Multiple validation errors

[TysonMN]

The INotifyDataErrorInfo interface supports multiple errors per property. Elmish.WPF is currently restricting this to a single error (see line 838).

Elmish.WPF/src/Elmish.WPF/ViewModel.fs

Lines 830 to 839 in db20825

	interface INotifyDataErrorInfo with
	[<CLIEvent>]
	member __.ErrorsChanged = errorsChanged.Publish
	member __.HasErrors =
	errors.Count > 0
	member __.GetErrors propName =
	log "[%s] GetErrors %s" propNameChain (propName |> Option.ofObj |> Option.defaultValue "<null>")
	match errors.TryGetValue propName with
	| true, err -> upcast [err]
	| false, _ -> upcast []

The branch demo/multiple_validation_errors contains a proof of concept that shows how to show multiple validation errors in WPF. See the screenshot below. The only hack there is the margin I added to the Submit button so that it wasn't covered by the additional validation errors.

Would anyone like this feature?

I think this feature can be added while remaining backwards compatible, so I think it is fine to exclude this feature from v4.

[cmeeren]

Hm, I think I remember considering this way back. Not sure why I landed the way I did. Are there any built-in or popular 3rd party controls that actually support showing multiple errors?

In any case, I'm not opposed to this.

[TysonMN]

Are there any built-in or popular 3rd party controls that actually support showing multiple errors?

Not sure. To repeat for clarity, I was able to display multiple errors without too much trouble. I added a screenshot of that to my previous comment.

Also, I meant to say that I left the tooltip as it was, which is still just showing the first error in the list. That can also be seen in the newly added screenshot.

[BentTranberg]

Showing multiple errors is very useful. E.g. if a password is too long and is missing certain classes of characters, you want to tell both facts simultaneously.

[TysonMN]

I created PR #315 to resolve this issue.