc()
             .AddJsonOptions(options => {
                options.SerializerSettings.NullValueHandling = NullValueHandling.Ignore;
     });
}

[.NET Core 3.1]

Instead of:

options.SerializerSettings.NullValueHandling = NullValueHandling.Ignore;

Use:

options.JsonSerializerOptions.IgnoreNullValues = true;

[.NET 5.0]

Instead of both variants above, use:

 options.JsonSerializerOptions.DefaultIgnoreCondition = JsonIgnoreCondition.WhenWritingNull;

The variant from .NET Core 3.1 still works, but it is marked as NonBrowsable (so you never get the IntelliSense hint about this parameter), so it is very likely that it is going to be obsoleted at some point.