在我的架構中，我有業務物件（從現在開始為 BO），它回傳 OperationResult<some_generic_type> 型別的標準結果，以便為每個 BO 結果提供檢測資訊（操作狀態成功/失敗、例外、操作別名、錯誤代碼.. 等等）。為了使這成為可能，每個 BO 呼叫都由一個名為“manager”的物件進行調解，該物件會將 BO 結果包裝到 OperationResult 中。
即使被管理器封裝，回傳型別總是在回傳時準備好，在我的專案中我不會使用延遲加載或延遲執行。
這就是說作為前提，我不明白奇怪的行為，其中兩個不同的串列應該指向相同的元素，但它們沒有（在評論中更詳細）：
var opResult = manager.Execute(userBo.FindUser, token, query);
//userBo.FindUser will return data inside a custom type that is "paged" list
//each page is not of type list but IEnumerable instead
if (opResult.Success && opResult.ReturnData != null && opResult.ReturnData.PageContent != null)
{
request.ItemCountAfterProcessing = opResult.ReturnData.ItemsCount;
request.ItemCountInPage = opResult.ReturnData.ActualItemsPerPage;
var users = opResult.ReturnData.PageContent.ToList();
//here i get the page as List, keep in mind that datasource was already a list but my custom
//'BasePageResults' type represent the page content as IEnumerable<T> for conveninece
//In the following instruction i decorate 'users' list with contact information about such users
//Everything work correctly and after decoration each user has its own contact information attached
var usersIds = users.Select(usr => usr.Id).ToList();
var contactQuery = new PagedQueryDto<tbl_usr_Contact> ( addr => usersIds.Contains(addr.USER_ID) );
var opContactFetchResult = manager.Execute(userBo.FindAddressBook, token, contactQuery);
if (opContactFetchResult.Success && opContactFetchResult.ReturnData != null && opContactFetchResult.ReturnData.PageContent != null)
{
Dictionary<int, ContactDto> indexedContacts = opContactFetchResult.ReturnData.GroupBy ( addr => addr.UserId )
.ToDictionary ( group => group.Key , group => group.FirstOrDefault() );
foreach (var user in users)
if (indexedContacts.ContainsKey(user.Id))
user.Contact = indexedContacts[user.Id];
}
var newListWithSameReference = opResult.ReturnData.PageContent.ToList();
//if now i inspect 'users' list i can find that each user has its contacts attached
//if now i inspect 'newListWithSameReference' the users appear to be in initial state (no contact information)
//What is not cler to me is that both variable point to the same list reference 'opResult.ReturnData.PageContent'
//In fact 'userBo.FindUser' return a paged list, where each page is a List<T> but is seen as IEnumerable<T>
//only due to the fact that i use the type BasePageResults in the signature (as return type)
result = opResult.ReturnData.PageContent.ToList().Select ( usr => new DestinationUserDto ( usr) ).ToList();
}
return result;
我知道我可能對所涉及的型別有點不清楚，為了清楚起見，我在這里添加了自定義分頁串列型別定義和 FindUser 方法
這里的分頁串列定義：
public class BasePageResults<TEntity> : IEnumerable<TEntity> where TEntity : new()
{
public TEntity this[int index]
{
get
{
if (index >= 0 && index < (this.PageContent?.Count() ?? 0))
this.PageContent.ElementAt(index);
throw new IndexOutOfRangeException();
}
set
{
if (index >= 0 && index < (this.PageContent?.Count() ?? 0))
{
var temporaryList = new List<TEntity>(this.PageContent);
temporaryList[index] = value;
this.PageContent = temporaryList;
}
throw new IndexOutOfRangeException();
}
}
/// <summary>
/// Content of the current query page
/// </summary>
public IEnumerable<TEntity> PageContent { get; set; }
/// <summary>
/// The current page number
/// </summary>
public int PageNumber { get; set; }
/// <summary>
/// Indicate how many items should be in the page
/// </summary>
public int ItemsPerPage { get; set; }
/// <summary>
/// Indicate how many items there are (actually) in the page
/// </summary>
public int ActualItemsPerPage { get { return this.PageContent?.Count() ?? 0; } }
/// <summary>
/// Define how many items match the query regardlss of how many items are currently placed in the current page
/// </summary>
public long ItemsCount { get; set; }
/// <summary>
/// Define how many page there are in total
/// </summary>
public int PagesCount { get { return this.ItemsPerPage <= 0 ? 0 : (int)Math.Ceiling((double)this.ItemsCount / (double)this.ItemsPerPage ); } }
public IEnumerator<TEntity> GetEnumerator()
{
return this.PageContent?.GetEnumerator();
}
IEnumerator IEnumerable.GetEnumerator()
{
return this.PageContent?.GetEnumerator();
}
}
這里的 FindUser 結構：
/// <summary>
/// Apply a query on user repository to find corresponding UserDto.
/// Result are presented in pages
/// </summary>
/// <param name="query">The query to apply to datasource</param>
/// <returns>The page searched of Users</returns>
[PermissionRequired(PermissionAttribute.Login | PermissionAttribute.Read)]
[Intent(IntentDescription.Read)]
public BasePageResults<UserDto> FindUser(PagedQueryDto<tbl_usr_User> query)
{
if (query == null)
throw new ExtendedArgumentException("query");
using (var context = ServiceLocator.ConnectionProvider.Instace<UserRoleDataContext>())
{
var repository = new UserRepository(context);
var dbQuery = repository.Read(query.Query);
var page = base.GenericPagedRead(dbQuery, query);
return new BasePageResults<UserDto> ()
{
ItemsCount = page?.ItemsCount ?? 0,
ItemsPerPage = page?.ItemsPerPage ?? 0,
PageNumber = page?.PageNumber ?? 0,
PageContent = page?.PageContent?.Select ( usr => (new UserDto()).Feed(usr) ) ?? new List<UserDto> ()
//page?.PageContent is already a list provided by ORM that will then mapped in DTO, so the return type is definitely a List and is not deferred or lazy loading here. ORM has already done his work when i get there
};
}
}
我真的無法理解為什么“用戶”和“newListWithSameReference”變數的行為就像它們是兩個深度復制的變數（如果我更改“用戶”的第一個元素中的屬性，它們應該是兩個不同串列中相同值的淺表副本列出'newListWithSameReference'的相應元素應該改變）
uj5u.com熱心網友回復：
有很多代碼使讀者很難找到相關位。為了證明您的問題，您只需呼叫opResult.ReturnData.PageContent.ToList();兩次，然后修改第一個串列的第一個元素并顯示它在第二個串列中保持不變。
現在解決問題。查看FindUser方法中的這一行。
return new BasePageResults<UserDto> ()
{
PageContent = page?.PageContent?.Select ( usr => (new UserDto()).Feed(usr) ) ?? new List<UserDto> ()
};
盡管page?.PageContent是一個串列，但每次訪問屬性BasePageResults<UserDto>.PageContent時IEnumerable<UserDto>都會創建新的。UserDtoPageContent
要獲得所需的行為，請在ToList之后添加Select
return new BasePageResults<UserDto> ()
{
PageContent = page?.PageContent?.Select ( usr => (new UserDto()).Feed(usr) ).ToList() ?? new List<UserDto> ()
};
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