Manual Validation with Data Annotations

Several people have asked me about using data annotations for validation outside of a UI framework, like ASP.NET MVC or Silverlight. The System.ComponentModel.DataAnnotations assembly contains everything you need to execute validation logic in the annotations. Specifically, there is a static Validator class to execute the validation rules. For example, let's say you have the following class in a console mode application:

public class Recipe
{
    [Required]
    public string Name { get; set; } 
}

You could validate the recipe with the following code:

var recipe = new Recipe();
var context = new ValidationContext(recipe, serviceProvider: null, items: null);
var results = new List<ValidationResult>();

var isValid = Validator.TryValidateObject(recipe, context, results);

if (!isValid)
{
    foreach (var validationResult in results)
    {
        Console.WriteLine(validationResult.ErrorMessage);
    }
}

Result: "The Name field is required".

You can construct the ValidationContext class without a service provider or items collection, as shown in the above code, as they are optional for the built-in validation attributes. The Validator also executes any custom attributes you have defined, and for custom attributes you might find the serviceProvider useful as a service locator, while the items parameter is a dictionary of extra data to pass along. The Validator also works with self validating objects that implement IValidatableObject.

public class Recipe : IValidatableObject 
{
    [Required]
    public string Name { get; set; }

    public IEnumerable<ValidationResult> Validate(ValidationContext validationContext)
    {
        // ...
    }
}

Like everything framework related, the Validator tries to provide an API that will work in a number of validation scenarios. I recommend you bend it to your will and build something that makes it easy to use inside your specific design and architecture. A class like the following would be a step towards hiding some complexity.

public class DataAnnotationsValidator
{
    public bool TryValidate(object @object, out ICollection<ValidationResult> results)
    {
        var context = new ValidationContext(@object, serviceProvider: null, items: null);
        results = new List<ValidationResult>();
        return Validator.TryValidateObject(
            @object, context, results, 
            validateAllProperties: true
        );
    }
}