Using DataAnnotations on Windows Forms project

Here's a simple example. suppose you have an object like the following

using System.ComponentModel.DataAnnotations;

public class Contact
{
    [Required(AllowEmptyStrings = false, ErrorMessage = "First name is required")]
    [StringLength(20, MinimumLength = 5, ErrorMessage = "First name must be between 5 and 20 characters")]
    public string FirstName { get; set; }

    public string LastName { get; set; }

    [DataType(DataType.DateTime)]
    public DateTime Birthday { get; set; }
}

And suppose we have a method that creates an instance of this class and tries to validate its properties, as listed below

    private void DoSomething()
    {
        Contact contact = new Contact { FirstName = "Armin", LastName = "Zia", Birthday = new DateTime(1988, 04, 20) };

        ValidationContext context = new ValidationContext(contact, null, null);
        IList<ValidationResult> errors = new List<ValidationResult>();

        if (!Validator.TryValidateObject(contact, context, errors,true))
        {
            foreach (ValidationResult result in errors)
                MessageBox.Show(result.ErrorMessage);
        }
        else
            MessageBox.Show("Validated");
    }

The DataAnnotations namespace is not tied to the MVC framework so you can use it in different types of applications. the code snippet above returns true, try to update the property values to get validation errors.

And make sure to checkout the reference on MSDN: DataAnnotations Namespace

Steve's example is a bit dated (though still good). The DataAnnotationsValidationRunner that he has can be replaced by the System.ComponentModel.DataAnnotations.Validator class now, it has static methods for validating properties and objects which have been decorated with DataAnnotations attributes.