topic Re: How to wait for an async method to complete inside a [LispFunction] ? in .NET Forum

How to wait for an async method to complete inside a [LispFunction] ?

bahman.jf.68 — Fri, 30 May 2025 11:04:20 GMT

Hi everyone,

I'm working on a .NET plugin for AutoCAD (2017+) and I'm facing an issue when calling an async method ((extractOSMImagesAsync)) from a [LispFunction].

I have a function that downloads satellite map tiles using HttpClient and displays a Windows Form circular progress bar (Modeless form) during the download. The actual download logic is asynchronous (async Task). When I call this logic from a [LispFunction], AutoCAD continues executing the rest of the code before the download finishes and before the progress form is closed.

While in this Command Method it works correct, it means "Alert dialog" shows after complete downloading process : (It should be done by LispFunction)

[CommandMethod("RunOSMMaps")] public async void RunOSMMapsCommand() { Point3d vpll = ...; // parsed from resBuf Point3d vpur = ...; if (CheckNet()) { // This is an async Task method await runmaps.extractOSMImagesAsync(vpll, vpur...); ); } Application.ShowAlertDialog(" Download completed!"); }

Any suggestions or examples are appreciated!

Thanks in advance,

Re: How to wait for an async method to complete inside a [LispFunction] ?

ActivistInvestor — Fri, 30 May 2025 15:46:14 GMT

Your CommandMethod works as you expect because it is marked as async and it uses await which is what causes the compiler to rewrite your code so that whatever follows the await-ed method call does not run until the await-ed method has returned.

Adding the same async and await keywords to the LispFunction should make it work like the CommandMethod::

[LispFunction("RunOSMMaps")] public async ResultBuffer RunOSMMaps(ResultBuffer resBuf) { Point3d vpll = ...; // parsed from resBuf Point3d vpur = ...; // Other parameters like image path, zoom, etc. ExtractImages runmaps = new ExtractImages(); if (CheckNet()) { // This is an async Task method await runmaps.extractOSMImagesAsync(vpll, vpur, ...); } // This message box is shown immediately before download completes Application.ShowAlertDialog(" Download completed!"); return null; }

The only difference is that the method is marked as async and the call to extractOSMImagesAsync() is preceded by await.

However, if you are expecting the calling LISP code to wait until extractOSMImagesAsync() has returned, that is not going to happen. The LispFunction will return to its caller before extractOSMImagesAsync() has returned, and the only way to prevent that is by not using async/await.

Re: How to wait for an async method to complete inside a [LispFunction] ?

BlackBox_ — Fri, 30 May 2025 13:48:54 GMT

What happens if you silently invoke the CommandMethod from your LispFunction?

Re: How to wait for an async method to complete inside a [LispFunction] ?

bahman.jf.68 — Fri, 30 May 2025 18:04:30 GMT

I checkde your method and gives this error :

Severity Code Description Project File Line Suppression State Error CS1983 The return type of an async method must be void, Task, Task<T>, a task-like type, IAsyncEnumerable<T>, or IAsyncEnumerator<T>

It seems [LispFunction] doesn't support "async ResultBuffer".

Just to clarify my previous setup:

When I was using the [extractOSMImages] method without async/await, and the progress form was shown as modal, everything worked fine. The entire [LispFunction] completed before returning control to the calling LISP code, and there were no issues.
I switched the progress form to modeless because in modal mode, the progress bar was flickering and not updating smoothly.
I also changed the download logic to be async so that the [HttpClient] requests could run asynchronously, which significantly improves download speed.

Now with both changes (modeless form and async download), the [LispFunction] returns immediately, before the download finishes or the form closes.

Re: How to wait for an async method to complete inside a [LispFunction] ?

bahman.jf.68 — Fri, 30 May 2025 18:13:50 GMT

I don't think invoking a [CommandMethod] from within a [LispFunction] would solve the issue. Even if the command is launched successfully, the [LispFunction] itself will return immediately and continue executing the remaining LISP code — while the [CommandMethod] is still running asynchronously. So we end up with the same problem: the LISP side doesn’t wait for the download or async task to complete.

Re: How to wait for an async method to complete inside a [LispFunction] ?

ActivistInvestor — Sat, 31 May 2025 01:04:20 GMT

@bahman.jf.68 wrote:
I checkde your method and gives this error :

It seems [LispFunction] doesn't support "async ResultBuffer".

A LispFunction can return void:

public async void RunOSMMaps(ResultBuffer resBuf) { ... }

However, based on your comments, I think you're expecting the calling LISP code to wait until the download operation is completed, which it will not. Even with async/await you cannot stop a LispFunction from returning before the await'ed method call returns. await only delays execution of the code in the async method that follows await, but the method itself will still return before the await'ed method call returns.

If there's progress dialog involved, things can be much more complicated.

Re: How to wait for an async method to complete inside a [LispFunction] ?

bahman.jf.68 — Sat, 31 May 2025 09:35:53 GMT

somewhat I knew that [LispFunction] runs as sync, actually I was looking for a way to stop running the rest of the Lisp code when calling [extractOSMImagesAsync] and before completing it by another way.
Thanks again.

Re: How to wait for an async method to complete inside a [LispFunction] ?

ActivistInvestor — Sat, 31 May 2025 19:23:50 GMT

@bahman.jf.68 wrote:
somewhat I knew that [LispFunction] runs as sync, actually I was looking for a way to stop running the rest of the Lisp code when calling [extractOSMImagesAsync] and before completing it by another way.
Thanks again.

You can prevent your LispFunction from returning until your asynchronous method call returns by just not using async/await. But in that case, you also have to deal with blocking the UI.

See if you can do something with this:

[LispFunction("RunOSMMaps")] public ResultBuffer RunOSMMaps(ResultBuffer resBuf) { Point3d vpll = ...; // parsed from resBuf Point3d vpur = ...; // Other parameters like image path, zoom, etc. ExtractImages runmaps = new ExtractImages(); if (CheckNet()) { var start = DateTime.Now; try // <- Not optional - If an exception leaks from this method, it can crash AutoCAD { var task = runmaps.extractOSMImagesAsync(vpll, vpur, ...); /// Don't allow continuation to run on worker thread: task.ConfigureAwait(false); var elapsed = DateTime.Now - start; // Set a timeout that aborts the operation if // it takes longer than a specified duration // (30 seconds in this example): var timeout = TimeSpan.FromSeconds(30); while(!task.IsCompleted) { elapsed = DateTime.Now - start; if(elapsed > timeout) { UpdateUI("\nError - operation timed out", 0); return null; } UpdateUI($"\rDownloading files {elapsed.TotalSeconds:F2}", 500); } task.GetAwaiter().GetResult(); // forces completion of the async operation } catch(System.Exception ex) { UpdateUI($"\nError: {ex.ToString()}"); return null; } var duration = (DateTime.Now - start).Seconds; UpdateUI($"\nDownload Completed in {duration:F2} seconds.\n"); } return null; } static void UpdateUI(string msg, int duration = 0) { Application.DocumentManager.MdiActiveDocument? .Editor.WriteMessage(msg); if(duration > 0) Thread.Sleep(duration); }