An easy Validation pattern implementation for any kind of application...

IValidator.cs

    //Generic interface for Validators
    public interface IValidator<T>
    {
        IEnumerable<ValidationResult> Validate(T businessEntity);
    }

UserManagement.cs

    //A simple example to implement validation rules.
    //I just wrote two seperate validators to clear out the pattern.
    //One validator or more validator can be implemented according to requirment.
    public class UserManagement
    {
        IValidator<User> _userValidator;
        IValidator<User> _ageValidator;
        public UserManagement()
        {
            //TODO apply some dependency injection and IoC 
            _userValidator = new UserNameValidator();
            _ageValidator = new UserAgeValidator();
        }

        public void SaveUser(User entity)
        {
            IEnumerable<ValidationResult> validationResults = _userValidator.Validate(entity);
            if (validationResults > 0)
                throw new SomeKindOfException(validationResults);

            //TODO do your business
        }

        public void CanAccess(User entity)
        {
            IEnumerable<ValidationResult> validationResults = _ageValidator.Validate(entity);
            if (validationResults > 0)
                throw new SomeKindOfException(validationResults);
        }
    }

ValidationResult.cs

    //Just a simple state transfer object for validation
    //This can be enhanced with additional properties
    public class ValidationResult
    {
        public string Message { get; private set; }

        public ValidationResult(string message)
        {
            Message = message;
        }
    }

Validators.cs

    public class UserAgeValidator : IValidator<User>
    {

        public IEnumerable<ValidationResult> Validate(User businessEntity)
        {
            if (businessEntity == null)
                yield return new ValidationResult("Entity can not be null");

            if(businessEntity.Age<18)
                yield return new ValidationResult(String.Format("User's age {0}, is invalid",businessEntity.Age.ToString()));

        }
    }

    //Another example for username check
    public class UserNameValidator : IValidator<User>
    {

        public IEnumerable<ValidationResult> Validate(User businessEntity)
        {
            if (businessEntity == null)
                yield return new ValidationResult("Entity can not be null");
            
            //Let's assume that we have a data repository to check username
            if (UserDataRepository.IsUserExists(businessEntity.UserName))
                yield return new ValidationResult("There is already a user with same username");
        }
    }