Simplify SendGridMessage internal code

[thinkingserious]

Issue Summary

Refactor repeated code out into utility functions. e.g. https://github.com/sendgrid/sendgrid-csharp/blob/master/src/SendGrid/Helpers/Mail/SendGridMessage.cs#L142

[pbolduc]

I have have a signed contributor agreement ready to send.
I have been working on a change for this issue. However, I am unclear on the expected behavior of these methods when you pass a non-zero personalizationIndex or a non-null personalization object. I have plans on create unit tests to validated the behavior before and after to ensure no regressions. For example, if I do:

    var emailAddress = new EmailAddress();
    var personalization = new Personalization();
    _sut.AddTo(emailAddress, 0, personalization);

This throws NullReferenceException because the Tos collection is not initialized.

    var emailAddress1 = new EmailAddress();
    var personalization = new Personalization();
    personalization.Tos = new List<EmailAddress>();
    _sut.AddTo(emailAddress1, personalization: personalization);

    var emailAddress2 = new EmailAddress();
    _sut.AddTo(emailAddress2, 2);

This throws ArgumentOutOfRangeException cause this code checks of the index is greater than the count and insert at the point. I think the expression should be personalizationIndex < this.Personalizations.Count (note, avoid using Linq methods when collections have a property) or it should just be an Add() call instead of Insert or it should check the parameters better.

Thoughts?

[SendGridDX]

Hello @pbolduc,

For the AddTo method, if you are passing in a personalization, I am assuming that personalization is already initialized. The idea is that you may have already defined several personalizations and now you want to go back and add an email to a particular one.

Regarding the second code example, when you execute _sut.AddTo(emailAddress2, 2); the code should create a new personalization that will be at position 2. I believe you are suggesting that simply append it to the next available location in the list. Perhaps the out of range issue is happening because we only have 1 personalization at this point, which should be at position 0.

[pbolduc]

On the AddTo, it would be possible to have a personalization already initialized but the associated collection Tos / Bccs / Ccs not initialized yet. It could simplify a bunch of things if each of the models that contain collections, that the collections are auto initialized with empty lists. The down side is extra memory is allocated and will need to be garbage collected. Also, during serialization, extra checks would be needed to ensure empty arrays are not serialized.

On the second example, you are correct in that I do no have that many items in the collection. Either the API could throw an error (recommended) or append the new personalization at the end of the collection. To me it would make sense to throw an error because caller would expect to be able to get the newly created personalization at message.Personalization[2]. this would throw index out of bounds exception.

Tightening up the API will make it easier and less error prone for the users. However, it must be consistent with the other language SDKs.

[SendGridDX]

@pbolduc,

How about adding a constructor here that initializes all the minimally required fields.