Linq query exception/error

Following is the code I get to achieve a list of Courses:

Dim Result As New List(Of WorkMateLib.CourseNameSpace.CoursesLib.Course)
    For Each cat As WorkMateLib.CourseNameSpace.CoursesLib.Category In Courses.CoursesOfferedMAIN.Values
        For Each c As WorkMateLib.CourseNameSpace.CoursesLib.Course In cat.Courses.Values
            Result.Add(c)
        Next
    Next

and it is working fine. However I am trying to a开发者_JAVA技巧ttempt to do the same with linq and the code is:

Result.AddRange(From k As WorkMateLib.CourseNameSpace.CoursesLib.Category In Courses.CoursesOfferedMAIN.Values Where Not k.CategoryName = "" Select k.Courses.Values.ToList())

But is throwing an error as follows:

Unable to cast object of type 'WhereSelectEnumerableIterator2[WorkMateLib.CourseNameSpace.CoursesLib+Category,System.Collections.Generic.List1[WorkMateLib.CourseNameSpace.CoursesLib+Course]]' to type 'System.Collections.Generic.IEnumerable`1[WorkMateLib.CourseNameSpace.CoursesLib+Course]'.

Could you please help me better understand it. Thank you.

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This should be equivalent. Note that there's no need to create a list yourself.

Dim Result = Courses.CoursesOfferedMAIN.Values                    _
                    .SelectMany(Function(cat) cat.Courses.Values) _
                    .ToList