Unable to implement custom IActionResult

[SoftCircuits]

I'm trying to do what I thought would be pretty simple. I want to create my own action result that implements IActionResult. Then I want to implement ExecuteResultsAsync() to write XML content to context.HttpContext.Response.Body.

The first problem is that the method has to be async. I'm thinking context.HttpContext.Response.Body is probably a MemoryStream? Either way, I don't see the need for this to run asynchronously and expect performance to suffer if I do.

So I instead derived from ActionResult, which has a virtual, non-async version of this method. But now I get the exception:

System.InvalidOperationException: 'Synchronous operations are disallowed. Call WriteAsync or set AllowSynchronousIO to true instead.'

So next I set XmlWriterSettings.Async = 2, and called only async methods of XmlWriter. I hoped the arguments would be the same, but the async methods all have different arguments. So I set all the additional arguments to null.

Now I still get the exception:

System.InvalidOperationException: 'Synchronous operations are disallowed. Call WriteAsync or set AllowSynchronousIO to true instead.'

I don't know where I would call WriteAsync, and I'm not sure the ramifications of change the application-wide AllowSynchronousIO value.

Question: How can I implement a custom IActionResult and implements ExecuteResultAsync() and writes XML to the output? What exactly does .NET Core want from me?

I'm using VS 2019 v16.7.2, ASP.NET 3.1, Razor Pages.