ObservableValidatorDemo

Instruction file imported from JDKoll1982/MTM_Waitlist (.github/instructions/demos/Mvvm/ObservableValidatorDemo.instructions.md). Copyright stays with the author.

ObservableValidator

What it teaches

Data-annotation validation wired into INotifyDataErrorInfo, so error text reaches the control.

How the demo is built

The page is a UserControl at Mvvm/ObservableValidatorDemo.xaml, paired with ObservableValidatorDemo.xaml.cs. The shell hosts it inside Shell/DemoPage.xaml, which also shows both files verbatim — the snippet viewer reads the same source the app compiles, so what is displayed is always what runs.

Layout. Root UserControl with 20 element(s).

Code-behind. Handlers OnErrorsChanged(). 82 line(s), including the comments that record what the control does that the docs do not say.

Anything the demo cannot show is explained on the page rather than faked: the same rules the rest of the repo follows are in .github/instructions/winui3-api-rules.instructions.md.

MTM use case

MTM_WIP_Application_WinForms › Inventory › transaction entry

A transaction is only valid when part, bin, quantity and operation are all present, so the commit command is disabled until the view model says otherwise. State changes have to reach the grid without the code-behind touching it.

Schema this mirrors: inv_transaction, inv_inventory, inv_inventory_batch_seq. Sample data for the page comes from src/WinUI3LearningLab.Core/LabData.cs — part numbers, bins, dunnage types and work orders, never Item 1 or John Doe.

Gotchas

• Validation runs on SetProperty only when you call the generated setter with validate: true.
• The XAML side needs the binding to surface errors; a missing Binding.ValidatesOnNotifyDataErrors leaves the message invisible.
• Validating on every keystroke marks a field invalid before the user finishes typing — validate on lost focus for text input.

Try this

• Run WinUI3LearningLab.App.exe --demo observable-validator. The page shows ObservableValidatorDemo.xaml and ObservableValidatorDemo.xaml.cs beside the live demo, so the next edit can be made against something already running.
• Change one value in the XAML and rebuild; the snippet viewer re-reads the file, so what is shown stays the code that compiled.

Documentation

• CommunityToolkit.Mvvm.ComponentModel.ObservableValidator
• WinUI 3 API rules for this repo
• Design system

Generated from DemoCatalog by --export-instructions. Do not edit by hand: change the descriptor in src/WinUI3LearningLab.Core/Catalog/Mvvm.cs or the demo source, then regenerate.