Question

I am pretty confused in collection and IEnumerable types. Can any one explain why the 1st query is wrong but the 2nd is correct-
1st- this gives error

ConditionFieldCollection conditionColl = (ConditionFieldCollection)Session["ConditionFieldCollection"];
ConditionFieldCollection cnd = new ConditionFieldCollection(); 
cnd = (from c in conditionColl
            where iq.QueryField == c.Expression
                select c);

2nd – works fine

ConditionFieldCollection conditionColl = (ConditionFieldCollection)Session["ConditionFieldCollection"];
List<ConditionField> cnd = (from c in conditionColl.OfType<ConditionField>()
                                       where iq.QueryField == c.Expression
                                       select c).ToList();

I know LINQ returns IEnumerable type of collection object but ConditionFieldCollection is also a Collection then why it gives me error at compile time. Is there any difference b/w Collecton and IEnumerable Collection??

Answer 1

You are attempting to assign an IEnumerable<ConditionField> to a variable with type ConditionFieldCollection – that can’t work. An enumerable is not a collection, and certainly isn’t that specific collection.

Many collections allow an enumerable constructor, so this may work:

ConditionFieldCollection cnd = new ConditionFieldCollection(
    from c in conditionColl
    where iq.QueryField == c.Expression
    select c);