using CsvHelper.Configuration;
using Microsoft.AspNetCore.Mvc;
using Microsoft.Extensions.DependencyInjection;
using System.Collections.Generic;
using System.Globalization;
using System.IO;
using System.Linq;

public class Startup
{
    //...
    public void ConfigureServices(IServiceCollection services)
    {
        //...
        services.AddMvc().AddMvcOptions(options =>
        {
            options.OutputFormatters.Add(new CsvOutputFormatter());
        });
    }
    //...
}

public class CsvOutputFormatter : TextOutputFormatter
{
    private readonly CsvConfiguration _csvConfiguration;

    public CsvOutputFormatter()
    {
        SupportedMediaTypes.Add("text/csv");
        SupportedEncodings.Add(Encoding.UTF8);
        _csvConfiguration = new CsvConfiguration(CultureInfo.InvariantCulture)
        {
            Delimiter = ",",
            HasHeaderRecord = true
        };
    }

    protected override bool CanWriteType(Type type)
    {
        if (typeof(IEnumerable).IsAssignableFrom(type) && type != typeof(string))
        {
            return true;
        }
        return base.CanWriteType(type);
    }

    public override async Task WriteResponseBodyAsync(OutputFormatterWriteContext context, Encoding selectedEncoding)
    {
        var response = context.HttpContext.Response;
        
        using (var writer = new StreamWriter(response.Body))
        {
            using (var csv = new CsvWriter(writer, _csvConfiguration))
            {
                var model = context.Object;
                var modelType = model.GetType();

                if (typeof(IEnumerable).IsAssignableFrom(modelType))
                {
                    csv.WriteRecords((IEnumerable