Linq Inheritance and subtypes (want to filter out base type)

OK I have a base query that is returning a list (ObjectQuery, actually but I don't think it matters)

I have about 5 subtypes based on the main type and I need to filter them. Everything works fine until I want the results to be of the main type (not a subclass)

Basically I have a query that includes everything, then I filter out the types that are not checked in a checkedbox list. (asp.net)

Lets say I have an Entity named Task, and SubTask1 and SubTask2 that inherit from Task

       Dim AllTasks As Objects.ObjectQuery(Of Demo.Task) = dbContext.CompletedTasks(ctx, anyID)  'calling a compiled query but it doesnt matter. 
    'It could be Dim AllTasks = from t in ctx.Tasks select t

    For Each itm As ListItem In chkTaskTypeList.Items
        If Not itm.Selected Then
            Select Case itm.Value
                Case "SubTask1"
                    'this works as expected, and eliminates tasks that are of the subtype Subtask1 
                    AllTasks = From ti 开发者_如何学GoIn AllTasks Where Not TypeOf ti Is SubTask1

                Case "SubTask2"
                    'This also works as I want by filtering out the various sub types
                    AllTasks = From ti In AllTasks Where Not TypeOf ti Is SubTask2 AndAlso Not TypeOf ti Is SubTaskWhatever

                Case "SubTask"
                    **'This works as expected, but not like I need. It removes all the types when what I really want is to remove** 
                    **'The results that are of the base type but NOT the ones that are of a subtype**
                    AllTasks = From ti In AllTasks Where Not TypeOf ti Is SubTask

            End Select
        End If
    Next

    lvwHistory.DataSource = AllTasks

If I had a limted number of subtypes I could probably say something like (Not TypeOf ti Is SubTask andalso TypeOf ti Is SubTask1) andalso ... but I am hoping for a better way (and one which doesn't break the page if a new subtype added)

You should compare if the types are equal

OnlyBaseClasses = From ti In AllTasks Where ti.GetType() = GetType(TheBaseClass)

This is difficult, but possible. However, you should rethink your design. Doing this usually violates the Liskov Substitution Principle.