Linq to SQL: object.Tolist manipulation not possible

When I try to work with the returned list from the query i can't change it's property
While the class has a get and set property

even when i create a copy i cant change the polygon property is there a way to create a copy without the readonly property.

so i can use the changed data further in my code

Thanks in advance

===========APPLICATION CODE============
=======================================
 Dim cityList = (From c In cities _
        Select c.name, c.polygon).Take(100).ToList
 
        Try
        Dim point As New ArrayList()
        '..... some manipulation                
        cityList(i).polygon = point
        Catch ex As Exception
 
        End Try
 
 
 
 
======================================
===========MAPPING CLASS==============
======================================
<Column(Storage:="_polygon", AutoSync:=AutoSync.Always, DbType:="NVarChar(MAX)", IsDbGenerated:=true, UpdateCheck:=UpdateCheck.Never)>  _
	Public Property polygon() As String
		Get
			Return Me._polygon
		End Get
		Set
			If (String.Equals(Me._polygon, value) = false) Then
				Me.OnpolygonChanging(value)
				Me.SendPropertyChanging
				Me._polygon = value
				Me.SendPropertyChanged("polygon")
				Me.OnpolygonChanged
			End If
		End Set
	End Property

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