Question

My program has decided to assign pointers instead of make copies of an object, and I’m not sure why. I have something like this:

    Public Class Foo
       Private myFooData As New List(Of FooData)

       Public Sub New(ByVal newFooData As List(Of FooData))
           myFooData = newFooData 
       End Sub

       Public Property FooValues() As List(Of FooData)
           Get
               Return myFooData 
           End Get
           Set(ByVal value As List(Of FooData))
                   myFooData = value
            End Set
       End Property
    End Class

And it’s used like this:

    Public Sub Dosomething()
        Dim mainFoo as new Foo

        For x = 1 to 10
            mainFoo.FooValues(x) = New FooData
        Next

        Dim originalFoo as new Foo
        originalFoo.FooValues = mainFoo.FooValues.Take(3).ToList

        Dim newFoo as new Foo
        newFoo.FooValues = originalFoo.FooValues

        newFoo.FooValues(1) += 1
    End Sub

Very simplified, but basically what I’m doing. So for some reason today when I change item in newFoo.FooValues, originalFoo.FooValues also changes, and mainFoo does not. I’ve tried assigning the entire objects as well and I get the same results. Any ideas why this may be happening and how to fix it?

Answer 1

This is how assignment in .Net is supposed to work.

When you called .ToList() in the middle of your second snippet, your code iterates over the set and makes copies into a whole new list. This is why your mainFoo object is “protected” — you created a new instance. If the FooData items being copied are themselves references to objects (hint: they probably are), then only the references are copied. The only exceptions are for strings and value types (primitives and structures), or if you code it by hand.

It’s usually a good idea for List properties to make the property readonly:

Public Property FooValues() As List(Of FooData)
   Get
       Return myFooData 
   End Get
End Property

This will still let you manipulate the list to your heart’s content, but prevents you from completely switching a list instance out from under the class. The same is true for other complex types exposed from a class as a property.

The one thing you don’t want to do is change this to be a Structure instead of a Class. This may seem to do what you want at first, but it will cause other problems for you later.