Add fake data to a List

Question

So I try to create a fake list of item

var items = new List<Item>();

var item = new Item 
{
    Name = "A";
} 

for(int i=0; i<3; i++)
{
    items.Add(item);
}

item.Name = "B";

for(int i = 0; i<2; i++)
{
    items.Add(item);
}

I expect it would return a list with 3 items Name A, and 2 items Name B, but it return 5 item Name B. Where am I wrong?

Answer 1

The problem in your code is

1. You created an item object with Name property as "A"
2. you inserted the same object into three spaces of the collection but all pointing to the same object
3. You changed the Name property to "B".
4. You inserted the same object into two new spaces of the collection
5. Since the variable item and all the 5 spaces of the collection points to the same object, any change going to any of these references will change the object and reflects in all references

For more details google reference typed variables

var items = new List<Item>();



for(int i=0; i<3; i++)
{
    var item = new Item() 
    {
        Name = "A";
    } 
    items.Add(item);
}



for(int i = 0; i<2; i++)
{     
    var item = new Item() 
    {
        Name = "B";
    } 
    items.Add(item);
}

Answer 2

In C# and most modern languages, objects are treated as reference types. This means that there is only one copy of the actual object in memory and each instance of that variable is just a reference pointing to this copy.

As a result when a reference of the object has been modified all instances where it is referenced get the new value. The code

var item = new Item() 
{
    Name = "A";
} 

and

item.Name = "B";

are modifying the same object. Hence, all the values in the list have the name A, which you then change to B and assign 2 more copies of this to the array - resulting in 5 total.

If you are looking to have 3 of A and 2 of B then you need to declare a new object and assign this. Try the following:

var items = new List<Item>();

var itemA = new Item() 
{
    Name = "A";
} 

for(int i=0; i<3; i++)
{
    items.Add(itemA);
}
// item list now has 3 A
var itemB = new Item() 
{
    Name = "B";
} 

for(int i = 0; i<2; i++)
{
    items.Add(itemB);
}
// itme list now has 3 A and 2 B

Further reading:

https://learn.microsoft.com/en-us/dotnet/csharp/programming-guide/classes-and-structs/objects and to contrast with reference types - structs or value types - https://learn.microsoft.com/en-us/dotnet/csharp/programming-guide/classes-and-structs/structs

Answer 3

The problem here is that you are adding references to you created object (item) to the list, not different objects. So the 5 items in your list, and your item variable are all the same object, meaning, if you change one, you change every other reference to it as well.

Try this for clarity:

var item = new Item
{
    Name "Hello "
};

var secondItem = item;
item.Name = "World";

// This will print 'WorldWorld'
Console.WriteLine(item.Name + secondItem.Name);

Sane principle is happening to your list items, just that instead of the separate variable secondItem your second (and third, and fourth, ...) item isn't in a variable, but being held by the list.

Specifically to solve your problem, you'll need to create a new object on every iteration of your loops. Like so:

var items = new List<Item>();
Item item;

for (int i = 0; i < 3; i++)
{
    item = new Item
    {
        Name = "A"
    };

    items.Add(item);
}

for (in i = 0; i < 2; i++)
{
    item = new Item
    {
        Name = "B";
    };

    items.Add(item);
}

This code can be reduced to a few lines (6 to be specific), but I left it this long as it is easier to read and more importantly easier to understand like this

Answer 4

var item = new Item() 
{
    Name = "A";
}  

Creates an Object. An Object has reference semantics, i.e. if you change the name, all references will see the same name. You have to add new objects to a list, e.g. create a new item in your loops.

Answer 5

In the list are stored references to the variable item. Changing item.Name changes name of the so when you look items in the list, all will have name set to "B", because there are references to the item object, which's name is now "B". If you want to have only the last two items name "B", instead of

item.Name = "B";

use this:

var item = new Item{
    Name = "B"
};