ZipManyFiles.cs

public async Task<byte[]> GenerateZipFileAsync()
        {
            using (var memoryStream = new MemoryStream())
            {
                using (var archive = new ZipArchive(memoryStream, ZipArchiveMode.Create, true))
                {
                    await AddFileToZipAsync(archive, "HzImpAccountsT.csv", file.Accounts);
                    await AddFileToZipAsync(archive, "HzImpAcctContactsT.csv", file.Account_Contacts);
                }

                return memoryStream.ToArray();
            }
        }

        protected async Task AddFileToZipAsync<TData>(ZipArchive archive, string filename, IEnumerable<TData> data)
        {
            var file = archive.CreateEntry(filename);

            using (var entryStream = file.Open())
            using (var streamWriter = new StreamWriter(entryStream))
            {
                streamWriter.Write(csvData);
            }

            return;
        }