Dialogs in Xamarin Forms can't RequestClose without a handler first being given to Show() [Bug] [XF]

[UnreachableCode]

Description

Hi everyone. I've been trying out the new dialog service in 7.2.0.1367 for Xamarin Forms. I've been able to get it working but only when providing a callback handler. Otherwise I get a vague null reference exception.

This isn't explained in the release notes or the spec or PR but I think it should be. I think it should also be possible to not provide a handler and for the dialog to just dismiss.

Steps to Reproduce

1. Create a dialog View and ViewModel as outlined in the release notes.
2. Open your dialog when the app is running using dialogService.ShowDialog("MyDialog")
3. Fire your close command which should call RequestClose(null).

Expected Behavior

Dialog should close.

Actual Behavior

Program crashes with null reference exception. If a callback is provided, then this doesn't happen, like so: dialogService.ShowDialog("MyDialog", CloseDialogCallback).

Basic Information

• Version with issue: Prism 7.2.0.1367
• Last known good version: N/A, new feature.
• Xamarin.Forms version: 4.1.0.618606
• IDE: Visual Studio for Mac Community 8.2.3

[brianlagunas]

Closing as no reproduction app provided.

[brianlagunas]

Also, make sure you check for null before calling the RequestClose. This is most likely your problem.

[UnreachableCode]

I'm afraid this doesn't fix it

[brianlagunas]

We'll need a repo app then

[UnreachableCode]

On it

[UnreachableCode]

It's going to be a little longer until I can reproduce this on a repo app.

[hro-it]

I'm afraid this doesn't fix it

Would like to second that I'm receiving the exact same NullReferenceException at the exact same line. Only solution is to provide a callback handler even if unnecessary.

[hro-it]

This issue needs to be reopened as I may have discovered the problem to it.

It appears that there is an extension helper interface known as IDialogServiceExtensions that aids the developer in offering overloads for a multitude of scenarios. However, they are merely calling the original method signature ShowDialog(string name, IDialogParameters parameters, Action<IDialogResult> callback.

The trick is that in this particular implementation of the interface IDialogService the callback parameter is always assumed to be valid instead of a potentially null argument.

Implementation of method signature ShowDialog(this IDialogService dialogService, string name) in IDialogServiceExtensions shows that you are deliberately just passing a null value. A check needs to be implemented into the DialogService class to ensure that the callback is valid prior to call when executing void DialogAware_RequestClose(IDialogParameters outParameters).

I do not want to perform a pull request as I do not have an extremely thorough understanding of all the code in this new feature yet, but hope that this will send everyone down the correct path in regards to debugging.

[dansiegel]

@hro-it if you'd like to provide a sample that reproduces the issue I'd be happy to reopen the issue

[hro-it]

@hro-it if you'd like to provide a sample that reproduces the issue I'd be happy to reopen the issue

https://github.com/hro-it/PrismDialogIssueApp

Sorry that took so long. Had some issues with Android emulation and the new Windows Sandbox interfering with one another. This problem is reproducible on both Android and iOS.

The project is just setup as a .zip file for you to download, build, and run.

[dansiegel]

@hro-it thanks for providing a sample. I'll look into this as soon as I get a moment.

[lock]

This thread has been automatically locked since there has not been any recent activity after it was closed. Please open a new issue for related bugs.