Test.cs

using System;
using System.IO;
using System.Collections.Generic;

public interface IWriterService
{
    Stream WriteTXT<T>(List<T> content, string delimiter,
                       string header, bool isHeader)
        where T : class;
}

public class WriterService : IWriterService
{
    public WriterService()
    {
    }

    public Stream WriteTXT<T>(List<T> content, string delimiter,
                              string header, bool isHeader)
        where T : class
    {
        var stream = new MemoryStream();
        // Implementation removed as irrelevant
        return stream;
    }
}

class Test
{
    static void Main()
    {
        List<string> list = new List<string>();
        var service = new WriterService();
        service.WriteTXT(list, "x", "y", false);
    }
}