public class student 
        { 
            public string name; 
            public int tel; 
        } 
        public class book 
        { 
            public student[] students; 
            public void initialize() 
            { 
                students = new student[3]; 
                students[0] = new student(); 
                students[0].name = "张三"; 
                students [0].tel=25252; 

                students[1] = new student(); 
                students[1].name = "李四"; 
                students[1].tel = 572957; 

            } 
            public int  this[string thename] 
            { 
                get 
                { 
                    if (students[0].name == thename) 
                    { 
                        return students[0].tel; 
                    } 
                    else 
                    { 
                        return students[1].tel; 
                    } 
                } 
            } 
          

        } 
        private void Form1_Load(object sender, EventArgs e) 
        { 
            book bk = new book(); 
            this.richTextBox1.Text = bk["张三"].ToString(); 
        } 
    } 

运行错误：指向students[0].name == thename，未将对象应用设置到对象的实例 
请问该怎么解决啊？