VB.NET linq extension methods such as ToList not working with generics - vb.net

Question

In a VB.NET class file, I'm trying to use extension methods such as ToList(), where the generic parameter is populated with what I know to be the subclass, in this context.
Dim specificOrders = request.Orders _
.Where(Function(x) x.GetType().Equals(GetType(SpecificOrderType))) _
.ToList(Of SpecificOrderType)()
However, I'm getting the error message "extension method is not generic or has no type parameters available". Any ideas as to why this is?
This method should be in the System.Linq namespace - I have it open and referenced in the file.

Answer 1

You don't use Where to filter by type. That's what the OfType method is for. It filters and casts:
Dim specificOrders = request.Orders.
OfType(Of SpecificOrderType)().
ToList()
In that case, OfType returns an IEnumerable(Of SpecificOrderType) and calling ToList on that returns a List(Of SpecificOrderType). That's how ToList works. It simply create a List(Of T) with the same generic type as the IEnumerable(Of T) that it's called on.
If you were going to use Where, you would use Cast to go from the base type to SpecificOrderType:
Dim specificOrders = request.Orders.
Where(Function(x) x.GetType().Equals(GetType(SpecificOrderType))).
Cast(Of SpecificOrderType)().
ToList()
One point to note about OfType is that it will match any item that can be cast as the specified type. That is usually what you want and probably the result that your original code would produce but it's worth noting that any item that was of a type that inherited SpecificOrderType would be excluded by your original code but included using OfType.

Answer 2

ToList() can not have type arguments, because that method is not generic.
So just use ToList()
Dim suborders = orders _
.Where(Function(x) x.GetType().Equals(GetType(nonspecificOrder))) _
.ToList()
Try below example.
'Order class
Public Class order
End Class
'specificOrder class
Public Class specificOrder
Inherits order
End Class
'nonspecificOrder class
Public Class nonspecificOrder
Inherits order
End Class
Usage:
Dim orders As List(Of order) = New List(Of [order])
Dim s1 As specificOrder = New specificOrder()
Dim s2 As specificOrder = New specificOrder()
Dim s3 As specificOrder = New specificOrder()
Dim s4 As specificOrder = New specificOrder()
Dim s5 As nonspecificOrder = New nonspecificOrder()
Dim s6 As nonspecificOrder = New nonspecificOrder()
orders.Add(DirectCast(s1, order))
orders.Add(DirectCast(s2, order))
orders.Add(DirectCast(s3, order))
orders.Add(DirectCast(s4, order))
orders.Add(DirectCast(s5, order))
orders.Add(DirectCast(s6, order))
Dim suborders = orders _
.Where(Function(x) x.GetType().Equals(GetType(nonspecificOrder))) _
.ToList()
Now the two "nonspecificOrder" type objects are returned.