Output better XAML errors

[grokys]

Avalonia XAML error messages are sometimes less than ideal. This can be a big issue particularly for new users as the error messages tend to confuse more than help. Here's a few problems with them:

• They duplicate the line/column numbers
• They include the csproj which isn't (ever?) useful
• The type information is noisy (always includes assembly and full namespace)
• Types/members not quoted
• They don't always give the correct column info (the open tag is marked as having the error when in reality the error occurs on e.g. a property)
• The error message isn't always clear
• They include irrelevant information
• Setting value to a readonly property should show a better error TextBox throws exception if CanCopy / CanCut / CanPaste / CanRedo / CanUndo being modified #12218

Some examples follow. This list isn't exhaustive but shows some of the more common errors:

Missing property

Invalid XAML

<Button Foo="Bar"/>

Avalonia

Unable to resolve suitable regular or attached property Foo on type Avalonia.Controls:Avalonia.Controls.Button (line 5 position 6) Line 5, position 6. [D:\projects\AvaloniaUI\Avalonia\samples\ControlCatalog\ControlCatalog.csproj]

WPF

The property 'Foo' does not exist in XML namespace 'http://schemas.microsoft.com/winfx/2006/xaml/presentation'. Line 4 Position 13.

Evaluation

• Duplicate line/column numbers
• csproj path is included
• Types/members not quoted
• This isn't attached property format so no need to mention "attached property"
• Avalonia.Controls:Avalonia.Controls.Button is overkill: Button would suffice
• The WPF error isn't much better to be honest

Better Error Message

Unable to resolve property 'Foo' on type 'Button'. Line 5, position 6.

Missing Event Handler

Invalid XAML:

<Button Click="Foo"/>

Avalonia:

Unable to find suitable setter or adder for property Click of type Avalonia.Controls:Avalonia.Controls.Button for argument netstandard:System.String, available setter parameter lists are: [D:\projects\AvaloniaUI\Avalonia\samples\ControlCatalog\ControlCatalog.csproj]
System.EventHandler`1<Avalonia.Interactivity.RoutedEventArgs> (line 5 position 13) Line 5, position 13. [D:\projects\AvaloniaUI\Avalonia\samples\ControlCatalog\ControlCatalog.csproj]

WPF:

'MainWindow' does not contain a definition for 'Foo' and no accessible extension method 'Foo' accepting a first argument of type 'MainWindow' could be found (are you missing a using directive or an assembly reference?)

Evaluation

• netstandard:System.String is overkill
• System.EventHandler``1<Avalonia.Interactivity.RoutedEventArgs> isn't particularly useful for new users
• Appears as two error messages
• Says "property Click" but Click is an event
• Again, WPF isn't much better though at least it's less verbose

Better Error Message

Unable to find suitable handler for event 'Button.Click'. Valid handler signature is 'void Button_Click(object sender, RoutedEventArgs e)'.

Invalid value

Invalid XAML:

<Button HorizontalAlignment="Foo"/>

Avalonia:

Unable to find suitable setter or adder for property HorizontalAlignment of type Avalonia.Layout:Avalonia.Layout.Layoutable for argument netstandard:System.String, available setter parameter lists are: [D:\projects\AvaloniaUI\Avalonia\samples\ControlCatalog\ControlCatalog.csproj]
Avalonia.UnsetValueType [D:\projects\AvaloniaUI\Avalonia\samples\ControlCatalog\ControlCatalog.csproj]
Avalonia.Data.IBinding [D:\projects\AvaloniaUI\Avalonia\samples\ControlCatalog\ControlCatalog.csproj]
Avalonia.Layout.HorizontalAlignment (line 5 position 13) Line 5, position 13. [D:\projects\AvaloniaUI\Avalonia\samples\ControlCatalog\ControlCatalog.csproj]

WPF:

Requested value 'Foo' was not found.

Evaluation

• Appears as three error messages
• The info about UnsetValueType/IBinding isn't very useful as this obviously isn't a binding

Better Error Message

'Foo' is not a valid value for 'HorizontalAlignment'.

[grokys]

Another from gitter: https://gitter.im/AvaloniaUI/Avalonia?at=5f32b56eb103e45c830e2c2d

[rstm-sf]

Maybe add warning support as well? For example, Obsolete

[maxkatz6]

"Unable to find the parent scope for AvaloniaProperty lookup" error can add a hint about where TemplateBinding can be used. And recommendation to use Bindings instead:
#15572

[maxkatz6]

This too: #16144

[maxkatz6]

This too #16580 (comment)

[maxkatz6]

#17562