Question

Similar to this question:
C# Constructor Design but this question is slight different.

I have a class Customer and a class CustomerManager. When an instance is created of the CustomerManager class I want to load all the customers. And this is where I got stuck. I can do this several ways:

1. Load all the customers in the constructor (I don’t like this one because it can take a while if I have many customers)

2. In every method of the CustomerManager class that performs database related actions, check the local list of customers is loaded and if not, load the list:

   public method FindCustomer(int id)
   {
     if(_customers == null)
     // some code which will load the customers list
   }
   

3. Create a method which loads all the customers. This method must be called before calling methods which performs database related actions:

   In the class:

   public LoadData()
   {
      // some code which will load the customers list
   }
   

   In the form:

   CustomerManager manager = new CustomerManager();
   manager.LoadData(); 
   Customer customer = manager.FindCustomer(int id);
   

What is the best way to do this?

EDIT:

I have the feeling that I am misunderstood here. Maybe it is because I wasn’t clear enough. In the CustomerManager class I have several methods which depends on the local list (_customers). So, my question is, where should I fill that list?

Answer 1

What you are describing is “lazy loading”.

A simple approach is to have a private property like this:

private Lixt<Customer> _customers;
private List<Customer> Customers
{
  get
  {
    if(_customers == null)
      _customers = LoadData();
    return _customers;
  }
}

Then, you refer to Customers internally. The customers will be loaded the first time they are needed but no earlier.

This is such a common pattern that .Net 4.0 added a Lazy<T> class that does this for you.

I that case, you just define it as a private like this:

private Lazy<List<Customer>> _customers = new Lazy<List<Customer>>(LoadData);

Then, you simply refer to your customers in code:

_customers.Value

The class will initialize the value with your LoadData() method.

If you are not on .Net 4.0 yet, the Lazy<T> class is very easy to implement.