VB.NET Extension methods error

''' <summary> 
''' Transforms an item to a list of single element containing this item.
''' '</summary> 
<Extension()> _
Public Function ToList(Of T)(ByVal item As T) As List(Of T)
  Dim tList As New List(Of T)
  tList.Add(item)
  Return tList
End Function

usage

Dim buttonControl As New System.Windows.Forms.Button
Dim controls = buttonControl.ToList(Of System.Windows.Forms.Control)()

compile time error (on the last line)

Extension method 'Public Function ToList() As System.Collecti开发者_JAVA百科ons.Generic.List(Of T)' defined in '...Utils' is not generic (or has no free type parameters) and so cannot have type arguments.

Was is das?

Try this:

<Extension()> _
Public Function ToList(Of TItem, TList As {New, List(Of TItem)})(ByVal item As TItem) As TList
  Dim tList As New TList
  tList.Add(item)
  Return tList
End Function

Basically your return type was a generic (declared as List (of T)). The function decaration here does it so that the return type is a list of the type that is being extended.

Try this.

<Extension()>
    Public Function ToList(Of T)(ByVal Item As Object) As List(Of T)
        Dim tlist1 As New List(Of T)
        tlist1.Add(Item)
        Return tlist1
    End Function