Class Custom Validators & Data Annotations

EXAMPLE 1:

using System.ComponentModel.DataAnnotations;

public class Book {
    [CustomValidation(typeof(Book), "ValidateContact")]
    public Contact PrimaryContact { get; set; }

    [CustomValidation(typeof(Book), "ValidateContact")]
    public Contact SecondaryContact { get; set; }

    [Required(ErrorMessage = "Book name is required")]
    public string Name { get; set; }

    public static ValidationResult ValidateContact(Contact contact, ValidationContext context) {
        ValidationResult result = null;

        if (contact == null) {
            result = new ValidationResult(string.Format("{0} is required.", context.DisplayName));
        } else if (string.IsNullOrWhiteSpace(contact.Name)) {
            result = new ValidationResult(string.Format("{0} name is required.", context.DisplayName));
        }

        return result;
    }
}

public class Contact {
    [Required(ErrorMessage = "Name is required")]
    public string Name { get; set; }
}

EXAMPLE 2:

public IEnumerable<ValidationResult> Validate()
{
   return Validate(new ValidationContext(this));
}

public IEnumerable<ValidationResult> Validate(ValidationContext validationContext)
{
   var results = new List<ValidationResult>();
   Validator.TryValidateObject(this, validationContext, results, true);
   return results;
}

EXAMPLE 3:

NO CUSTOMIZED:

[RegularExpression(@"^.{5,}$", ErrorMessage = "Minimum 3 characters required")]
[Required(ErrorMessage = "Required")]
[StringLength(30, MinimumLength = 3, ErrorMessage = "Invalid")]

CUSTOMIZED:

public class MyStringLengthAttribute : StringLengthAttribute
{
    public MyStringLengthAttribute(int maximumLength)
        : base(maximumLength)
    {
    }

    public override bool IsValid(object value)
    {
        string val = Convert.ToString(value);
        if (val.Length < base.MinimumLength)
            base.ErrorMessage = "Minimum length should be 3";
        if (val.Length > base.MaximumLength)
            base.ErrorMessage = "Maximum length should be 6";
        return base.IsValid(value);
    }
}

public class MyViewModel
{
    [MyStringLength(6, MinimumLength = 3)]
    public String MyProperty { get; set; }
}