.Where method not defined on generic typed list? - vb.net

Question

When I try to use the .Where() method on a list, this does method does not seem to be defined if the list is of a generic type:
In my program, I have a class called Warning, and in another class, a list of warnings, defined as:
Dim warningList As List(Of Warning)
When I try to manipulate this list as:
Dim item = warningList.Where(Function(x) x.GetName() = "Foo").FirstOrDefault()
This works completely fine, but when I try it like this:
Dim itemList
if(type = "Warning") Then 'Please note that this condition is true...
itemList = warningList
End If
Dim item = itemList.Where(Function(x) x.GetName() = "Foo").FirstOrDefault()
I get an exception, stating that method .Where() is not defined for class Warning
Can anybody tell me why this is?
Thank you!

Answer 1

Now that you've edited your question it's clear.
You declare itemList without a type, so it's Object implicitly(in VB.NET with option strict set to Off which i strongly recommend against).
Now that you have declared a variable of type Object you can asssign any type to it. But you would have to cast it back to its real type List(Of Warning) to be able to use list or LINQ methods(which extend IEnumerable(Of T).
But instead declare it with the correct type:
Dim itemList As List(Of Warning)
if(type = "Warning") Then
itemList = warningList
End If
Dim item = itemList.Where(Function(x) x.GetName() = "Foo").FirstOrDefault()
Including to comment to explain why Warning is not related to this problem:
That's not the real code. If warningList is really a List(Of Warning)
you should be able to use Enumerable.Where(if LINQ is
imported). The fact that you assign this instance to another variable
(on declaration) doesn't change anything because that variable's type
is also a List(Of Warning). So itemList.Where should work too. Warning
has nothing to do with it because the type which is extended by Where
is IEnumerable(Of T), T can be any type(even Object). Since List(Of T)
implements IEnumerable(Of T) you can use Enumerable.Where on any list
(or array).
If you actually have multiple types and Warning is just one of it, you should implement a common interface. Here's an example:
Public Enum NotificationType
Warning
Info
[Error]
End Enum
Public Interface INamedNotification
ReadOnly Property Type As NotificationType
Property Name As string
End Interface
Public Class Warning
Implements INamedNotification
Public Sub New( name As String )
Me.Name = name
End Sub
Public Property Name As String Implements INamedNotification.Name
Public ReadOnly Property Type As NotificationType Implements INamedNotification.Type
Get
Return NotificationType.Warning
End Get
End Property
End Class
Now you can declare a List(Of INamedNotification) and fill it with whatever implements this interface, like the Warning class:
Dim notificationList As List(Of INamedNotification)
if type = "Warning" Then
itemList = warningList
Else If type = "Info"
itemList = infoList
End If
Dim item = notificationList.Where(Function(x) x.Name = "Foo").FirstOrDefault()