Lazy loading isn't a new concept, it's a pattern that been around for a while to improve the performance of your apps by only loading objects when they are going to be used. For example if you have an object that contains a property of a list of customers then you only really need to populate it when you access the property not when the object was initially created, as it may never be used. At the same time though you don't want to be going off to the database every time access the property. So the simple solution is to have another private variable that stores if the customers property is populated or not and then check that in the property's get to determine if the data needs to be loaded or not.
Well now in .NET 4, lazy loading has been built into the framework with System.Lazy. Instead of the above all you need to do now is write something like this...
Lazy<Customers> _customers = new Lazy<Customers>();
What this will do is create you a customers object but only run the constructor when you actually access the objects Value property which will be of type Customers. e.g.
It's that simple, but can get even better. The constructor may not be the only thing you want to run when you access the property the first time. In this case you would write something like...
_customers = new Lazy<Customers>(() =>
// Write any other initialization stuff in here
return new Customers();
I must point out though while as great as this is, it does have some limitations so you probably won't want to use it in all scenarios.
For more information check out the Lazy initialization page on MSDN