How canI pass a path of a file fromUI to filestream?

Hy guys,

My task is to take some string from GUI (which is supposed to be a path to the file) and to read a file from the filestream.

pretty simle, but I failed with the mission.

the code snippet is like this:

 public class BI
    {

        string _path;

        FileStream stream = new FileStream(_path, FileMode.Open, FileAccess.Read);
        StreamReader reader = new StreamReader(stream);



        public LottoMashine(string path)
        {
            _path = Path.Combine(path);
        }
    }

What do I get?

Error2 A field initializer cannot reference the non-static field, method, or property 

What did I do the wrong pay?