How to query the DbSets of all types that implement an interface? - entity-framework

Question

Many of my data models use this interface:
public interface IHasPrimaryImageProperty
{
PrimaryImageDataModel PrimaryImage { get; set; }
int? PrimaryImageId { get; set; }
}
Where PrimaryImageDataModel is:
public class PrimaryImageDataModel
{
public int Id { get; set; }
public string ImageFile { get; set; }
public int TotalItemsUsingImage { get; set; }
}
I want populate PrimaryImageDataModel.TotalItemsUsingImage, by performing counts on all data models that implement IHasPrimaryImageProperty.
So far I have managed to get a list of types that implement the IHasPrimaryImageProperty.
But I haven't been able to get the total for each Type.
Please see the example below for a demonstration of what I would like to acheive.
public static PrimaryImageDataModel GetImageUsageTotals(PrimaryImageDataModel image)
{
var typesUsingImage = GetTypesWithPrimaryImageProperty();
int totalUsingImage = 0;
foreach (Type typeUsingImage in typesUsingImage)
{
// I WOULD LIKE TO DO SOMETHING LIKE THIS
totalForType = db.Set<typeUsingImage>()
.Where(x => x.PrimaryImageId == image.Id)
.Count()
totalUsingImage += totalForType;
}
image.TotalItemsUsingImage = totalUsingImage;
return image;
}
public static IEnumerable<Type> GetTypesWithPrimaryImageProperty()
{
var currentAssembly = Assembly.GetExecutingAssembly();
foreach (Type type in currentAssembly.GetTypes())
{
if (type.GetInterfaces().Contains(typeof(IHasPrimaryImageProperty)))
{
yield return type;
}
}
}

Answer 1

The simplest I see (works in both EF6 and EF Core) is to create a generic method and call it via reflection.
For instance:
static int CountUsage<T>(DbContext db, PrimaryImageDataModel image)
where T : class, IHasPrimaryImageProperty
{
return db.Set<T>()
.Where(x => x.PrimaryImageId == image.Id)
.Count();
}
static readonly MethodInfo CountUsageMethodInfo = typeof(YourClass)
.GetMethod("CountUsage", BindingFlags.NonPublic | BindingFlags.Static);
public static PrimaryImageDataModel GetImageUsageTotals(PrimaryImageDataModel image)
{
var args = new object[] { db, image };
image.TotalItemsUsingImage = GetTypesWithPrimaryImageProperty()
.Sum(type => (int)CountUsageMethodInfo.MakeGenericMethod(type).Invoke(null, args));
return image;
}

Answer 2

IQueryable is covariant. See Variance in Generic Interfaces (C#) This allows an IQueryable<SomeEntity> to be cast to IQueryable<InterfaceType>, for interfaces implemented by that Entity type.
So if you put this method on your EF6 DbContext type:
public IQueryable<T> GetQuery<T>(Type EntityType)
{
return (IQueryable<T>)this.Set(EntityType);
}
Or like this for EF Core:
public IQueryable<T> GetQuery<T>(Type EntityType)
{
var pq = from p in this.GetType().GetProperties()
where p.PropertyType.IsGenericType
&& p.PropertyType.GetGenericTypeDefinition() == typeof(DbSet<>)
&& p.PropertyType.GenericTypeArguments[0] == EntityType
select p;
var prop = pq.Single();
return (IQueryable<T>)prop.GetValue(this);
}
Then you can write
foreach (Type typeUsingImage in typesUsingImage)
{
// I WOULD LIKE TO DO SOMETHING LIKE THIS
totalForType = db.GetQuery<IHasPrimaryImageProperty>(typeUsingImage)
.Where(x => x.PrimaryImageId == image.Id)
.Count()
totalUsingImage += totalForType;
}