Validation 2.0

MultiModelValidator.cs

public class AccountValidator : Validator<Account>
{
    public AccountValidator()
    {
        base.AddSpecifications(
            new EmailAddressFormatIsValidSpec(),
            new PasswordIsRequiredSpec(),
            new StringIsNotEmpty<User>(model => model.Username),
            new StringIsNotEmpty<Account>(model => model.AccountNumber),
            new UsernameIsRequiredSpec());
    }
}

StringHasAtLeast8Characters`1.cs

/// <summary>
/// Specification to ensure a string property on a model has a value
/// </summary>
public class StringHasAtLeast8Characters<TEntity> : ISpecification<TEntity>
{
    // Store the property selector
    private readonly Func<TEntity, string> propertySelector;

    // Require the validator to pass in a property selector.
    public StringHasAtLeast8Characters(Func<TEntity, string> propertySelector)
    {
        this.propertySelector = propertySelector;
    }

    public string ErrorMessage { get; private set; }

    public bool IsSatisfiedBy(TEntity entity)
    {
        // Invoke the delegate so we can get the value from the property specified by the validator.
        string propertyValue = this.propertySelector(entity);

        // Validate
        return propertyValue.Length >= 8;
    }
}

StringIsNotEmpty`1.cs

public class StringIsNotEmpty<TEntity> : ISpecification<TEntity>
{
    // Store the property selector
    private readonly Func<TEntity, string> propertySelector;

    // Require the validator to pass in a property selector.
    public StringIsNotEmpty(Func<TEntity, string> propertySelector)
    {
        this.propertySelector = propertySelector;
    }

    public string ErrorMessage { get; private set; }

    public bool IsSatisfiedBy(TEntity entity)
    {
        // Invoke the delegate so we can get the value from the property specified by the validator.
        string propertyValue = this.propertySelector(entity);

        // Validate
        return !string.IsNullOrEmpty(propertyValue);
    }
}

UserValidator.cs

public class UserValidator : Validator<User>
{
    public UserValidator()
    {
        base.AddSpecifications(
            new EmailAddressFormatIsValidSpec(),
            new PasswordIsRequiredSpec(),
            new StringHasAtLeast8Characters<User>(model => model.Password),
            new StringIsNotEmpty<User>(model => model.Username),
            new UsernameIsRequiredSpec());
    }
}