sample.cs

using Chr.Avro.Abstract;
using Chr.Avro.Confluent;
using Chr.Avro.Resolution;
using Confluent.Kafka;
using Confluent.SchemaRegistry;
using System;
using System.Diagnostics;
using System.Threading.Tasks;

namespace ConsoleApp1
{
	public class Foo
	{
		public string Bar { get; set; }
		public DateTime Date { get; set; }
	}

	class Program
	{
		static async Task Main(string[] args)
		{
			var schemaRegistryConfig = new SchemaRegistryConfig
			{
				SchemaRegistryUrl = "http://localhost:8081/"
			};

			var producerConfig = new ProducerConfig
			{
				BootstrapServers = "localhost"
			};

			var stopwatch = new Stopwatch();
			stopwatch.Start();

			Console.Out.WriteLine("Start CreateProducerForType");
			var fooProducer = await CreateProducerForType<string, Foo>(producerConfig, schemaRegistryConfig);
			Console.Out.WriteLine("Stop CreateProducerForType");

			Console.Out.WriteLine($"CreateProducerForType took: {stopwatch.Elapsed}");

			stopwatch.Restart();

			Console.Out.WriteLine("Start CreateNullProducer");
			var nullProducer = await CreateNullProducer<string>(fooProducer.Handle, schemaRegistryConfig);
			Console.Out.WriteLine("Stop CreateNullProducer");

			Console.Out.WriteLine($"CreateNullProducer took: {stopwatch.Elapsed}");

			stopwatch.Stop();

			await fooProducer.ProduceAsync("datetest4", new Message<string, Foo> { Key = "123", Value = new Foo { Bar = "123", Date = DateTime.Now } });
			await nullProducer.ProduceAsync("datetest4", new Message<string, Null> { Key = "123", Value = null });

			Console.ReadLine();
		}

		private static async Task<IProducer<TKey, TValue>> CreateProducerForType<TKey, TValue>(ProducerConfig producerConfig, SchemaRegistryConfig schemaRegistryConfig)
		{
			var builder = new ProducerBuilder<TKey, TValue>(producerConfig);

			using (var registry = new CachedSchemaRegistryClient(schemaRegistryConfig))
			{

				await Task.WhenAll(
					builder.SetAvroKeySerializer(
						new SchemaRegistrySerializerBuilder(registry, 
							new SchemaBuilder(null, new DataContractResolver(resolveReferenceTypesAsNullable: true), TemporalBehavior.EpochMilliseconds)),
						$"datetest4-avro-key", registerAutomatically: true),
					builder.SetAvroValueSerializer(
						new SchemaRegistrySerializerBuilder(registry, 
							new SchemaBuilder(null, new DataContractResolver(resolveReferenceTypesAsNullable: true), TemporalBehavior.EpochMilliseconds)),
						$"datetest4-avro-value", registerAutomatically: true)
				);
			}

			return builder.Build();
		}

		private static async Task<IProducer<TKey, Null>> CreateNullProducer<TKey>(Handle handle, SchemaRegistryConfig schemaRegistryConfig)
		{
			var builder = new DependentProducerBuilder<TKey, Null>(handle);

			using (var registry = new CachedSchemaRegistryClient(schemaRegistryConfig))
			{
				await Task.WhenAll(
					builder.SetAvroKeySerializer(new SchemaRegistrySerializerBuilder(registry, 
						new SchemaBuilder(null, 
							new DataContractResolver(resolveReferenceTypesAsNullable: true), TemporalBehavior.EpochMilliseconds)),
						$"datetest4-avro-key", registerAutomatically: true)
				);
			}

			return builder.Build();
		}
	}
}