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LINQ查询（按组分组）？

c#linq
3

考虑以下对象：

public class Address { public string city; public string state; public string country; }

如果我有一个地址列表，我该如何使用LINQ来获取城市、州和国家相匹配的计数列表。
所以我的结果可能是这样的:
  • "普林斯顿" "新泽西州" "美国" 122
  • "奥斯汀" "德克萨斯州" "美国" 44
  • "洛杉矶" "加利福尼亚州" "美国" 1
  • "普林斯顿" "na" "英国" 3
  • ....
谢谢！
4个回答
9
比Marc的回答更进一步了（他在我发帖之前就编辑了！）。哈哈
var qry = from addr in addresses
          group addr by new { addr.city, addr.state, addr.country } into grp
          select new
          {
            city = grp.Key.city,
            state = grp.Key.state,
            country = grp.Key.country,
            count = grp.Count(),
          };
2
var qry = addresses.GroupBy(addr => new { addr.city, addr.state, addr.country});

那么你可以执行以下操作（例如）：
foreach(var row in qry) {
    Console.WriteLine("{0} {1} {2} \t {3}",
                 row.Key.city, row.Key.state, row.Key.country, row.Count());
}

例如，每个组的.Key是复合键 new { addr.city, addr.state, addr.country}，每个组还是IEnumerable<Address>。请注意，对于数据库查询，您应该提前告诉它您的全部意图，例如：
var qry = from addr in db.Addresses
          group addr by new { addr.city, addr.state, addr.country} into grp
          select new {
              grp.Key.city, grp.Key.state,
              grp.Key.country, Count = grp.Count()
          };

这样它就有最好的机会进行合理的查询（而不是n+1）。

1

这是我在LINQPAD中尝试过的，并且它运行良好（类似于Enigmativity的答案）：

void Main()
{

    List<Address>   lst = new List<Address>();
    lst.Add(new Address{ city = "ny", state = "cy", country = "india"});
    lst.Add(new Address{ city = "ny", state = "cy", country = "india"});
    lst.Add(new Address{ city = "by", state = "cy", country = "india"});
    lst.Add(new Address{ city = "ny", state = "fr", country = "india"});
        var qry = from addr in lst
        group addr by new { addr.city, addr.state, addr.country } into grp
        select new
        {
            city = grp.Key.city,
            state = grp.Key.state,
            country = grp.Key.country,
            count = grp.Count(),
        };
        qry.Dump();
}
public class Address { public string city; public string state; public string country; }

输出： 
纽约，Cy，印度 2
柏林，Cy，印度 1
纽约，法兰克福，印度 1
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