Question

I have a collection class which inherits from BindingList and I can use the Sum function over it such as myList.Sum(x=>x.Quantity).

But when I implement the interface IEnumerable<SqlDataRecord> I can’t use it anymore, x.Quantity is not given as an option. How can I resolve this issue?

class item
{
    public decimal Quantity { get; set; }
}

class items : BindingList<T>
{
}

items newItems = new items();
items.Sum(x=>x.Quantity);

The above code works but when I add the following code it doesn’t work anymore. It says the class doesn’t have a definition of Sum. What am I doing wrong?

class items : BindingList<T>, IEnumerable<SqlDataRecord>
{
}
items.Sum(x=>x.?);

Answer 1

When your class only inherited BindingList<T>, myList.Sum(...) was unambiguously calling Enumerable<T>.Sum(myList, ...) via BindingList<T>‘s inheritance from Collection<T>.

Now you have made your class directly implement IEnumerable<SqlDataRecord>, it’s no longer clear which type of Enumerable<?> to call Sum on – is ? meant to be T from your BindingList inheritance or SqlDataRecord from your direct implementation?

(For some reason, my VS reports this as “does not contain a definition…” rather than as an ambiguous call.)

Thus if you want to make your call as before you have to explicitly call myList.Sum<T>(...).