C＃list.remove（item）从我想要的其他列表中删除项目

Question

I have following code:

List<T> firstList; //list with some items
List<T> secondList = firstList //copyOfFirstList


List<T> elementsToDelete = secondList.FindAll(whichShouldIDelete);


foreach(var item in elementsToDelete){
     secondList.Remove(item);
}

The problem is that Remove() method does not delete items only from secondList. Items are deleted from both firstList and secondList. So when I remove 20 items from secondList, the same elements are removed from firstList. What may be the cause of this situation?

Answer 1

The problem is with this line:

List<T> secondList = firstList //copyOfFirstList

It is not creating a copy of the list, instead there are now two references pointing to the same instance of list.

You can do:

List<T> secondList = firstList.ToList(); // explicitly creating a new list. 

or

List<T> secondList = new List<T>(firstList);

But remember, if your List<T> has a reference type (class) then the object of each list would point to the same instance. (A shallow copy of object will be available). So if you do:

firstList[0].SomeProperty = 2;

and if that item exists in secondList then you will see the property change in second list as well.

Answer 2

Your assumption:

List<T> secondList = firstList //copyOfFirstList

is only partially right. Is it a shallow reference copy.

The secondList refers to the same object in a different variable.

Google for DeepCopy List to find answers that may match your expectation

Answer 3

As List<T> is reference type, not value one, then List<T> secondList = firstList; is not a copy, but just a reference to the same object.

Answer 4

It is because both lists point to the same "address", so editing one affects the other.

try

List<T> firstList = new List<T>();
List<T> secondList = new List<T>();
secondList.AddRange(firstList);

Answer 5

public static List<T> CloneList<T>(List<T> oldList)
    {
        BinaryFormatter formatter = new BinaryFormatter();
        MemoryStream stream = new MemoryStream();
        formatter.Serialize(stream, oldList);
        stream.Position = 0;
        return (List<T>)formatter.Deserialize(stream);
    } 

    private void button1_Click(object sender, EventArgs e)
    {
        List<string> firstList = new List<string>() { "1", "2", "3"}; //list with some items
        List<string> secondList = CloneList(firstList); //copyOfFirstList

        List<string> elementsToDelete = new List<string>() { "1" }; // do some logic to find all items you need to delete

        foreach (var item in elementsToDelete)
        {
            secondList.Remove(item);
        }
    }