List.Contains returns false, even though it seems it should return true

Question

Sub pageload() Handles Me.Load
    Dim bom As New List(Of Car)

    Dim car1 As New Car With {.Name = "Pea", .Year = 2}
    Dim car2 As New Car With {.Name = "Pea", .Year = 2}

    bom.Add(car1)

    MsgBox(bom.Contains(car2))
End Sub

WHY??? I mean the object has the exactly same data, so why does it say it is not contained?

Answer 1

The Contains method tests equality by calling the Equals method on objects. Unless you override the Equals method in the Car class and implement your own means of comparison, instances of Car will be considered equal by means of referential identity (being the exact same object) not based on their contents being equal.

Those objects you created are definitely not the same object. Proof: changing one will not change the other.

Answer 2

The key is that car is a reference type, not a value type. The two car objects you create are completely separate things. Each points to a different place in memory, even though they happen to have all the same properties. Think of them like identical twins. One gets into a truck. If you ask if the other one is in the truck, the answer is no, even though in a sense they are exactly the same.

If you had defined car2 and then set it equal to car1 (Dim car2 As New Car = car1), then you would have to pointers to the exact same place in memory. Your result would have been true. Both variables would have referred to the same car object.

Now, if this was a value type, like an integer or date (or structure. And a string behaves this way also.), then any values that are the same would be considered the same. You put $10 in a car, and I ask if $10 is in the car. The answer is yes, and it doesn't matter if it was my $10 or your $10.

You example is a great example of a key difference in how reference types and value types behave.

For reference types by default, the Equals property check to see if two objects are references to the exact same place in memory, not whether all the values are the same. (Contains uses the Equals property.) You could override that behavior for particular objects if you wished.

Answer 3

Your two cars are completely independent one of each other.

Contains checks whether any list item points to the same object like the parameter you provide. And not whether these two objects are logically equal.

If you want it to do this, you should override the Equals-method of your Car