Dictionary<T> look-up or create made simpler

The design of a Dictionary<T> lends itself well to a caching or identification mechanism and as a result you often see code that looks like this:

private static Dictionary<string, Employee> employees = new Dictionary<string, Employee>();

public static Employee GetByName(string name) {
  Employee employee;
  if (!employees.TryGetValue(name, out employee)) {
    employee = new Employee(whatever);
    employees.Add(name, employee);
  }
  return employee;
}

It’s not that it is particularly difficult but it can be a bit error prone and when you’re doing it over and over. What would be nicer is something that let you do:

public static Employee GetByName(string name) {
  return employees.GetOrAdd(name, () => new Employee(whatever));
}

Here’s an extension method to drop-in to a static class of your choosing that achieves just that.

public static TDictionaryValue GetOrAdd<TKey, TDictionaryValue>(
  this IDictionary<TKey, TDictionaryValue> dictionary,
  TKey key,
  Func<TDictionaryValue> newValue
) {
  TDictionaryValue value;
  if (!dictionary.TryGetValue(key, out value)) {
    value = newValue.Invoke();
    dictionary.Add(key, value);
  }
  return value;
}

[)amien