找到一個DateTime和一個DateTime

Question

嗨，我已經寫的區別LINQ查詢這樣

IssuedBooks = (from transaction in db.BookTransaction 
    join tag in db.BookTagMaster on transaction.BookTagID equals tag.ID 
    where tag.IsTagActive == true 
    join book in db.BookMaster on tag.BookID equals book.ID 
    join author in db.AuthorMaster on book.AuthorID equals author.ID 
    join category in db.CategoryMaster on book.CategoryID equals category.ID 
    join publisher in db.PublisherMaster on book.PublisherID equals publisher.ID 
    select new BookIssuedView 
    { 
     ID = transaction.ID, 
     EmployeeName = transaction.EmployeeName, 
     IssuedDate = transaction.IssuedDate, 
     ReturnDate = transaction.ReturnDate, 
     BookName = book.Name, 
     AuthorName = author.Name, 
     CategoryName = category.Name, 
     PublisherName = publisher.Name, 
     SiteID = tag.SiteID, 
     BuildingID = tag.BuildingID, 
     LateFees = transaction.LateFees, 
     DueDate = transaction.DueDate, 
     LateBy = (!transaction.IsReturned)?0:(transaction.ReturnDate - transaction.DueDate).TotalDays 
    }).ToList();   

但我ReturnDate是一個可空變量，DateTime類型的？和交貨期只是日期時間，因此編譯器拋出一個錯誤，這卡恩不能做任何機構可以幫助我角落找尋這個

Answer 1

工作，你可以檢查的爲空的日期時間HasValue屬性，然後比較Value屬性（在這種情況下，DATETIME）與交貨期：

IssuedBooks = (from transaction in db.BookTransaction 
join tag in db.BookTagMaster on transaction.BookTagID equals tag.ID 
where tag.IsTagActive == true 
join book in db.BookMaster on tag.BookID equals book.ID 
join author in db.AuthorMaster on book.AuthorID equals author.ID 
join category in db.CategoryMaster on book.CategoryID equals category.ID 
join publisher in db.PublisherMaster on book.PublisherID equals publisher.ID 
select new BookIssuedView 
{ 
    ID = transaction.ID, 
    EmployeeName = transaction.EmployeeName, 
    IssuedDate = transaction.IssuedDate, 
    ReturnDate = transaction.ReturnDate, 
    BookName = book.Name, 
    AuthorName = author.Name, 
    CategoryName = category.Name, 
    PublisherName = publisher.Name, 
    SiteID = tag.SiteID, 
    BuildingID = tag.BuildingID, 
    LateFees = transaction.LateFees, 
    DueDate = transaction.DueDate, 
    LateBy = (!transaction.IsReturned && !transaction.ReturnDate.HasValue)?0:(transaction.ReturnDate.Value - transaction.DueDate).TotalDays 
}).ToList();