With the new TwiML.ToXDocument() method (PR) from the Twilio C# helper library, we can now avoid serializing the XML to a big string before writing it to the response body.

ASP.NET Core does require writes to the response to be async, and in older .NET Core versions XDocument.SaveAsync is not available. In that case, the XML is serialized to a string and written to the response.

For ASP.NET, there's only XDocument.Save available but there's no need for asynchronous writes.

Additionally, I replaced null checks with string.IsNullOrEmpty for BaseUrlOverride because sometimes the setting is an empty string.

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