逻辑上为什么不能更改对象的ParentID,而必须自己更改父对象?

//Set Parent ID for the rest of the Reports data sources
this.ReportDataSources.ToList().ForEach(rds => rds.Parent = reportDataSource);

为什么不能直接设置父ID?是什么让LINQ阻止了这种行为

//Set Parent ID for the rest of the Reports data sources
this.ReportDataSources.ToList().ForEach(rds => rds.ParentID = reportDataSource.ID);

此处引发异常

[Column(Storage="_ParentID", DbType="Int")]
public System.Nullable<int> ParentID
{
    get
    {
        return this._ParentID;
    }
    set
    {
        if ((this._ParentID != value))
        {
            if (this._Parent.HasLoadedOrAssignedValue)
            {
                throw new System.Data.Linq.ForeignKeyReferenceAlreadyHasValueException();
            }
            this.OnParentIDChanging(value);
            this.SendPropertyChanging();
            this._ParentID = value;
            this.SendPropertyChanged("ParentID");
            this.OnParentIDChanged();
        }
    }
}