如何将 linq 查询转换为 lambda 表达式答案

【问题标题】：how to convert linq query into lambda expression如何将 linq 查询转换为 lambda 表达式
【发布时间】：2020-06-01 13:19:20
【问题描述】：

我已经写了 Linq Query，我想把它做成 Lambda 表达式。如何编写 Lambda 表达式 代替 return 语句 Linq 查询。实体表是entities.Users,entities.Users,entities.ponds

查询表达式：

from pond in Ponds
join customerdevice in CustomerDevices on pond.Imei equals customerdevice.Imei
join user in Users on customerdevice.CustomerId equals user.CustomerId
where user.Username=="user1"
select new { temp = pond.Temp, imei = pond.Imei,timestamp=pond.Timestatmp }

Lambda 表达式：

 public async Task<IHttpActionResult> GetAllData(int deviceid)
        {
            using (smartpondEntities entities = new smartpondEntities())
            {
                try
                {
                    return Ok(await entities.ponds.Where(u=>u.deviceid==deviceid).OrderByDescending(u => u.timestatmp).ToListAsync());
                }
                catch (Exception Ex)
                {
                    return BadRequest("Sorry Error Found!!!");
                }
            }
        }

【问题讨论】：

我已经修改了我的问题，你可以检查一下

标签： c# linq lambda

【解决方案1】：

你的 lamda 应该是：

var ponds = from pond in Ponds
join customerdevice in CustomerDevices on pond.Imei equals customerdevice.Imei
join user in Users on customerdevice.CustomerId equals user.CustomerId
where user.Username=="user1"
select new { temp = pond.Temp, imei = pond.Imei,timestamp=pond.Timestatmp }

然后查询（我用imei替换了deviceid）。

return Ok(await entities.ponds.Where(u=>u.imei==deviceid).OrderByDescending(u => u.timestatmp).ToListAsync());

【讨论】：

【解决方案2】：

这几乎是 1-1 映射。

entities.Ponds
    .Join(entities.CustomerDevices, pond => pond.Imei, device => device.Imei, new {pond,device});
    .Join(entities.Users, devicePond => devicePond.deviceCustomerId, user => user.CustomerId, new {devicePond.pond, devicePond.device, user})
    .Where( e => e.user.Username == "user1")
    .Select( e => new { temp = e.pond.Temp, imei = e.pond.Imei,timestamp=e.pond.Timestatmp })
    .ToListAsync();

但是，我建议您在实体上添加一些导航属性以缩短 Linq 查询并让 EF 为您生成连接。

public class Ponds {
    public string Imei {get;set}
    [ForeignKey(nameof(Imei))]
    public virtual CustomerDevice Device {get;set;}
}

public class CustomerDevices {
    [Key]
    public string Imei {get;set}
    public int CustomerId {get;set;}
    public int DeviceId {get;set;}
    [ForeignKey(nameof(CustomerId))]
    public virtual User Customer {get;set;}
}

public class Users { 
    [Key]
    public int CustomerId {get;set;}
}

将查询缩短到：

this.entites.Ponds.Where ( 
   e => e.Device.Customer.Username == "user1" ||
        e.Device.DeviceId == 1
)
.Select( e => new { temp = e.Temp, imei = e.Imei,timestamp=e.Timestatmp })
.FirstOrDefaultAsync();

如果您使用 MS DB 进行查看，则应考虑使用 SQLProfiler 之类的工具，然后优化 EF-SQL 查询。

【讨论】：