linq - 如果 LINQ to entity 查询没有返回值，如何返回默认值

Question

在这样的 LINQ to 实体表达式中：

var vote = (from vote in db.Vote where
    vote.Voter.Id == user.Id
    select v).FirstOrDefault();

如何添加 DefaultIfEmpty 值，以便在没有投票时获得默认值？

Answer 1

Another approach, if Vote is a reference type and thus uses null as its default value, would be to use the null coalescing operator:

var vote = (db.Vote
   .Where(v => v.Voter.Id == user.Id)
   .FirstOrDefault()) ?? defaultVote;

Answer 2

添加您自己的扩展方法。例如：

public static class Extension
{
    public static T FirstOrDefault(this IEnumerable<T> sequence, T defaultValue)
    { 
        return sequence.Any() ? sequence.First() : defaultValue;
    }
}

在该类的范围内，您可以说：

var vote = (from vote in db.Vote where
    vote.Voter.Id == user.Id
    select v).FirstOrDefault(yourDefaultValue);

当然，您的方法也可以有一个返回 default(T) 的重载，如果那是您要查找的内容。内置 Extension 类中已经定义了一个 DefaultIfEmpty 扩展方法，所以我在示例中将该方法命名为“FirstOrDefault”，这样看起来更合适。

Answer 3

只需在获取第一个元素之前添加默认值即可。

var vote = db.Vote
    .Where(v => v.Voter.Id == user.Id)
    .DefaultIfEmpty(defaultVote)
    .First();

请注意，您现在可以安全地使用First()而不是FirstOrDefault().

更新

LINQ to Entity does not recognize the DefaultIfEmpty() extension method. But you can just use the null coalescing operator.

var vote = db.Vote.FirstOrDefault(v => v.Voter.Id == user.Id) ?? defaultVote;

Answer 4

I ended up going for a very simple approach which was recommended by an answer here that was latter erased:

var vote = (from vote in db.Vote 
            where vote.Voter.Id == user.Id
            select v).FirstOrDefault();
if (vote == null) {
    vote = new Vote() { .... };
    db.AddToVoteSet(vote);
}

Answer 5

For some reason if I turn the resultset into a List, the Defaultifempty() works I don't know if I've inadvertantly crossed over into Linq area.

var results = (from u in rv.tbl_user
                        .Include("tbl_pics")
                        .Include("tbl_area")
                        .Include("tbl_province")
                        .ToList()
                        where u.tbl_province.idtbl_Province == prov
                       select new { u.firstName, u.cellNumber, u.tbl_area.Area, u.ID, u.tbl_province.Province_desc,
                                    pic = (from p3 in u.tbl_pics
                                           where p3.tbl_user.ID == u.ID
                                           select p3.pic_path).DefaultIfEmpty("defaultpic.jpg").First()
                                               }).ToList();