Linq to object Right outer join

本文关键字：outer join Right object to Linq | 更新日期: 2023-09-27 18:21:33

我已经编写了以下linq查询，但没有得到预期的结果。我的预期结果是第一个和列表之间的所有匹配记录以及列表中的所有不匹配记录例如

我想要以下结果

a、b、c、d、e、f

public class Com : IEqualityComparer<DuplicateData>
    {
        public bool Equals(DuplicateData x, DuplicateData y)
        {
            return x.address.Equals(y.address);
        }
        public int GetHashCode(DuplicateData obj)
        {
            return obj.address.GetHashCode();
        }
    }
static void Run ()
{
    List<string> lst = new List<string>();
    lst.Add("a");
    lst.Add("b");
    lst.Add("c");
    lst.Add("p");
    List<DuplicateData> list = new List<DuplicateData>()
    {
        new DuplicateData{address="a"},
        new DuplicateData{address="a"},
        new DuplicateData{address="a"},
        new DuplicateData{address="b"},
        new DuplicateData{address="b"},
        new DuplicateData{address="c"},
        new DuplicateData{address="d"},
        new DuplicateData{address="e"},
        new DuplicateData{address="f"},
    };
    var dup = list.Distinct(new Com());
    var RightJoin = from x in dup
                    join y in lst
                    on x.address equals y
                    into right
                    from z in right
                    select new
                    {
                        UniqueAddress = z,
                    };
}

Linq to object Right outer join

这样试试：

var RightJoin = from x in dup
                join y in lst
                on x.address equals y
                into right
                from z in right.DefaultIfEmpty(x.address)
                select new
                {
                    UniqueAddress = z,
                };

结果是（a，b，c，d，e，f）

工作样品：http://ideone.com/MOIhZH

解释

要在linq中进行左/右联接，必须使用DefaultIfEmpty方法，当没有匹配时（联接结果为空），该方法将产生（默认）结果。但是，字符串的默认值是null，所以您必须提供"左侧"集合的默认值才能在结果集中看到它

替代方法

这可能是更方便的方法。您将从x.address（联接的左侧）中进行选择，而不是从z中进行选择并提供默认值

var RightJoin = from x in dup
                join y in lst
                on x.address equals y
                into right
                from z in right.DefaultIfEmpty()
                select new
                {
                    UniqueAddress = x.address,
                };

这应该有效：

var res = from x in list
          join y in lst on x.address equals y into right 
          from xy in right.DefaultIfEmpty()
          select new { UniqueAddress = xy };

结果：

{ UniqueAddress = a },
{ UniqueAddress = a },
{ UniqueAddress = a },
{ UniqueAddress = b },
{ UniqueAddress = b },
{ UniqueAddress = c },
{ UniqueAddress = null },
{ UniqueAddress = null },
{ UniqueAddress = null }
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