I am trying to write a linq to entity extension method that takes a Func to select a property Id and compare it against a list of ids.
Classes
public class A
{
public int AId { get; set; }
}
public class B
{
public int BId { get; set; }
}
Extension Method
public static IQueryable<T> WithId<T>(this IQueryable<T> entities,
Func<T, int> selector, IList<int> ids)
{
Expression<Func<T, bool>> expression = x => ids.Contains(selector(x));
return entities.Where(expression); // error here (when evaluated)
}
Calling Method
var ids = new List<int> { 1, 2, 3 };
DbContext.EntityAs.WithId(e => e.AId, ids);
DbContext.EntityBs.WithId(e => e.BId, ids);
The problem I am experiencing is that it is trying to Invoke the function which is not allowed in Entity Framework.
How can I use a property selector (Func) to evaluate the query?
You'll have to pass an Expression<Func<T, int>>
instead of an Func<T, int>
and build up the complete expression yourself. This will do the trick:
public static IQueryable<T> WithId<T>(this IQueryable<T> entities,
Expression<Func<T, int>> propertySelector, ICollection<int> ids)
{
var property =
(PropertyInfo)((MemberExpression)propertySelector.Body).Member;
ParameterExpression parameter = Expression.Parameter(typeof(T));
var expression = Expression.Lambda<Func<T, bool>>(
Expression.Call(
Expression.Constant(ids),
typeof(ICollection<int>).GetMethod("Contains"),
Expression.Property(parameter, property)),
parameter);
return entities.Where(expression);
}
When you try to keep your code DRY when working with your O/RM, you will often have to fiddle with expression trees. Here's another fun example.