Why doesn't TryAsync wrap up this exception?

[MrYossu]

I'm trying to use TryAsync to ensure that my methods return something the consumer can handle neatly. I thought that the point of TryAsync was that if an exception was thrown in the code inside, it would return the exception, which could be handled in the appropriate action passed to Match.

However, I have the following code. This makes calls to the Google Drive API, but the details of that aren't relevant...

    public TryAsync<DriveFile> GetParentFolder(string folderId) =>
      TryAsync(() => GetParentFolderDo(folderId));

    private async Task<DriveFile> GetParentFolderDo(string folderId) {
      FilesResource.GetRequest request = _service.Files.Get(folderId);
      request.Fields = "id, name, parents";
      DriveFile folder = await request.ExecuteAsync();
      request = _service.Files.Get(folder.Parents[0]);
      return await request.ExecuteAsync();
    }

This is fine when the folderId is not for the root folder. However, if I pass in the Id of the root folder, then the folder.Parents collection is null (why not empty? Don't ask me). This means that the penultimate line of code throws an NRE.

I was expecting the NRE to be caught and handled by TryAsync, but instead, my code crashes.

What am I doing wrong? Thanks

[Protiguous]

Don't mind me, I'm just a bystander on this project, but I don't think the issues you are having has anything to do with language-ext.
Your questions would be better asked somewhere like StackOverflow.com, in my opinion.
I would also suggest breaking down your lines so you're not trying to do everything in one function.

[MrYossu]

@Protiguous Thanks for the reply. Why don't you think this is to do with LanguageExt? As I understand it, the whole point of Try and TryAsync is that they catch exceptions and return a monad that wraps either the desired result or the exception. That's not a C# feature, it's part of the monad that is in this package.

Please clarify if I misunderstood. Thanks again.

[Protiguous]

Can you step into the function, line by line, and see when the NRE happens? It may be before language-ext even has a chance.

[MrYossu]

@Protiguous Duh, just realised that the code I posted isn't even using the TryAsync stuff properly! I had posted an old version of the code.

I've updated the question, and now you can see that all the action happens inside the call to TryAsync(), so the NRE, which happens when it tries to access the null Parents property of the folder variable is in fact within the scope of the LanguageExt call.

Sorry about the confusion, does the correct code clarify? Thanks again.

[MrYossu]

@Protiguous Duh part II.

Now I tried again with my updated code, it works fine!

I think I need more sleep! Thanks for the replies.

[Protiguous]

Hah, I'm glad you fixed it. Good luck on future code!

[MrYossu]

@Protiguous Thanks - with this track record I'm going to need it!