How to test whether a lazy loaded child collection has been loaded - entity-framework-6

Question

Suppose I have users with tags being lazy loaded:
public class User
{
public int UserId { get; set; }
public virtual ICollection<Tag> Tags { get; set; }
}
If I get a user this way, I know that the tags aren't loaded:
User myUser;
using (var context = new MyContext())
{
myUser = context.Users.Find(4);
}
How do I test the Tags collection presence outside of the using clause?
if (myUser.Tags == null) // throws an ObjectDisposedException
I could use a try/catch but there must be a better way.

Answer 1

The only way I can think of is to be able to do a non virtual call (similar to when you do base.Something in a derived class) to the class property getter. Since there is no way to do that with pure C# or reflection, I've ended up with the following helper method which utilizes System.Reflection.Emit LCG (lightweight code generation) to emit Call IL instruction instead of the normal Callvirt:
using System;
using System.Linq.Expressions;
using System.Reflection;
using System.Reflection.Emit;
public static class Utils
{
public static TValue GetClassValue<TSource, TValue>(this TSource source, Expression<Func<TSource, TValue>> selector)
where TSource : class
{
Func<TSource, TValue> getValue = null;
if (source.GetType() != typeof(TSource))
{
var propertyAccessor = selector.Body as MemberExpression;
if (propertyAccessor != null)
{
var propertyInfo = propertyAccessor.Member as PropertyInfo;
if (propertyInfo != null)
{
var getMethod = propertyInfo.GetGetMethod();
if (getMethod != null && getMethod.IsVirtual)
{
var dynamicMethod = new DynamicMethod("", typeof(TValue), new[] { typeof(TSource) }, typeof(Utils), true);
var il = dynamicMethod.GetILGenerator();
il.Emit(OpCodes.Ldarg_0);
il.EmitCall(OpCodes.Call, getMethod, null);
il.Emit(OpCodes.Ret);
getValue = (Func<TSource, TValue>)dynamicMethod.CreateDelegate(typeof(Func<TSource, TValue>));
}
}
}
}
if (getValue == null)
getValue = selector.Compile();
return getValue(source);
}
}
It can be used for both single and collection type navigation properties like this:
if (myUser.GetClassValue(x => x.Tags) == null)

Answer 2

Another solution using a naive try/catch, not sure about the performance compared to the other answer:
using System;
public static class EntityFrameworkExtensions
{
public static bool IsCollectionLoaded<TSource, TValue>(this TSource source, Func<TSource, TValue> selector)
where TSource : class
{
try
{
return (selector(source) != null);
}
catch (ObjectDisposedException)
{
return false;
}
}
}
usage:
if (myUser.IsCollectionLoaded(x => x.Tags))