Copy list of items to another list

Question

I am trying to copy the list of items from list1 to another list list2. I'm able to do that. However I don't want the changes made in list2 tobe reflected in list1.

class Program
{
    static void Main(string[] args)
    {
        List<MyClass> list1 = new List<MyClass>();
        list1.Add(new MyClass(){ID = 1, Name = "name1"});
        list1.Add(new MyClass(){ID = 2, Name = "name2"});
        list1.Add(new MyClass(){ID = 3, Name = "name3"});

        //Copy items from list1 to list2
        List<MyClass> list2 = new List<MyClass>(list1);

        list1.ForEach(x => Console.WriteLine(x.Name));   //It shows the name as added

        //Empty items in list2
        list2.ForEach(x => x.Name = string.Empty);

        //Print items in list1
        list1.ForEach(x => Console.WriteLine(x.Name));   //It shows the name is empty
        Console.ReadKey();
    }
}

class MyClass
{
    private int id;

    public int ID
    {
        get { return id; }
        set { id = value; }
    }

    private string name;

    public string Name
    {
        get { return name; }
        set { name = value; }
    }
}

I hope there should be an easy way.

What you need is just a deep copy. Try doing that yourself. – Nikhil Girraj Oct 16 '15 at 09:56 — Nikhil Girraj, Oct 16 '15 at 09:56

Tim Schmelter · Accepted Answer · 2015-10-16T10:54:11.290

Since this is a reference type you are changing all. You need to create a copy of that instance:

public class MyClass 
{
    private int id;

    public int ID
    {
        get { return id; }
        set { id = value; }
    }

    private string name;

    public string Name
    {
        get { return name; }
        set { name = value; }
    }

    public MyClass Copy()
    { 
        return new MyClass
        {
            ID = this.ID,
            Name = this.Name
        };
    }
}

Now you can create the second list in this way:

List<MyClass> list2 = new List<MyClass>(list1.Select(x => x.Copy()));

Of course you don't need that method. You could do that also on-the-fly in the LINQ query:

List<MyClass> list2 = new List<MyClass>(list1.Select(x => new MyClass { ID = x.ID, Name = x.Name }));

Another similar approach is a copy-constructor. A copy-constructor is used to initialize an instance by providing another instance of the same type:

public class MyClass
{
    private int id;

    public MyClass(MyClass instance)
    {
        this.id = instance.ID;
        this.name = instance.Name;
    }

    public int ID
    {
        get { return id; }
        set { id = value; }
    }

    private string name;

    public string Name
    {
        get { return name; }
        set { name = value; }
    }
}

Now you could fill the list with copies in this way:

List<MyClass> list2 = new List<MyClass>(list1.Select(x => new MyClass(x)));

or with List.ConvertAll:

List<MyClass> list2 = list1.ConvertAll(x => new MyClass(x));

Thanks @Tim. . It worked for me. I hope this is the copy constructor. Not sure :) — Gopichandar, Oct 16 '15 at 10:33
@Gopichandar: what is a _copy constructor_? However, i'm glad that it helped you. — Tim Schmelter, Oct 16 '15 at 10:33
@Gopichandar: it's not a copy-contructor since it's not a cosntructor but a normal instance method. It's similar to a factory method. A copy-constructor is used to initialize an instance by providing another instance of the same type. You could do that also. It's a similar but different approach. — Tim Schmelter, Oct 16 '15 at 10:43
@Gopichandar: i have edited my answer to add your copy-construcor ;) — Tim Schmelter, Oct 16 '15 at 10:47

score 1 · Answer 2 · answered Oct 16 '15 at 09:50

If you want to use this feature in other places too, I guess the best way to do is, to create an extention method for IList like this:

static class Extensions
{
   public static IList<T> Clone<T>(this IList<T> sourceList) 
      where T: ICloneable
   {
       return sourceList.Select(item => (T)item.Clone()).ToList();
   }
}

And to use it, you should change your class and make it ICloneable:

class MyList : ICloneable
{
    public MyList(int idParam, string nameParam)
    {
        ID = idParam;
        Name = nameParam;
    }
    public object Clone()
    {
        return new MyList(ID, Name);
    }

    private int id;
    public int ID
    {
        get { return id; }
        set { id = value; }
    }

    private string name;
    public string Name
    {
        get { return name; }
        set { name = value; }
    }
}

[Why no ICloneable?](http://stackoverflow.com/questions/536349/why-no-icloneablet) ... [Should we Obsolete ICloneable](http://blogs.msdn.com/b/brada/archive/2004/05/03/125427.aspx) — Tim Schmelter, Oct 16 '15 at 09:55

score 1 · Answer 3 · answered Oct 16 '15 at 09:56

1

If your elements are reference types and type implement ICloneable interface you clould do something like this:

list1.ForEach((item) =>
{
    list2.Add((ICloneable)item.Clone());
});

If your element type doesn't implement ICloneable, you clould create "copy" constructor and do something like this:

list1.ForEach((item)=>
    {
        list2.Add(new MyList(item.ID, item.Name));
    });

answered Oct 16 '15 at 09:56

Mariusz

442
5
16

1

The `IClonable` contract does not specify the type of the clone performed, different classes have different implementations. A consumer cannot rely on `ICloneable` to let them know whether an object is deep-cloned or not. For that reason it should be avoided. – Tim Schmelter Oct 16 '15 at 10:06

Copy list of items to another list

3 Answers3