Filtering IQueryable based on dynamic field - entity-framework

Question

I'm trying to implement a generic item filter using Entity Framework based on a lambda expression telling me the id field. The following code compiles but of course does not work since EF does not understand the generic function:
public static IQueryable<T> Authorize<T>(this IQueryable<T> items, Func<T, Guid> idGetter) where T : class
{
return items.Where(i => idGetter(i) == new Guid("4A6FE5AF-AB63-4BB3-9D32-88766CF242CC"));
}
var result = context.Items.Authorize(i => i.Id);
How to do this using EF? How to use the expression tree to tell him what field to compare? How to use a generic name in a query that can be handled by Entity Framework?

Answer 1

If your id field is always of type Guid (as your code is suggesting), your classes could implement an interface with a member call Id of type Guid:
public interface MyInterface
{
Guid Id { get; set; }
}
public class Item : MyInterface
{
public Guid Id { get; set; }
// ...
}
Then your Authorize method could look like this:
public static IQueryable<T> Authorize<T>(this IQueryable<T> items) where T : MyInterface
{
return items.Where(i => i.Id == new Guid("4A6FE5AF-AB63-4BB3-9D32-88766CF242CC"));
}
And you can use this as follows:
var result = items.AsQueryable().Authorize();

Answer 2

Based on your comments and using reflection this is my approach:
First, your (simple) Attribute to decorate your "id fields":
[AttributeUsage(AttributeTargets.Property)]
public class IdFieldAttribute : Attribute
{
public IdFieldAttribute()
{
}
}
Then your classes could look like this:
public class Item
{
[IdFieldAttribute]
public Guid YourIdField{ get; set; }
// ...
}
A Reflection helper class for getting your desired field:
public static class ReflectionHelper
{
public static PropertyInfo GetPropertyInfoByAttribute<T>(T o, Type attributeType) where T : class
{
var type = o.GetType();
var propertyInfo = type.GetProperties(BindingFlags.Public | BindingFlags.Instance)
.FirstOrDefault(pi => Attribute.IsDefined(pi, attributeType));
return propertyInfo;
}
}
Once you have this, your Authorize method could look like this:
public static IQueryable<T> Authorize<T>(this IQueryable<T> items) where T : class
{
return items.Where(i => (Guid)ReflectionHelper.GetPropertyInfoByAttribute(i, typeof(IdFieldAttribute)).GetValue(i) == new Guid("4A6FE5AF-AB63-4BB3-9D32-88766CF242CC"));
}
And you can use it as follows:
var result = items.AsQueryable().Authorize();
I don't know if what you're looking for is something like this. Maybe this approach could help you.