Shared Validation

StringHasValueSpecification`1.cs

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

    private readonly int minimumLength;

    // Require the validator to pass in a property selector.
    // Having something pass in the minimum length violates the concept of a "specification"
    // because now the spec does nothing but do a compare.
    // The specification doesn't actually act as a spec by specifying what the minimum length is.
    public MinimumLengthSpecification(Func<TEntity, string> propertySelector, int minimumLength)
    {
        this.propertySelector = propertySelector;
        this.minimumLength = minimumLength;
    }

    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 >= this.minimumLength;
    }
}

UserValidator.cs

public class UserValidation : Validator<User>
{
    public UserValidation()
    {
        base.AddSpecifications(
            new EmailAddressFormatIsValidSpec(),
            new PasswordIsRequiredSpec(),
            new MinimumLengthSpecification<User>(model => model.Password, minimumLength: 8),
            new MinimumLengthSpecification<User>(model => model.Username, minimumLength: 6),
            new UsernameIsRequiredSpec());
    }
}