Dictionary Values copy

[George2]

Hello everyone,


Here is my code to make a duplicate value copy of all the Values in a dictionary instance -- not just reference. So, I use CopyTo method.

My current issue is, I think my code is stupid but do not know how to solve it. My specific concern is,

1.

the code Foo[] FooArray = new Foo[3] creates 3 new instances of Foo, the code dic.Values.CopyTo(FooArray, 0) will overwrite the 3 new instances of Foo? So, seems it is stupid to create 3 new instances of Foo in Foo[] FooArray = new Foo[3], but never use them and overwrite them? Any ideas? Is it an issue?

2.

Maybe I am wrong, the code Foo[] FooArray = new Foo[3] creates only references variable of Foo class? Not class instances variable themselves?

    class Foo
    {
        public int abc;

        public Foo (int i)
        {
            abc = i;
        }
        
    }
    
    public static void Main()
    {

        Dictionary<int, Foo> dic = new Dictionary<int,Foo>();
        dic.Add(1, new Foo(10));
        dic.Add(1, new Foo(20));
        dic.Add(3, new Foo(30));

        Foo[] FooArray = new Foo[3];

        dic.Values.CopyTo(FooArray, 0);

        return;
    }

thanks in advance,
George

[cilu]

Foo[] FooArray = new Foo[3];

That does not create 3 Foo instances. It only creates an array that has 3 elements set to null. Have you tried to run it in debugger and look at the values?

You could also put a message in the constructor of Foo and see that it's not printed.

   public Foo(int i)
   {
      abc = i;
      Console.WriteLine("Constructs Foo({0})", i);
   }

[George2]

Thanks cilu,


So this code Foo[] FooArray = new Foo[3] will only create 3 references and make them refer to NULL? Then code dic.Values.CopyTo(FooArray, 0) will create 3 instances of Foo class, and assign the 3 NULL references variables in FooArray to point to the instances?

Foo[] FooArray = new Foo[3];

That does not create 3 Foo instances. It only creates an array that has 3 elements set to null. Have you tried to run it in debugger and look at the values?

You could also put a message in the constructor of Foo and see that it's not printed.

   public Foo(int i)
   {
      abc = i;
      Console.WriteLine("Constructs Foo({0})", i);
   }

regards,
George

[cilu]

You know, most of the time, the best way is to try it.

[George2]

Thanks cilu!


I have tested CopyTo will only copy the reference, not the references, to my surprise. :-)

    class Foo
    {
        public int abc;

        public Foo (int i)
        {
            abc = i;
        }
        
    }
    
    public static void Main()
    {

        Dictionary<int, Foo> dic = new Dictionary<int,Foo>();
        dic.Add(1, new Foo(10));
        dic.Add(2, new Foo(20));
        dic.Add(3, new Foo(30));

        Foo[] FooArray = new Foo[3];

        dic.Values.CopyTo(FooArray, 0);

        // change the 1st one
        FooArray[0].abc = 40;

        // output 40
        Console.WriteLine(dic[1].abc);

        return;
    }

You know, most of the time, the best way is to try it.

regards,
George

[torrud]

C# works in most cases only with references. For getting duplicates you should implement ICloneable in your business class and call the clone() method for getting a second instance of the object. Thats the usual way in C# as far as I know.

[cilu]

I have tested CopyTo will only copy the reference, not the references, to my surprise. :-)

Hm, Foo is a class right? That makes it a reference type. So of course it behaves as you observed. In .NET there are two kind of types: value types, copied/passed by value and reference types, copied/passed by reference.

Make Foo a struct and then see what happens.