Question

Think this is a very basic question, but it’s my first LINQ query and I’m completely stuck:

I have a dictionary with string key and list value (see definition below) and want to pull out elements of a list of a particular type having selected the list by the dictionary key.

IDictionary<string, IList<MyBaseType>> dataItemMap;

Where MySubType extends MyBaseType.

My dodgy query is:

string identCode = "foo";

IEnumerable<MySubType> query = 
    from entry in dataItemMap
    where entry.Key == identCode
    select entry.Value.OfType<MySubType>();

And the error message (from LinqPad):

Cannot implicitly convert type
'System.Collections.Generic.IEnumerable<System.Collections.Generic.IEnumerable<MySubType>>'
to 'System.Collections.Generic.IEnumerable<MySubType>'.
An explicit conversion exists (are you missing a cast?)

The problem is clearly in the entry.Value.OfType<> but how can I specify the lists elements? I’m looking for something like entry.Value.Items.OfType<> ?

thanks.

Answer 1

I think you want something like this:

IEnumberable<MySubType> query = dataItemMap[identCode].OfType<MySubType>();

This will get the list with the given key, and then filter it to return only MySubType elements.