FileStream Assignment Inside a Static Method

Question

I would like to keep a filestream alive for the duration of my app to lock a file from being used. However I cannot seem to assign the MyFileStream field from within my app's logic. The below code reproduces the issue. Why does the MyFileStream assignment fail?

class Program
{        

    public class MyClass
    {
        public FileStream MyFileStream;

        public MyClass()
        {
            MyFileStream = null;
            setFS(MyFileStream);
        }
    }

    static void Main(string[] args)
    {

        MyClass classinstance = new MyClass();
        //str is null
        FileStream str = classinstance.MyFileStream;
    }

    private static void setFS(FileStream fs)
    {
        reallysetFS(fs);
    }

    private static void reallysetFS(FileStream fs)
    {
        fs = new FileStream(@"C:\myfile.xml", FileMode.Open, FileAccess.ReadWrite, FileShare.None);
    }

}