Variables refer to the same instance - vb.net

Question

Within my learning curve I play around with converting List and IEnumerable between each other.
What I am surprised with is that after executing EditMyList procedure MyIEnumerable contains the same data for each DBTable object as MyList. However I have modified MyList only, without assigning it to MyIEnumerable once List has been modified.
Can you explain what happened here and why MyList and MyEInumerable refer to the same instance?
Public Class DBTable
Public Property TableName As String
Public Property NumberOfRows As Integer
End Class
Public Sub EditMyList
Dim MyList As New List(Of DBTable)
MyList.Add(New DBTable With {.TableName = "A", .NumberOfRows = 1})
MyList.Add(New DBTable With {.TableName = "B", .NumberOfRows = 2})
MyList.Add(New DBTable With {.TableName = "C", .NumberOfRows = 3})
Dim MyIEnumerable As IEnumerable(Of DBTable) = MyList
For Each item In MyList
item.NumberOfRows += 10
Next
End Sub
UPDATE: string case where at the end b is not equal to a. String is also reference type, so assigning one variable to other one we shall copy just reference. However at the end there is different result than in the first example (explained by #Sefe)
Dim a As String
Dim b As String
a = "aaa"
b = "bbb"
a = b
' At this point a and b have the same value of "bbb"
a = "xxx"
' At this point I would expect a and b equal to "xxx", however a="xxx" but b="bbb"

Answer 1

A List is a reference type. That means it is created on the heap and your MyList variable contains just a reference (sometimes incorrectly called "pointer") to the list. When you assign MyList to MyEnumerable you don't copy the whole list, you just copy the reference. That means all changes you make to the (the one) list, is reflected by all the references.
If you want a new list you need to create it. You can use the list constructor:
Dim MyIEnumerable As IEnumerable(Of DBTable) = New List(Of DBTable)(MyList)
Since you don't need a list, but an IEnumerable you can also call the list's ToArray method:
Dim MyIEnumerable As IEnumerable(Of DBTable) = MyList.ToArray
You can also use LINQ:
Dim MyIEnumerable As IEnumerable(Of DBTable) = MyList.ToList
As far as the behavior of String is concerned, strings in .net are immutable. That means once created, they can not be changed. String operations (for example concatinations) will always create new strings. In other words: the copy operation you have to do manually for your lists is done automatically for strings. That's why you see similar behavior for strings as for value types.
Also, the assignment operation in your question would also still behave the same if strings were mutable. When you assign a = "xxx", you update the reference of afrom "bbb" to "xxx". That however does not affect b, which still keeps its old reference.

Answer 2

Use ToList() extension method for creating another List
Dim newCollection = MyList.ToList()
But notice that instances of DBTable still will reference to the same items
For creating "full" copy you need create new instances of DBTable for every item in the collection
Dim newCollection = MyList.Select(Function(item)
return new DBTable
{
.TableName = item.TableName,
.NumberOfRows = item.NumberOfRows
}
End Function).ToList()
For Each item in MyList
item.NumberOfrows += 10 ' will not affect on the newCollection items
Next