Implementing Missing Features in Entity Framework Core – Part 6: Lazy Loading
This will be the sixth post in my series of posts about bringing the features that were present in Entity Framework pre-Core into EF Core. The others are:
Part 1: Introduction, Find, Getting an Entity’s Id Programmatically, Reload, Local, Evict
Part 2: Explicit Loading
Part 3: Validations
Part 4: Conventions
Part 5: Getting the SQL for a Query
As you may know, the second major version of Entity Framework Core, 1.1, was released recently, however, some of the features that used to be in the non-Core versions still didn’t make it. One of these features is lazy loading of collections, and I set out to implement it… or, any way, something that I could use instead of it! 
Here’s what I came up with. First, let’s define a class that will act as a proxy to the collection to be loaded. I called it CollectionProxy<T>, and it goes like this:
internal sealed class CollectionProxy<T> : IList<T> where T : class
{
private bool _loaded;
private bool _loading;
private readonly DbContext _ctx;
private readonly string _collectionName;
private readonly object _parent;
private readonly List<T> _entries = new List<T>();
public CollectionProxy(DbContext ctx, object parent, string collectionName)
{
this._ctx = ctx;
this._parent = parent;
this._collectionName = collectionName;
}
private void EnsureLoaded()
{
if (this._loaded == false)
{
if (this._loading == true)
{
return;
}
this._loading = true;
var entries = this
._ctx
.Entry(this._parent)
.Collection(this._collectionName)
.Query()
.OfType<T>()
.ToList();
this._entries.Clear();
foreach (var entry in entries)
{
this._entries.Add(entry);
}
this._loaded = true;
this._loading = false;
}
}
IEnumerator<T> IEnumerable<T>.GetEnumerator()
{
this.EnsureLoaded();
return this._entries.GetEnumerator();
}
IEnumerator IEnumerable.GetEnumerator()
{
return (this as ICollection<T>).GetEnumerator();
}
int ICollection<T>.Count
{
get
{
this.EnsureLoaded();
return this._entries.Count;
}
}
bool ICollection<T>.IsReadOnly
{
get
{
return false;
}
}
void ICollection<T>.Add(T item)
{
this.EnsureLoaded();
this._entries.Add(item);
}
void ICollection<T>.Clear()
{
this.EnsureLoaded();
this._entries.Clear();
}
bool ICollection<T>.Contains(T item)
{
this.EnsureLoaded();
return this._entries.Contains(item);
}
void ICollection<T>.CopyTo(T[] array, int arrayIndex)
{
this.EnsureLoaded();
this._entries.CopyTo(array, arrayIndex);
}
bool ICollection<T>.Remove(T item)
{
this.EnsureLoaded();
return this._entries.Remove(item);
}
T IList<T>.this[int index]
{
get
{
this.EnsureLoaded();
return this._entries[index];
}
set
{
this.EnsureLoaded();
this._entries[index] = value;
}
}
int IList<T>.IndexOf(T item)
{
this.EnsureLoaded();
return this._entries.IndexOf(item);
}
void IList<T>.Insert(int index, T item)
{
this.EnsureLoaded();
this._entries.Insert(index, item);
}
void IList<T>.RemoveAt(int index)
{
this.EnsureLoaded();
this._entries.RemoveAt(index);
}
public override string ToString()
{
this.EnsureLoaded();
return this._entries.ToString();
}
public override int GetHashCode()
{
this.EnsureLoaded();
return this._entries.GetHashCode();
}
}
You can see that, in order to be as compliant as possible, I made it implement IList<T>; this way, it can be easily compared and switched with, for example, ICollection<T> and, of course, the mother of all collections, IEnumerable<T>. How it works is simple:
- It receives in its constructor a pointer to a DbContext, the collection’s parent, and the collection-to-be-made-lazy’s name;
- There is an EnsureLoaded method that essentially checks if the collection has already been loaded, and, if not the case, does so, through the new (in EF Core 1.1) explicit loading API; it populates an internal list with the loaded collection’s items;
- Inner fields _loading and _loaded act as defenses to prevent the collection to be loaded twice, or to enter an infinite loop (stack overflow);
- I implemented all inherited methods and properties as explicit implementations, but there was no need for that, just a personal preference; all of them ensure that the collection is loaded (EnsureLoaded) before delegating to its internal field list;
- ToString and GetHashCode delegate to the internal list as well.
I created as well an extension method to make it’s usage more simple:
public static class CollectionExtensions
{
public static void Wrap<TParent, TChild>(this DbContext ctx, TParent parent, Expression<Func<TParent, IEnumerable<TChild>>> collection) where TParent : class where TChild : class
{
var prop = ((collection.Body as MemberExpression).Member as PropertyInfo);
var propertyName = prop.Name;
prop.SetValue(parent, new CollectionProxy<TChild>(ctx, parent, propertyName));
}
}
As you can see, I kept it very simple – no null/type checking or whatever, that is left to you, dear reader, as an exercise! 
Finally, here’s how to use it:
using (var ctx = new MyContext())
{
var parentEntity = ctx.MyParentEntities.First();
ctx.Wrap(parentEntity, x => x.MyChildren); //sets up the proxy collection
var childEntitiesCount = parentEntity.MyChildren.Count(); //forces loading
foreach (var child in parentEntity.MyChildren) //already loaded, so iterate in memory
{
child.ToString();
}
}
Hope you like it! Let me know your thoughts!