C#中的表达式 [英] Expressions in C#

问题描述

大家好，

如何转换为Func返回类型?

在下面的代码中，我在提到错误"的地方出现错误.因为我无法从Employee的IEnumerable转换为Expression< Func< Employee>>.我该如何转换呢?

公共静态类PredicateClass
{
    公共静态表达式< Func< List< Employee>>> GetByDeptId(int deptId)
    {
      雇员obj = new Employee();
      return obj.GetAllEmployees().Where(x => x.DeptId == deptId); //错误
    }
}

班级计划
{

静态void Main(string [] args)
{
  PredicateClass.GetByDeptId(2); //错误
} 

解决方案

重要的是要认识到Expression和IEnumerable是两个完全不同的东西，所以您不能只是转换". >

表达式是给定表达式的树数据结构的表示.在您的特定情况下，该函数的数据结构将返回雇员列表.

换句话说，它描述的是函数的功能，而不是实际执行的功能.

也就是说，要返回一个表示您的函数的表达式，那么我认为您可以这样做:

返回()=> obj.GetAllEmployees().其中​​(x => x.DeptId == deptId); 

(您可能需要在该return语句的末尾粘贴一个'.ToList()').

Hi All,

How do I convert to Func return type?

In the below code I get error in the places where I have mentioned "Error" because I am not able to convert from IEnumerable of Employee to Expression<Func<Employee>>. How do I convert to that?

public static class PredicateClass
{
    public static Expression<Func<List<Employee>>> GetByDeptId(int deptId)
    {
      Employee obj = new Employee();
      return obj.GetAllEmployees().Where(x=>x.DeptId == deptId); // Error
    }
}

class Program
{

static void Main(string[] args)
{ 
  PredicateClass.GetByDeptId(2); // Error 
}

解决方案

It's important to recognise that an Expression and and an IEnumerable are two completely different things so you cannot just 'convert'.

The Expression is a representation of the tree data structure for a a given expression. In your particular case, a data structure for a function that returns a list of Employees.

In other words, it describes what the function does rather than actually doing it.

That said, to return an expression representing your function then I think you could do:

return ()=>obj.GetAllEmployees().Where(x=>x.DeptId == deptId); 

(You may need to stick a '.ToList()' on the end of that return statement).

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