Data gets overwritten when adding class object to generic list - asp.net

Question

I'm trying to add a class object (I think that's what it's called) to a generic list of my class. In the following code, whenever I change the values in TempQuoteReturnFromExecuteEstimate, it changes those values in all the rows of the generic list. I guess it's just referencing the object. How can I copy the values to the generic list? Thanks
Dim AllEstimatesReturn As New List(Of QuoteReturnData)
Dim TempQuoteReturnFromExecuteEstimate As New QuoteReturnData
...
AllEstimatesReturn.Add(TempQuoteReturnFromExecuteEstimate)

Answer 1

Your assumption is correct. When you create a class-type (reference-type) variable, it merely references an object. You can have many different variables all referencing the same object at the same time.
In this case, each item in your AllEstimatesReturn list is like a separate QuoteReturnData variable. Each item is merely a reference to an object, not a copy. So, theoretically, every item in a list can all reference the same object, although, usually that's not what you want.
If you want to make separate objects for each item in the list, you will need to create them, each time, using the New keyword:
'Add the first item
Dim TempQuoteReturnFromExecuteEstimate As New QuoteReturnData
...
AllEstimatesReturn.Add(TempQuoteReturnFromExecuteEstimate)
'Add the second item
TempQuoteReturnFromExecuteEstimate = New QuoteReturnData ' Use New to create another object
...
AllEstimatesReturn.Add(TempQuoteReturnFromExecuteEstimate)
Note that in the above example, I am reusing the same variable and just changing it's value so that it is referencing a new object. The old object is still there because it is referenced by the list object. Alternatively, you could declare a new variable each time, but, unless you want to do so for clarity, there is no technical reason why you need to do so.

Answer 2

Class types are reference types. Therefore this adds 3 references to the same object into the list:
Dim AllEstimatesReturn As New List(Of QuoteReturnData)
Dim TempQuoteReturnFromExecuteEstimate As New QuoteReturnData
AllEstimatesReturn.Add(TempQuoteReturnFromExecuteEstimate)
AllEstimatesReturn.Add(TempQuoteReturnFromExecuteEstimate)
AllEstimatesReturn.Add(TempQuoteReturnFromExecuteEstimate)
Either create new objects every time
Dim AllEstimatesReturn As New List(Of QuoteReturnData)
AllEstimatesReturn.Add(New QuoteReturnData())
AllEstimatesReturn.Add(New QuoteReturnData())
AllEstimatesReturn.Add(New QuoteReturnData())
Or implement a method that clones QuoteReturnData:
Class QuoteReturnData
' Creates an exact copy of the current QuoteReturnData object.
Public Function SwallowCopy() As QuoteReturnData
Return DirectCast(Me.MemberwiseClone(), QuoteReturnData)
End Function
End Class
MemberwiseClone is inherited from Object and does exactly what we need; however, this method is protected and can only be called from within the class. Therefore we wrap it into a public function.
Now you can do this
Dim AllEstimatesReturn As New List(Of QuoteReturnData)
Dim TempQuoteReturnFromExecuteEstimate As New QuoteReturnData
AllEstimatesReturn.Add(TempQuoteReturnFromExecuteEstimate.SwallowCopy())
AllEstimatesReturn.Add(TempQuoteReturnFromExecuteEstimate.SwallowCopy())
AllEstimatesReturn.Add(TempQuoteReturnFromExecuteEstimate.SwallowCopy())
Note that this only creates a shallow clone. If the object contains references to other objects, these other objects won't be cloned automatically. It's up to you to decide whether this is okay or whether you need a deep clone.

Answer 3

Objects are stored in references. Only value types are copied (i.e. structs).
In order to have a copy of your object, you need to clone it. Unless you do this, you'll be always creating many references to the same object. Your list isn't an exception.
I'd start implementing the ICloneable interface present in the .NET Framework.
Update
If you don't like the ICloneable interface because Clone() returns the object typed as object, you can implement an interface like this:
Public Interface ICloneable(Of T)
{
Function Clone() As T;
}
Your code adding items to the list will look something like this (if QuoteReturnData implements ICloneable(Of T), of course):
AllEstimatesReturn.Add(TempQuoteReturnFromExecuteEstimate.Clone());