LINQ表达式中的Func参数

Question

I have problem with Func delegate in LINQ expression. This is the problematic part of the method (repository.Items is IQueryable<T>) :

public static ActionResult XXX<T>(IRepository<T> repository,
        Func<T, int> keyExtractor, int id = 0)
    {
        if (id == 0) return ...
        T item = repository.Items.Where(x => keyExtractor(x) == id).
           FirstOrDefault();
        if (item == null) return ...
        try {
            repository.DeleteItem(item);
            return ...
        } catch (Exception e) {
            return ...
        }
    }

But when i run the method, i get error like type of node is not supported in LINQ entities. I tried also version with predikate but i had no luck at all.

Any ideas how to fix it?

I find out one possible way. LINQ performs delayed execution so i must first enforce execution like this:

T item = repository.Items.AsEnumerable().Where(x => keyExtractor(x) == id)

Answer 1

If you want to work on a IQueryable<T> , your argument must be an Expression<Func<T, int>> , not a Func<T, int> .

Func<T, int> will work with an IEnumerable<T> .

Where extensions methods has the same name for IQueryable<T> and IEnumerable<T> , but not the same arguments...

(by the way you could write repository.Items.FirstOrDefault(x => keyExtractor(x) == id) ;