Lists getting cleared unexpectedly after assignment [duplicate]

Answer 1

 if (a is System.ValueType)
 {
   //never
    Console.WriteLine("List is value type");
 }
 if ('s' is System.ValueType)
 {
     //always
     Console.WriteLine("char is value type");
 }

I think you know, char is value type, but list is reference type.

Even code like this; it would be same.

List<char> a = new List<char>();
a.Add('s');
List<char> c = a;
a.Clear();
Console.WriteLine(c.Count);

Answer 2

Since you are passing a reference instead of the list itself, you will get 0 after clearing your list.

What you have to do is passing to the class a new List containing the other list's values:

cl c = new cl(new List<char>(a)); 

This way, even if you clear the 'main' list, in your class you'll have 1 as items count.

Hope this helps.

Answer 3

List and other classes are reference types. In few words, it means you have an object somewhere in memory and a reference(s) on it.

this.l = l; means you copied the reference to the first list to the class field. So you have one list and two references on it. And when you clear the list via a variable, no matter how you address it after clearing - via a or cl.l. Your single list is cleared already.

If you want to avoid this, you need to create a copy of list in your constructor:

public cl(List<char> l)
    {
        this.l = new List<char>();
        this.l.AddRange(l);
    }
}

I recommend you to read more information about reference types. They are used widely and knowledge about them will give you a good base for programming skills.