c# - Join 语句中的 Lambda where 条件

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我必须过滤Employee基于他们的department 。我可以用 LINQ 做同样的事情。

Linq和lambda编译得到相同的结果。编译器将查询表达式更改为等效的 Lambda expression编译之前，所以生成的IL是完全一样的。 Source

var deptCollection = new List<Dept>();
var employeeCollection = new List<Employee>();

employeeCollection.Add(new Employee { Id = 1, Name = "Eldho" });

deptCollection.Add(new Dept { DepetarmentName = "a", EmployeeId = 3 });
deptCollection.Add(new Dept { DepetarmentName = "a", EmployeeId = 1 });

var empinadept = (from e in employeeCollection
                  from dep in deptCollection
                  where e.Id == dep.EmployeeId
                  && dep.DepetarmentName == "a"
                  select e)
                 .ToList();

I can't able to add .Where Clause in this lambda

var empindeptLamda = employeeCollection
                     .Join(deptCollection,
                     emp => emp.Id, dep => dep.EmployeeId,
                     (em, dep) => em.Id == dep.EmployeeId
                      && dep.DepetarmentName == "a")
                     .ToList();

class Employee
{
    public int Id { get; set; }
    public string Name { get; set; }
}

class Dept
{
    public int EmployeeId { get; set; }
    public string DepetarmentName { get; set; }
}

Q1。上述 linq 的等效 lambda 语句是什么？(如何在方法语法中添加 linq 中的 where 子句

Answer 1

相当于:

var empinadept = (from e in employeeCollection
              from dep in deptCollection
              where e.Id == dep.EmployeeId
              && dep.DepetarmentName == "a"
              select e)
             .ToList();

这是:

var result = employeeCollection.Join(deptCollection,
        e => e.Id,
        dep => dep.EmployeeId,
        (e,dep) => new { e, dep })
    .Where(item => item.dep.DepetarmentName == "a")
    .Select(item => item.e)
    .ToList();

更好的选择是:

var result = employeeCollection.Join(
            deptCollection.Where(dep => dep.DepetarmentName == "a"),
            e => e.Id,
            dep => dep.EmployeeId,
            (e,dep) => e)
       .ToList();

最接近查询语法(但我想说这不太好基于意见)是:

var result = employeeCollection.Join(
        deptCollection,
        e => new { e.Id, "a" },
        dep => new { dep.EmployeeId, dep.DepartmentName },
        (e,dep) => e).ToList();