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();

        using var reader = XmlReader.Create(new StringReader(@"

<xsl:stylesheet version=""1.0"" xmlns:xsl=""http://www.w3.org/1999/XSL/Transform"">

    <xsl:output method=""html"" indent=""yes"" doctype-system=""html""/>

    <xsl:template match=""/"">

        <bar/>

    </xsl:template>

</xsl:stylesheet>"));

        transform.Load(reader);

​

        var settings = transform.OutputSettings.Clone();

        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();

    }

}