Is there a built in way of using snake case as the naming policy for JSON in ASP.NET Core 3?

Update

.Net 8 has been released and it has built in support for different naming policies.

var serializeOptions = new JsonSerializerOptions
{
    // This can be changed to other naming policies like SnakeCaseLower, KebabCaseLower
    PropertyNamingPolicy = JsonNamingPolicy.CamelCase,
    WriteIndented = true
};
jsonString = JsonSerializer.Serialize(weatherForecast, serializeOptions);

Old Answer

Just slight modification in pfx code to remove the dependency on Newtonsoft Json.Net.

String extension method to convert the given string to SnakeCase.

public static class StringUtils
{
    public static string ToSnakeCase(this string str)
    {
        return string.Concat(str.Select((x, i) => i > 0 && char.IsUpper(x) ? "_" + x.ToString() : x.ToString())).ToLower();
    }
}

Then in our SnakeCaseNamingPolicy we can do

public class SnakeCaseNamingPolicy : JsonNamingPolicy
{
    public static SnakeCaseNamingPolicy Instance { get; } = new SnakeCaseNamingPolicy();

    public override string ConvertName(string name)
    {
        // Conversion to other naming convention goes here. Like SnakeCase, KebabCase etc.
        return name.ToSnakeCase();
    }
}

The last step is to register our naming policy in Startup.cs

public void ConfigureServices(IServiceCollection services)
{
    services.AddControllers()            
        .AddJsonOptions(
            options => { 
                options.JsonSerializerOptions.PropertyNamingPolicy = 
                    SnakeCaseNamingPolicy.Instance;
            });
}

Using the model:

public class WeatherForecast
{
    public DateTime Date { get; set; }

    public int TemperatureCelcius { get; set; }

    public int TemperatureFahrenheit { get; set; }

    public string Summary { get; set; }
}

Json output:

{
  "date": "2019-10-28T08:26:14.878444+05:00",
  "temperature_celcius": 4,
  "temperature_fahrenheit": 0,
  "summary": "Scorching"
}