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)
    {
        // ...
    }
}

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