c# - LINQ 加入 : Object reference not set to an instance of an object

Question

我有一个从 Linq to SQL 生成的列表。为简单起见，它看起来像这样:

var myItems = (from my in db.MyTable
                select new
                {
                    UniqueID = my.ID,
                    UserName = my.UserName,
                    CreatedOn = my.CreatedOn
                }).ToList();

此列表包含 4 个项目。

我还有一个:

var grid = (from q in AnotherLinqQuery
                select new
                {
                    UniqueID = q.ID,
                    Department = q.Department,
                    Comments = q.Comments
                }).ToList();

此列表包含 20 个项目。myItems 中的所有 ID 都显示在网格中。

现在我想用左连接将它连接起来。

var q = from A in grid
        from B in myItems.Where(x => x.UniqueID == grid.UniqueID).DefaultIfEmpty()
        select new 
        {
           UniqueID = A.UniqueID,
           Department = A.Department,
           CreatedOn = B.CreatedOn
        }

当我执行这个时，我得到

Object reference not set to an instance of an object.

我也尝试过其他连接，例如

from A in grid
            from B in myItems.Where(x => x.UniqueID != null && x.UniqueID == grid.UniqueID).DefaultIfEmpty()

Answer 1

您没有正确加入。试试这个:

var q = from A in grid
        join B in myItems on A.UniqueId equals B.UniqueId into LB
        from B in LB.DefaultIfEmpty()
        select new 
        {
            UniqueID = A.UniqueID,
            Department = A.Department,
            CreatedOn = B.CreatedOn
        };

您可能需要引用 documentation有关加入 linq 的更多信息。