Question

我有一个具有以下属性的对象列表

public class Client
{
Id Guid,
FromDate datetime
}

客户列表

  List<Clients> clients = new List<Clients>();

此列表中有许多相同的ID具有不同的FromDates。如何获取每个Id的Id和最小FromDate列表？

例如：

Id    FromDate
1       2015-01-01
1       2016-01-01
2       2015-02-02
2       2015-12-31

我希望最终得到一个包含以下内容的列表：

Id    FromDate
1       2015-01-01
2       2015-02-02

Answer 1

您可以申请group by：

var query=clients.GroupBy(c=>c.Id)
                 .Select(g=>new {Id=g.Key, FromDate=g.Min(c=>c.FromDate)});

Answer 2

RKObjectManager* objectManager = [RKObjectManager managerWithBaseURL:url];
RKObjectMapping *userMapping = [RKObjectMapping requestMapping]; 
NSDictionary *objMapping = @{@"id": @"MyId"};

[userMapping addAttributeMappingsFromDictionary:objMapping];
RKRequestDescriptor *requestDescriptor = [RKRequestDescriptor requestDescriptorWithMapping:userMapping objectClass:[MyObject class] rootKeyPath:nil method:RKRequestMethodPOST];
[objectManager addRequestDescriptor:requestDescriptor];

objMapping = @{@"id": @"MyId"};

[userMapping addAttributeMappingsFromDictionary:objMapping];
requestDescriptor = [RKRequestDescriptor requestDescriptorWithMapping:userMapping objectClass:[AnotherObject class] rootKeyPath:nil method:RKRequestMethodPOST];
[objectManager addRequestDescriptor:requestDescriptor];

Answer 3

有两种不同的方式

Distinct 您必须实施自己的IEqualityComparer<Client>（https://msdn.microsoft.com/en-us/library/bb338049.aspx）而且你可以致电：
```
IEnumerable<Client> noduplicates = products.Distinct(new ClientComparer());
```

GroupBy

var result = clients
                .GroupBy(c => c.Id)
                .Select(g => new { Id = g.Key, FromDate = g.Min(c => c.FromDate) });

var result = from c in clients
    group c by new { id = c.Id } into g
    select new { id = g.Key, FromDate = g.Min(c => c.FromDate) };

Linq找到每个对象的最小值

3 个答案: