卡住了“EntityCollection已经被初始化。”错误

工具： Visual Studio 2010中
语言： C＃卡住了“EntityCollection已经被初始化。”错误

我刚开始学习实体框架，我被困在一个问题，whenver我用编号＃1，它工作正常，但每当我用CODE＃2，我得到错误（贴在下面）

标题： InvalidOperationException异常是未处理的用户代码
错误消息“”EntityCollection已经被初始化。该InitializeRelatedCollection方法只应调用对象图的反序列化期间初始化新EntityCollection“

//SchoolModel.Designer.cs 
public EntityCollection<Course> Courses 
{ 
    get 
    { //Blah blah code } 
    set 
    { 
     if ((value != null)) 
     {//Below statement is pointed by Visual Studio as Exception Thrower 
      ((IEntityWithRelationships)this).RelationshipManager.InitializeRelatedCollection<Course>("SchoolModel.CourseInstructor", "Course", value); 
     } 
    } 
}

CODE＃1：

List<string> list = new List<string>(); 
var prs = new Person(); 
using (var myEntity = new SchoolEntities()) 
{ 
    var result = myEntity.People; 
    foreach (var ppl in result) 
    { 
     list.Add(ppl.PersonID+","+ppl.FirstMidName); 
    } 
}

CODE＃2：

List<string> list = new List<string>(); 
List<Person> prsList = new List<Person>();//when using this list,problem started 
var prs = new Person(); 
using (var myEntity = new SchoolEntities()) 
{ 
    var result = myEntity.People; 
    foreach (var ppl in result) 
    { 
     list.Add(ppl.PersonID+","+ppl.FirstMidName); 

     //New code which raised exceptions 
     prs.PersonID = ppl.PersonID; 
     prs.FirstMidName = ppl.FirstMidName; 
     prs.LastName = ppl.LastName; 
     prs.Courses = ppl.Courses; 
     prsList.Add(prs); 
     //New code end 
    } 
}

数据库图表：
Database Diagram from tutorial