Azure Functions custom serializer

functions-customserialization.cs

[assembly: FunctionsStartup(typeof(FunctionApp1.Startup))]
namespace FunctionApp1
{
    public class Startup : FunctionsStartup
    {
        public override void Configure(IFunctionsHostBuilder builder)
        {
            builder.Services.AddSingleton<ICosmosDBSerializerFactory, MyCosmosDBSerializerFactory>();
        }
    }

    public class MyCosmosDBSerializerFactory : ICosmosDBSerializerFactory
    {
        public CosmosSerializer CreateSerializer()
        {
            var options = new JsonSerializerOptions();

            // return a custom implementation of CosmosSerializer
            return new CosmosSystemTextJsonSerializer(options);
        }
    }
}