如何在单个LinQ查询中编写OrderByDescending和where子句

Question

I want to show the latest UpdateDevice -value of a particular DeviceId . When writing a LinQ query using OrderByDescending and Where , I'm getting the error

Cannot execute text selection: CS1061 'int' does not contain a definition for 'OrderByDescending' and no extension method 'OrderByDescending' accepting a first argument of type 'int' could be found

Datatypes

Id - int32
UpdatedDate- datetime

LinQ

from a in Attendances
where a.DeviceId == 1 
.OrderByDescending(x=>x.Id)
.Take(1)
select a.UpdatedDate

Answer 1

You need to put parenthesis for you predicate ie where clause.

Either use query syntax or lambda expression completely. The following should work:

(from a in Attendances
where a.Deviceid == 1 
select a)
.OrderByDescending(x=>x.Id)
.Take(1)
.Select(x=>x.UpdatedDate)

or use lambda expression syntax:

Attendances.Where(a =>  a.Deviceid == 1) 
.OrderByDescending(x=>x.Id)
.Take(1)
.Select(a => a.UpdatedDate);

Side Note:

If single item is intended to be returned, then you can use FirstOrDefault() or First() , you can read about the difference of both :

var latestDate = Attendances.Where(a =>  a.Deviceid == 1) 
                      .OrderByDescending(x=>x.Id)
                      .FirstOrDefault()
                      ?.UpdatedDate;