新手求助：如何正确调用方法
我设置了score[]来存放类score的数据，想用input output 和 toave完成输入、输出和求平均分，但是总出现空值引用异常，求教

using System;
using System.Collections.Generic;
using System.Text;

namespace ConsoleApplication1
{
  public class score
  {
  public string name;
  public int code;
  public int csharp, english, mathm;
  public float avers;
   
  public void toave(ref score st)
  {
  st.avers = (st.csharp + st.english + st.mathm) / 3;
  }
  public score (int n1,score st1)
  {
  Console.WriteLine("下面输入第{0}个学生信息", n1 + 1);
  Console.WriteLine("输入姓名、学号、C#成绩、英语、数学");
  st1.name = Console.ReadLine();
  st1.code = Convert.ToInt32(Console.ReadLine());
  st1.csharp = Convert.ToInt32(Console.ReadLine());
  st1.english = Convert.ToInt32(Console.ReadLine());
  st1.mathm = Convert.ToInt32(Console.ReadLine());
  }
  public void output(ref score st2)
  {
  Console.WriteLine("{0,6} {1,4} {2,4} {3,4} {4,4} {5,4}", st2.name, st2.code, st2.csharp, st2.english, st2.mathm, st2.avers);
  }
  }


  class Program
  {
  static void Main(string[] args)
  {
  int n;
  Console.WriteLine("请输入人数");
  n=Convert.ToInt32(Console.ReadLine());
  score[] s=new score[n];
  for (int i = 0; i < n; i++)
  {
  s[i].name = "1";
  s[i].code = 1;
  s[i].csharp = 1;
  s[i].english = 1;
  s[i].mathm = 1;
  }
  for (int i = 0; i < n; i++)
  {
  s[i].score(n,s[i]);
  }
  for(int i=0;i<n-1;i++)
  for(int j=i+1;j<n;j++)
  {
  score temp;
  if(s[i].avers>s[j].avers)
  {
  temp=s[j];
  s[j]=s[i];
  s[i]=temp;
  }
  }
  for(int i=0;i<n;i++)
  s[i].toave(ref s[i]);
  for(int i=0;i<n;i++)
  s[i].output(ref s[i]);
  Console.ReadKey();
  }
  }
}


------解决方案--------------------
写得好乱阿~按你的意思改了一下~~你的面向对象思想有点问题啊~还有ref的用法最好去看看~~
using System;
using System.Collections.Generic;
using System.Text;

namespace ConsoleApplication1
{
public class score
{
private string name;
private int code;
private int csharp, english, mathm;
private float avers;

public float Avers
{
get { return avers; }
set { avers = value; }
}

public void toave()
{
this.avers = (this.csharp + this.english + this.mathm) / 3;
}
public score()
{
this.name = "";
this.code = 0;
this.csharp = 0;
this.english = 0;
this.mathm = 0;
this.avers = 0;