c# - 找出 DateTime 和 DateTime 之间的区别

Question

嗨，我写了一个这样的 linq 查询

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();         

但我的 ReturnDate 是 DateTime 类型的可为空变量？并且 DueDate 只是 DateTime 因此编译器抛出一个错误，这是无法完成的，任何人都可以帮助我解决这个问题

Answer 1

您可以检查可空日期时间的 HasValue 属性，然后将 Value 属性（在本例中为 DateTime）与 DueDate 进行比较：

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();