Add item.md

Add value to enumeration

Summary

1. Check all usages of the enum type
2. Check all usages of the containing class

Introduction and terminology

We have a class Account, which contains a property AccountType The account type is Credit or Debit, and we want to add a type Mortgage

1. Check existing behaviour

Inclusive filters

When we want to search for debt a person has, we now return Accounts where Type is Credit.

var debt = accounts.Where(acc => acc.AccountType == AccountType.Credit)

This logic breaks when we add Mortgage type, because a mortgage is also a form of debt.

To resolve this:

1. Search for all usages of the AccountType enum and see where logic needs to be updated

Exclusive filters

There's also the option where the query could be setup like this:

var debt = accounts.Where(acc => acc.AccountType != AccountType.Debit)

Note the != here

1. Check all usages of the Account class (all classes that use AccountType), and see where new logic needs to be implemented

2. Introduce new behaviour

Example: to query the balance of the account, we query all Accounts for a person. However, we only want to see liquid funds, so we should add an introduction of a filter to only return Accounts where Type is Credit or Debit

Imagine this code:

var spendableBalance = accounts.Sum(x => x.Balance);

There is no usage of the AccountType here, so using #1 doesn't apply here, as it doesn't show up in our search results. The way to find where the logic needs to be introduced is to search for all usages of the Account class.

2. Find where the enum is used, and query all usages of those classes. Where logic needs to be introduced, introduce it.

TODO

1. When introducing logic, you can add equality and inequality checks. When is Type == Credit || Type == Debit better than Type != Mortgage
2. What about libraries? You don't know the ways a consumer uses your code.