Use Lazy Initialization To Delay Object Instantiation

Question

It is a common practice to declare object variables as the class members and initialize them in the constructor of the class. At times, however, you need to delay the object instantiation until its first use. One way to achieve such an deferred object instantiation is to write custom code that implements Lazy Load pattern. An easy alternative is to use .NET framework's inbuilt lazy initialization feature. To that end this article explains the later approach.

Suppose you wish to create Customer class as shown below:

public class Customer
{
    public string CustoemrID { get; set; }
    public string CompanyName { get; set; }
    public string ContactName { get; set; }
    public string Country { get; set; }

    public List<Order> Orders { get;  }
    public List<Product> Products { get;  }

    public Customer()
    {
        this.Orders = new List<Order>();
        this.Products = new List<Product>();

        //fill Orders and Products here
        //through some lengthy processing
    }
}

The Customer class consists of six properties - CustomerID, CompanyName, ContactName, Country, Orders and Products. The first four properties are quite straightforward. The Orders property holds a List of Order objects representing orders placed by a customer so far. The Products property holds a list of Product objects representing a List of unique products ordered by a customer so far. These properties are initialized and filled in the constructor of the Customer class.

Now further assume that the Customer class is being used in three modules - Contact manager module, Order processing and tracking module and Product tracking module. When used in the contact manager module the Orders and Products properties won't play any role. Similarly order processing module may not need the Products property. Thus Orders and Products properties may waste the resources. Moreover, initializing these properties in the constructor might also cause performance penalty to the instantiation of the Customer class. That is where lazy loading comes into the picture.

The .NET framework's System.Lazy class provides an easy way to implement lazy loading in your applications. The Lazy<T> class wraps an object and instantiates it only when an object is accessed. This way there won't be any wastage of resources since an object is being created only if it is needed. Moreover, it frees the constructor from the lengthy initialization improving the instantiation time.

Let's rewrite the Customer class using the Lazy<T> class.

public class CustomerLazy
{
    public string CustoemrID { get; set; }
    public string CompanyName { get; set; }
    public string ContactName { get; set; }
    public string Country { get; set; }

    private Lazy<List<Order>> lazyOrders = 
                 new Lazy<List<Order>>(() => {
        List<Order> orders = new List<Order>();
        //lengthy processing that fills Orders
        return orders;
    });

    private Lazy<List<Product>> lazyProducts = 
                 new Lazy<List<Product>>(() => {
        List<Product> products = new List<Product>();
        //lengthy processing that fills Products
        return products;
    });

    public  List<Order> Orders
    {
        get
        {
            return lazyOrders.Value;
        }
    }


    public List<Product> Products
    {
        get
        {
            return lazyProducts.Value;
        }
    }
}

Notice the code marked in bold letters. The code declares two private member variables - lazyOrders and lazyProducts - at the top. These Lazy<T> objects wrap List<Order> and List<Product> respectively. Moreover, the code that fills the order and product lists is moved inside a delegate function passed to the constructor of the Lazy class.

The Orders and Products properties are then declared with property getters. The property getters uses the Value property of Lazy<T> class to get an instance of the List<Order> and List<Product>. These instances are created when the getter properties are accessed for the first time. While creating these instances the code inside the delegate function is also executed. Thus the object instantiation is delayed until an instance is actually needed.

If you don't wish to execute any code during the object creation you can use the Lazy<T> as follows:

private Lazy<List<Order>> lazyOrders = new Lazy<List<Order>>();
private Lazy<List<Product>> lazyProducts = new Lazy<List<Product>>();

In this case List<Order> and List<Product> are created by Lazy<T> for you when Value is accessed for the first time.