Question

考虑以下情况：

类

public class Nested
{
    public Nested(int id){ Id = id; }
    public int? Id {get; set;}  
}

public class Pair
{
    public Nested NestedType {get; set;}
    public string Name {get; set;}  
}

public class Match
{
    public int? Id {get; set;}
    public string Name {get; set;}  
}

实例

var p1 = new Pair();
p1.NestedType = new Nested(1);
p1.Name = "a";          

var p2 = new Pair();
p2.NestedType = null;
p2.Name = "b";

var p3 = new Pair();
p3.NestedType = new Nested(3);
p3.Name = "c";

List<Pair> pairs = new List<Pair>() {p1, p2, p3};

var m1 = new Match();
m1.Id = 1;
m1.Name = "AA";

var m2 = new Match();
m2.Id = null;
m2.Name = "BB";

var m3 = new Match();
m3.Id = 3;
m3.Name = "CC";

List<Match> matches = new List<Match>(){m1, m2, m3};

查询

var query = from p in pairs
            join m in matches on p.NestedType.Id equals m.Id into pm
            from m in pm.DefaultIfEmpty()                               
            select new
            {
               PairName = p.Name,
               MatchId = m != null ? m.Id : null
               MatchName = m != null ? m.Name : null
            };

当equals为空时，查询将在p.NestedType语句行上抛出异常。

我想要实现的目标是：当p.NestedType为空时，应放置null值而不是它;就像分配了p.NestedType但其Id为空。可以吗？

编辑：我必须使用C＃5.0。

Answer 1

使用where(...)子句，您可以防止在您的案例中抛出空引用异常，并检查这是否会为您的案例产生所需的结果：

var query = from p in pairs
            where p.NestedType!=null //Notice the where condition added
            join m in matches on p.NestedType.Id equals m.Id into pm
            from m in pm.DefaultIfEmpty()
            select new
            {
                PairName = p.Name,
                MatchId = m != null ? m.Id : null,
                MatchName = m != null ? m.Name : null
            };

或

如果您想要始终获取所有左侧值，例如，如果结果看起来像下面这样，您可以使用Null-Conditional Operator in C# 6.0 +：

var query = from p in pairs join m in matches on p?.NestedType?.Id equals m.Id into pm //Notice Null-Conditional operator: p?.NestedType?.Id from m in pm.DefaultIfEmpty() select new { PairName = p.Name, MatchId = m != null ? m.Id : null, MatchName = m != null ? m.Name : null };

或

如果我们需要在下面评论的C＃5中进行，您可以这样做：

var query = from p in pairs let nestedTypeId = (p != null && p.NestedType != null) ? p.NestedType.Id : null join m in matches on nestedTypeId equals m.Id into pm from m in pm.DefaultIfEmpty() select new { PairName = p.Name, MatchId = m != null ? m.Id : null, MatchName = m != null ? m.Name : null };

当等于左边的类可以为null时，Linq离开了join

1 个答案: