ReadOnly Property Gets "Unintentional" Change - arrays

Question

I understand that List, Array, Object etc. types "copied" by reference. However my natural and ordinary intend is to just have a "copy" of it in this context where I intentionally use ReadOnly instead of Read/Write property. In below sample the ReadOnly 'Extensions' property get change through 'm_extensions' reference change. Regardless, I think this behavior is incorrect and I have to do extra work to prevent ReadOnly properties from being overwritten. Is there any built in keyword to use for 'm_extensions' value protection?
Public Classs A
' more properties and methods here...
Private m_extensions() As String = {"*.abc", "*.def"}
Public ReadOnly Property Extensions() As String()
Get
Return m_extensions
End Get
End Property
End Class
Public Classs B
' more stuff here...
Private Function BuildFilter() As String
Dim l() As String = A.Extensions
Dim s As String = String.Empty
For m As Integer = 0 To l.Length - 1
Select Case l(m).ToLower
Case "*.*" : s = "All Files"
Case "*.abc" : s = "ABC File"
Case "*.def" : s = "DEF File"
Case Else : s = "XYZ File " + m.ToString
End Select
l(m) = String.Format("{1} ({0})|{0}", l(m), s)
Next
Return String.Join("|", l)
End Function
End Class

Answer 1

Readonly modifier means that anything using the property cannot change the reference that you protected this way (e.g. cannot set it to Nothing). It doesn't prevent changing the values in the array returned from that property.
One way around it could be to copy the array inside the property. This will prevent modifications of the original array:
Public ReadOnly Property Extensions() As String()
Get
Return m_extensions.Clone()
End Get
End Property