using System.IO;

using System.Threading.Tasks;

using System.Xml;

using System.Xml.Xsl;

public class Program

{

    public static void Main()

    {

        Test().GetAwaiter().GetResult();

    }

    public static async Task Test()

    {

         var transform = new XslCompiledTransform();

            var settings = new XmlWriterSettings();

            settings.CloseOutput = false;

            settings.Async = true;

​

            var method = settings.OutputMethod;

​

            using var stream = new MemoryStream();

            using (var writer = XmlWriter.Create(stream, settings))

            {

                await writer.WriteStartDocumentAsync();

                await writer.WriteStartElementAsync(null, "foo", null);

                await writer.WriteEndElementAsync();

                await writer.WriteEndDocumentAsync();

            }

            stream.Position = 0;

            var content = new StreamReader(stream).ReadToEnd();

    }

}