Return a view as download html file (netcore 3.1)

HtmlFileResult.cs

	public class HtmlFileResult : ActionResult
	{
		private readonly string fileDownloadName;
		private readonly ViewResult viewToRender;

		public HtmlFileResult(ViewResult viewToRender, string fileDownloadName)
		{
			if (string.IsNullOrWhiteSpace(fileDownloadName))
			{
				throw new ArgumentException($"'{nameof(fileDownloadName)}' cannot be null or whitespace.", nameof(fileDownloadName));
			}
			this.viewToRender = viewToRender ?? throw new ArgumentNullException(nameof(viewToRender));
			this.fileDownloadName = fileDownloadName;
		}

		public override async Task ExecuteResultAsync(ActionContext context)
		{
			const int BufferSize = 64 * 1024;
			var viewEngine = viewToRender.ViewEngine
				?? (IViewEngine)context.HttpContext.RequestServices.GetService(typeof(ICompositeViewEngine));
			var viewResult = viewEngine.FindView(context, viewToRender.ViewName, true);
			if (viewResult is null)
			{
				throw new InvalidOperationException($"'{viewToRender.ViewName}' does not match any available view.");
			}

			var response = context.HttpContext.Response;
			response.StatusCode = 200;
			response.ContentType = "text/html";
			var contentDisposition = new ContentDispositionHeaderValue("attachment");
			contentDisposition.SetHttpFileName(fileDownloadName);
			response.Headers.Add("Content-Disposition", contentDisposition.ToString());
			using (TextWriter textWriter = new StreamWriter(response.Body, Encoding.UTF8,
				bufferSize: BufferSize, leaveOpen: true))
			{
				IView view = viewResult.View;
				var viewContext = new ViewContext(context, view, viewToRender.ViewData, viewToRender.TempData, textWriter, new HtmlHelperOptions());
				await view.RenderAsync(viewContext);
				await textWriter.FlushAsync();
			}
		}
	}