This post is a brief update to the .NET 3.5 article I posted about P.I. lazy loading. The only major change I have made to the code is to use the new Lazy<T> class that was introduced in .NET 4.0. This considerably cleans up the LazyLoadingList<T> class from the previous post.
Here is the new LazyLoadingList<T>:
public class LazyLoadingList<T> : IList<T>
{
private Lazy<IList<T>> _lazyList;
public LazyLoadingList(Lazy<IList<T>> lazyList)
{
_lazyList = lazyList;
}
#region Implementation of IEnumerable
public IEnumerator<T> GetEnumerator()
{
return _lazyList.Value.GetEnumerator();
}
IEnumerator IEnumerable.GetEnumerator()
{
return _lazyList.Value.GetEnumerator();
}
#endregion
#region Implementation of ICollection<T>
public void Add(T item)
{
_lazyList.Value.Add(item);
}
public void Clear()
{
_lazyList.Value.Clear();
}
public bool Contains(T item)
{
return _lazyList.Value.Contains(item);
}
public void CopyTo(T[] array, int arrayIndex)
{
_lazyList.Value.CopyTo(array, arrayIndex);
}
public bool Remove(T item)
{
return _lazyList.Value.Remove(item);
}
public int Count
{
get
{
return _lazyList.Value.Count;
}
}
public bool IsReadOnly
{
get
{
return ((ICollection<T>)_lazyList.Value).IsReadOnly;
}
}
#endregion
#region Implementation of IList<T>
public int IndexOf(T item)
{
return _lazyList.Value.IndexOf(item);
}
public void Insert(int index, T item)
{
_lazyList.Value.Insert(index, item);
}
public void RemoveAt(int index)
{
_lazyList.Value.RemoveAt(index);
}
public T this[int index]
{
get
{
return _lazyList.Value[index];
}
set
{
_lazyList.Value[index] = value;
}
}
#endregion
}
Here are the changes to the invoking code:
public class CompanyDAO : ICompanyDAO
{
List<Company> _companiesInDatabase = new List<Company>
{
new Company(){Name = "ACME"},
new Company(){Name = "Hardees"}
};
#region Implementation of ICompanyDAO
public Company GetByName(string name)
{
// Write to console to demonstrate when loading is happening
Console.WriteLine("---Loading Company---");
// Pretend we are calling / mapping from a store procedure
var company = _companiesInDatabase.Where(x => x.Name == name).First();
// Create / add the lazily loaded collection
if (company != null)
{
var lazyLoader = new Lazy<IList<Employee>>(
() =>
{
var employeeDAO = new EmployeeDAO();
return employeeDAO.GetByCompanyName(name).ToList();
}
);
company.Employees = new LazyLoadingList<Employee>(lazyLoader);
}
return company;
}
#endregion
}
The full source can be found here: http://github.com/dauger/BlogSamples
