Combining code contracts and regex

Question

So I have a very simple class with one string as property. This string has to have a certain pattern. I'm trying to enforce this using code contracts. The class looks something like this:

class SimpleClass
{
    public Property { get; set; }

    public SimpleClass(string prop)
    {
      Contract.Requires(IsValid(prop));
      this.Property = prop;
    }

    [ContractInvariantMethod]
    void ObjectInvariant()
    {
      Contract.Invariant(IsValid(Property));
    }

    bool IsValid(string arg)
    {
      // Use regex to check if arg is a valid string
    }
}

Very straightforward. However, this throws an unreadable exception and another one saying that 'Member SimpleClass.IsValid has less visibility than the enclosing method SimpleClass.#ctor(System.String)'. Why is this illegal? Should I copy/paste the regex into both methods? That seems to be the opposite of right. Please help me understand!

Answer 1

Another way is avoid 'primitive obsession' and use a class tailored to your purpose, e.g.:

public SimpleClass(Email address)
{
    // no need to check, it must be valid :)
}

... and then encapsulate all your validation logic in the Email class. You'll still have the "string format" issues about validation, but I think a better idiom for this is to create a method called Email.TryParse, and fashion it along the lines of int.TryParse.