Proposal: Data Annotation Validation Behavior for .NET MAUI (Contribution)

[jglstrausss]

I’d like to start a discussion around contributing a new "DataAnnotationValidationBehavior" to the CommunityToolkit for .NET MAUI.

Data annotation attributes (like [Required], [StringLength], [EmailAddress], etc.) are a natural fit for .NET developers coming from MVC or MVVM validation patterns. Currently, MAUI provides ValidationBehavior, but it doesn’t support model-based data annotation validation out of the box.

To fill this gap, I’ve created a working prototype behavior that:

• Uses System.ComponentModel.DataAnnotations.Validator to validate bound objects or properties.
• Displays validation errors via the existing ValidationBehavior visual cues.
• Supports IValidatableObject for more complex validation scenarios.
• Integrates well into MVVM patterns.

I'd like to propose adding this behavior into the toolkit under CommunityToolkit.Maui.Behaviors, making data annotation-based validation more accessible to MAUI devs.

Before I go further, I’d love to:

• Hear your thoughts on including this kind of behavior in the toolkit.
• Understand any constraints or design patterns you'd prefer I follow.

If there's interest, I’d be happy to clean up the code, write tests, and follow contribution guidelines.

[bijington]

I really like this idea and would love to see the prototype. The only thing that I would want to highlight is to that we aim to avoid using any reflection when possible. Code that looks up usages of attributes may be ok here though

[jglstrausss]

In my current prototype I'm specifying the property name and path and do some reflection to get the property. I can reduce the reflection to bind the property on the behavior, but I will still need the following to lookup the attributes:

var validationAttributes = this.sourceProperty
    .GetCustomAttributes(typeof(ValidationAttribute), true)
    .Cast<ValidationAttribute>()
    .ToList();

[pictos]

Instead of reflection it can use SG. Keep in mind that asp.net runs on super computers at cloud connect directly to a source power and mobile apps can be very poor machines that uses battery, so avoid these overheads in hot paths is always good

[jglstrausss]

Using SG to generate a lookup store for the validation attributes on a property?

[pictos]

You want to validate if the value set is valid based on conditions configured by attributes, so with partial properties you can override the set and validate that directly.

I think something like that

// user code

[Required]
[MaxLength(10)]
public partial string Name {get; set;}

// SG

public partial string Name
{
	get => field;
	set
	{
		if (string.IsNullOrEmpty(value))
			throw new Exception(); // Or trigger an event or method or whatever
		if (string.Length > 10)
			throw new Exception();

		field = value;
	}
}

With that, do you need a look up?

[jglstrausss]

Thanks for the input, I’m not opposed to the partial property approach at all, it’s a solid idea. My main goal is to avoid runtime reflection, and I’m exploring whether a source generator could produce a centralized store of validation attributes to keep things performant and DRY, especially for larger forms.

I’ll investigate the pros and cons of both approaches. The one thing I want to ensure is that we don’t lose the flexibility of custom setters or inline validation logic where it’s needed.