PropertyValidator is a versatile library designed to simplify property validation for classes that implement the INotifyPropertyChanged interface. It offers a straightforward way to define and apply validation rules to properties, making it easier to ensure the integrity of your data.

You can install the PropertyValidator library via NuGet:

Install-Package PropertyValidator

using PropertyValidator;

class ViewModel : INotifyPropertyChanged, INotifiableModel
{
    // We are just converting the function to readonly property. This will be accessed by XAML later.
    public IDictionary<string, string?> Errors => validationService.GetErrors();

    // Implement INotifiableModel so it propagates changes to XAML
    public void NotifyErrorPropertyChanged() => RaisePropertyChanged(nameof(Errors));

    // I have only included this here for clarity. Substitute with your own implentation.
    private void RaisePropertyChanged([CallerMemberName] string? propertyName = null)
    {
        PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
    }
}

var validationService = new ValidationService();

// Register your INPC instance for validation
validationService.For(this, delay: TimeSpan.FromSeconds(0.7))
    .AddRule(e => e.FirstName, new StringRequiredRule(), new MinLengthRule(2))
    .AddRule(e => e.LastName, new StringRequiredRule(), new MaxLengthRule(5))
    .AddRule(e => e.EmailAddress, new StringRequiredRule(), new EmailFormatRule(), new RangeLengthRule(10, 15))

<Entry
    Text="{Binding EmailAddress}"
    Keyboard="Email"
    Placeholder="Email address" />
<Label
    Text="{Binding Errors[EmailAddress]}"
    TextColor="Red"
    FontSize="Small" />
...

Test.Maui

PropertyValidator offers a range of features to streamline your validation process:

• Simple and intuitive API for defining validation rules.
• Supports both single-property rules and multi-property rules.
• Allows you to create custom and simple to create validation rules.
• Provides options to manually trigger validation or automatically ensure properties are valid.
• Offers event-based error handling through the PropertyInvalid event.
• Supports delayed validation to enhance user experience.
• Format error messages using SetErrorFormatter(errorMessages => ...).

ValidationPack contains a set of common validation rules to cover popular input validation scenarios. The ValidationPack includes the following rules:

• StringRequiredRule: Ensures that a string property is not empty or null.
• MaxLengthRule: Validates that a string does not exceed a specified maximum length.
• MinLengthRule: Checks that a string meets a specified minimum length.
• RangeLengthRule: Validates that a string falls within a specific length range.
• EmailFormatRule: Ensures that a string follows a valid email format.

PropertyValidator allows you to create custom validation rules to suit your specific validation requirements. To create a custom validation rule, follow these steps:

1. Create a new class that inherits from ValidationRule<T>, where T is the type of the property you want to validate.

2. Implement the IsValid(T value) method in your custom rule class. This method should return true if the value is valid according to your rule, and false otherwise.

3. Override the ErrorMessage property to provide a custom error message that will be displayed when the validation fails.

Here's an example of creating a custom validation rule to check if a string contains only alphanumeric characters:

using PropertyValidator;

public class AlphanumericRule : ValidationRule<string>
{
    public override string ErrorMessage => "Only alphanumeric characters are allowed.";

    public override bool IsValid(string value)
    {
        if (string.IsNullOrEmpty(value))
            return true;

        // Use regular expression to check if the string contains only alphanumeric characters
        const string pattern = "^[a-zA-Z0-9]*$";
        return Regex.IsMatch(value, pattern);
    }
}

Now, you can use your custom validation rule in your validation service. Here's an example of how to use the AlphanumericRule:

var validationService = new ValidationService();

// Register your model for validation and include the custom rule
validationService.For(this)
    .AddRule(e => e.Username, new AlphanumericRule());

Feel free to contribute to the project, report issues, or provide feedback to help us improve PropertyValidator.