卡住了「EntityCollection已經被初始化。」錯誤

工具： Visual Studio 2010中
語言： C＃卡住了「EntityCollection已經被初始化。」錯誤

我剛開始學習實體框架，我被困在一個問題，whenver我用編號＃1，它工作正常，但每當我用CODE＃2，我得到錯誤（貼在下面）

標題： InvalidOperationException異常是未處理的用戶代碼
錯誤消息「」EntityCollection已經被初始化。該InitializeRelatedCollection方法只應調用對象圖的反序列化期間初始化新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 
    } 
}

數據庫圖表：
Database Diagram from tutorial