Nullable object must have a value in IComparable.CompareTo - asp.net

Question

I have a custom class that needs to be sortable using a Nullable(of Double) property. I have defined the class, and implemented iCompare. Within my code, I thought I had placed sufficient checks for Null values, but I am suddenly seeing this error...
Nullable object must have a value.
...on this line:
Return Me.SomeDouble.Value.CompareTo(CType(obj, MatrixCellData).SomeDouble.Value)
Can anyone please explain why this is happening? I know that usually this error can be easily avoided by using Double.HasValue, which I have included. The full sample code to reproduce is below.
Namespace MyApp
Public Class MatrixCellData
Implements IComparable
Public Property SomeDouble As Nullable(Of Double)
Public Function CompareTo(obj As Object) As Integer Implements System.IComparable.CompareTo
If obj Is Nothing OrElse IsDBNull(obj) OrElse Not TypeOf (obj) Is MatrixCellData Then
Return 1
Else
If Not Me.SomeDouble.HasValue Then
Return -1
Else
Return Me.SomeDouble.Value.CompareTo(CType(obj, MatrixCellData).SomeDouble.Value)
End If
End If
End Function
End Class
End Namespace
Breaking code...
Dim lst As New List(Of MyApp.MatrixCellData)
lst.Add(New MyApp.MatrixCellData With {.SomeDouble = 75})
lst.Add(New MyApp.MatrixCellData With {.SomeDouble = 25})
lst.Add(New MyApp.MatrixCellData With {.SomeDouble = Nothing})
lst.Sort() ' Triggers error

Answer 1

If you don't use
If Not Me.SomeDouble.HasValue Then
Then there's no check to stop Me.SomeDouble from being equal to Nothing.
When you call the sort function on your list, you will be calling the CompareTo function at least once for each item. Effectively, the framework tries to place your last item somewhere in the ordered list the call looks similar to this:
(New MatrixCellData With {.SomeDouble = Nothing}).CompareTo(New MatrixCellData With {.SomeDouble = 25})
All the following checks are false:
If obj Is Nothing OrElse IsDBNull(obj) OrElse Not TypeOf (obj) Is MatrixCellData Then
Obj isn't Nothing, (Me.SomeDouble is Nothing)
This means your last line in the CompareTo method
Return Me.SomeDouble.Value.CompareTo(CType(obj, MatrixCellData).SomeDouble.Value)
In the case where Me.SomeDouble is Nothing, will actually be:
Return Nothing.Value.CompareTo(CType(obj, MatrixCellData).SomeDouble.Value)
Which of course doesn't make sense since Nothing does not have a 'Value' method or property.
I would re-write this as
Public Function CompareTo(obj As Object) As Integer Implements System.IComparable.CompareTo
If obj Is Nothing OrElse Not TypeOf (obj) Is MatrixCellData OrElse CType(obj, MatrixCellData).SomeDouble Is Nothing Then
Return 1
Else If Me.SomeDouble is Nothing Then
Return -1
End If
Return Me.SomeDouble.Value.CompareTo(CType(obj, MatrixCellData).SomeDouble.Value)
End Function