Darp.CommunityToolkit.MvvmValidator 0.1.0

Darp.CommunityToolkit.MvvmValidator

An AOT- and trimming-compatible replacement for ObservableValidator from CommunityToolkit.Mvvm. It builds on the released toolkit; no fork needed.

ValidatingObservableObject derives from ObservableObject and implements INotifyDataErrorInfo. A source generator writes the validation for each derived type, so no reflection runs at runtime.

using System.ComponentModel.DataAnnotations;
using CommunityToolkit.Mvvm.ComponentModel;
using Darp.MvvmValidator;

public partial class SignUpViewModel : ValidatingObservableObject
{
    public SignUpViewModel() => ValidateAllProperties();

    [ObservableProperty]
    [Required]
    [StringLength(20, MinimumLength = 3)]
    [Display(Name = "User name")]
    public partial string? UserName { get; set; }

    [ObservableProperty]
    [Required]
    public partial string? Password { get; set; }

    [ObservableProperty]
    [Compare(nameof(Password))]
    public partial string? ConfirmPassword { get; set; }

    [CustomValidation(typeof(SignUpViewModel), nameof(ValidateInvite))]
    public string? InviteCode
    {
        get;
        set => SetProperty(ref field, value);
    }

    public static ValidationResult? ValidateInvite(string? code, ValidationContext context) =>
        code is null or "WELCOME" ? ValidationResult.Success : new ValidationResult("Unknown invite code");
}

Behavior

• Every property with validation attributes is validated whenever it raises PropertyChanged, before subscribers are notified. This covers [ObservableProperty] fields and partial properties as well as hand-written SetProperty setters. [NotifyDataErrorInfo] and SetProperty(..., validate: true) are not needed.
• Validation runs like Validator.TryValidateProperty: [Required] runs first and stops validation if it fails, then all other attributes run.
• ValidateAllProperties() validates the current values, for example from the constructor.
• SetErrors(propertyName, errors) publishes results from asynchronous or server-side validation. The next validation of that property replaces them. ErrorsChanged is only raised when the messages change.
• ClearErrors() removes all errors, ClearErrors(propertyName) those of one property. Both forget that the properties were validated, so a reset form stays free of errors until its properties change again.
• [CustomValidation] calls the method directly, and [Compare] reads the other property directly. Both work without reflection.
• Messages and display names can come from resources (ErrorMessageResourceType/ErrorMessageResourceName, [Display(ResourceType = ...)]). The resource properties are read with reflection that the trimmer can prove through [DynamicallyAccessedMembers], so they also work with Native AOT.
• When the target of a [Compare] changes, the comparing property is validated again, but only if it was validated before. An untouched "repeat password" field stays free of errors while the user types the password.

Requirements

• .NET 10. Older frameworks don't have a trim-safe ValidationContext constructor.
• Types that declare validated properties, and their containing types, must be partial and must not be file-local.
• Attributes that can only target properties, like [Compare], need the property: target on [ObservableProperty] fields: [property: Compare(nameof(Password))].
• Generated members start with __dmv_. Don't declare members with that prefix in view models.

Diagnostics

The package turns off the toolkit's MVVMTK0006, which only accepts ObservableValidator, and reports these instead:

The trim analyzer reports IL2026 where [Compare], [MinLength], [MaxLength] or [Length] is applied. The package suppresses these warnings when the generated code never reaches the reflection they warn about.

Preview packages

Every pull request publishes a preview package to the GitHub Packages feed https://nuget.pkg.github.com/rosslight/index.json. The pull request comment names the version. The feed requires a GitHub token with read:packages access.

Development

• Format with CSharpier: dotnet tool restore and dotnet csharpier format .
• tests/Darp.CommunityToolkit.MvvmValidator.Tests runs the runtime behavior. CI also publishes it with Native AOT and /warnaserror, which proves the generated validation is trim safe: dotnet publish tests/Darp.CommunityToolkit.MvvmValidator.Tests -c Release -r win-x64 /warnaserror
• tests/Darp.CommunityToolkit.MvvmValidator.Generator.Tests covers the analyzer and snapshots the generated code with Verify. Review changed snapshots before accepting them.
• Releases are cut by release-please from Conventional Commits on main.