繼承對象訪問出錯，未将对象引用设置到对象的实例

Question

public abstract class TaskBasic
{
       public int _ProjectClass;
       [DisplayName("項目級別")]
        public int ProjectClass
        {
            get
            {
                return _ProjectClass;
            }
            set
            {
                _ProjectClass=value;
            }
        }
        //定義一個集合
        [Association("Master-Detl"), Aggregated]
        public XPCollection<ProjectTask> Detl
        {
            get
            {
                return GetCollection<ProjectTask>("Detl");
            }
        }
}
public class ProjectTask : TaskBasic
{
       private TaskBasic _ParentProjectNo;
        [Association("Master-Detl")]
        public TaskBasic ParentProjectNo
        {
            get
            {
                return _ParentProjectNo;
            }
            set
            {
                _ParentProjectNo=value;
            }
        }       
       ProjectClass=ParentProjectNo.ProjectClass+1;--->>>>>運行到這ParentProjectNo.ProjectClass就出錯了，未将对象引用设置到对象的实例
}

Answer 1

補充下問題，ProjectClass=ParentProjectNo.ProjectClass+1;這句實在創建的新的ProjectTask 對象時報錯，public override void AfterConstruction()
        {
            base.AfterConstruction();
            ProjectClass=ParentProjectNo.ProjectClass+1
        }

Answer 2

你的ParentProjectNo都没实例化，当然报错...public class ProjectTask : TaskBasic
{
  public ProjectTask()
  {
    this.ParentProjectNo = new ProjectTask();实例化
  }
  private TaskBasic _ParentProjectNo;
  [Association("Master-Detl")]
  public TaskBasic ParentProjectNo
  {
    get
    {
      return _ParentProjectNo;
    }
    set
    {
      _ParentProjectNo=value;
    }
  }
}