How can I pass an object instance in a method call in C#?

Question

I have a class Club which has a list _list as an argument, while 'Player' is another class ... I have this method for adding the club members, thas is implemented this way:

public void AddPlayer(Player p)
{
    if(_list.Contains( p)) 
        throw new Exception("The Player " + p.Name + " " + p.Surname + " is already a         member of this club!");
    _list.Add(p);
}

Now, when I do this in my main program:

Player p = new Player("Christiano", "Ronaldo", 1993);
Club club = new Club("ManUtd", coach);
club.AddPlayer(p);

It throws an exception that says that object reference is not set as an instance of an object.

Answer 1

(Constructor code grabbed from OP comment.)

In your constructor, it appears that you just initializing a local variable within that method, not the field _list.

public Club(string clubName, Coach mycoach) 
{ 
     List<Player> _list = new List<Player>(); 
     ClubName = clubName; 
     Mycoach = mycoach; 
}

Try changing

 List<Player> _list = new List<Player>(); 

to:

 _list = new List<Player>();