How can i validate this attribute(annonation) ?

Question

I am new about attributes. I just try it on my console application.

So how can i validate my person instance below example ?

class Person
    {
        [StringLength(8,ErrorMessage="Please less then 8 character")]
        public string Name { get; set; }

    }

Answer 1

The only function that Attribute can handle is describe, provide some descriptive data with member. They are purely passive and can't contain any logic. (There are some AOP frameworks that can make attributes active). So if you want logic you have to create another class that will read attributes using MemberInfo.GetCustomAttributes and do the validation and return results.

Answer 2

Below code shows how to determine validation for only properties and give idea validation for methods ,classes etc.

public class DataValidator
    {
        public class ErrorInfo
        {
            public ErrorInfo(string property, string message)
            {
                this.Property = property;
                this.Message = message;
            }

            public string Message;
            public string Property;
        }

        public static IEnumerable<ErrorInfo> Validate(object instance)
        {
            return from prop in instance.GetType().GetProperties()
                   from attribute in prop.GetCustomAttributes(typeof(ValidationAttribute), true).OfType<ValidationAttribute>()
                   where !attribute.IsValid(prop.GetValue(instance, null))
                   select new ErrorInfo(prop.Name, attribute.FormatErrorMessage(string.Empty));
        }
    }

After adding this code to project we can use it like:

    var errors =DataValidator.Validate(person);

    foreach (var item in errors)
    {
        Console.WriteLine(item.Property +" " + item.Message);
    }

Answer 3

Here is simple code example without reflection.

class Program
{
    static void Main(string[] args)
    { 
        var invalidPerson = new Person { Name = "Very long name" };
        var validPerson = new Person { Name = "1" };

        var validator = new Validator<Person>();

        Console.WriteLine(validator.Validate(validPerson).Count);
        Console.WriteLine(validator.Validate(invalidPerson).Count);

        Console.ReadLine();
    }
}

public class Person
{
    [StringLength(8, ErrorMessage = "Please less then 8 character")]
    public string Name { get; set; }
}

public class Validator<T> 
{
    public IList<ValidationResult> Validate(T entity)
    {
        var validationResults = new List<ValidationResult>();
        var validationContext = new ValidationContext(entity, null, null);
        Validator.TryValidateObject(entity, validationContext, validationResults, true);
        return validationResults;
    }
}