https://learn.microsoft.com/ja-jp/dotnet/api/system.io.windowsruntimestreamextensions.asoutputstream?view=dotnet-uwp-10.0
WindowsRuntimeStreamExtensions.AsOutputStream(Stream) Method (System.IO) | Microsoft Learn
Converts a managed stream in the .NET for Windows Store apps to an output stream in the Windows Runtime.
streammethodsystemiomicrosoft