Namespace System.Linq.Dynamic
    Public Module DynamicQueryable

        <Extension()> _
        Public Function Where(Of T)(ByVal source As IQueryable(Of T), ByVal predicate As String, ByVal ParamArray values() As Object) As IQueryable(Of T)
            Return DirectCast(Where(DirectCast(source, IQueryable), predicate, values), IQueryable(Of T))
        End Function

        <Extension()> _
        Public Function Where(ByVal source As IQueryable, ByVal predicate As String, ByVal ParamArray values() As Object) As IQueryable

Imports System.Linq.Dynamic 'ошибка

Public Class Class1

    Sub Main()
        Dim x As IEnumerable
        Dim y As IQueryable = x
        y.Where("fsdf") 'ошибка
    End Sub

End Class

Imports ClassLibrary2.System.Linq.Dynamic