Lambda表达式树 [英] Lambda Expression Tree

问题描述

我正在做一些改进项目。



我想修改lambda表达式树，以便我可以编译它。按照现在的逻辑：



假设我想要一些学生的数据，从第8天和第9天开始。所以当前的表达式树构建如下：



std == 8th std and student = student 1

std == 8th std and学生=学生2

std == 8th std and student = student 3.



std == 9th std and student = student 1

std == 9th std and student = student 2

std == 9th std and student = student 3.





因为这很耗时所以我想修改为：



std == 8th和学生（学生1，学生） 2，学生3）

std == 9th和学生（学生1，学生2，学生3）是这样的。



任何想法怎么做？

解决方案

你的问题没有定论：你是否要求像 dynamix LINQ [ ^ ]？或者我的CP示例：发明您自己的动态LINQ解析器 [ ^ ] ？

它们都接受一个字符串并将其转换为一个编译的表达式树，可以用于查询等（例如用于LINQ查询）。

干杯

Andi

使用这种表达方式：

 Student.Where（ c => c.std ==8&& c => c.student ==student1）; 

是你的student1，student2，student3总是这3个值（或一些 3个特定值）？

或者是这样的情况你有一些集合的价值？

  //  如果ID值总是相同： 
  private   static   readonly 列表< string> ID =  new 列表< string>（）{  student1 ，  student2，  student3}; 
  public   static   bool  MatchingStudent （ StudentType 学生， int  std）
 {
  //  添加对学生为空的检查!!  
  //  根据您的需要使用适当的StringComparer  
  return  student.std == std&& IDs.Contains（student.StudentID，StringComparer.CurrentCultureIgnoreCase）; 
} 
  //  如果ID值在某个集合中： 
  public   static   bool  MatchingStudent（ StudentType 学生， int  std，IEnumerable< string> studentIDs）
 {
  //  添加对student和studentID为空的检查!!  
  //  根据您的需要使用适当的StringComparer  
  return  student.std == std& &安培; studentIDs.Contains（student.StudentID，StringComparer.CurrentCultureIgnoreCase）; 
} 

然后用法为：

  var  matching = Student.Where（s = >  MatchingStudent（s， 8 ） ）; 
  //  或者，视上述情况而定 
  var  matching = Student.Where（s = >  MatchingStudent（s， 8 ， IDsCollection ））; 

如果你有std值可以的情况成为众多之一，然后你可以扩展上面的 IEnumerable< int> stds 而不是 int std ，并将 student.std == std 更改为 stds.Contains（student.std）

Hi,

I am doing some enhancement project.

I want to modify lambda expression tree so that I can compile it. In present logic :

Suppose I want data of some students from 8th std & 9th std. So the current expression tree is building like this:

std== 8th std and student= student 1
std== 8th std and student= student 2
std == 8th std and student = student 3.

std== 9th std and student= student 1
std== 9th std and student= student 2
std == 9th std and student = student 3.


As this is time consuming so I want to modify as:

std==8th and student in (student 1, student 2, student 3)
std==9th and student in (student 1, student 2, student 3) like this.

Any idea how to do this?

解决方案

Your question is not conclusive: Do you ask for somethinng like dynamix LINQ[^]? Or an example of mine on CP: Invent your own Dynamic LINQ parser[^]?
They both take a string and convert it into an expression tree that is compiled and can be used for queries and alike (e.g. for LINQ queries).
Cheers
Andi

use this kind of expressions :

Student.Where(c => c.std == "8" && c=>c.student=="student1");

Are your "student1", "student2", "student3" always these 3 values (or some 3 specific values)?
Or is the case that you have some collection of values?

// if the ID values are always the same:
private static readonly List<string> IDs = new List<string>(){ "student1", "student2", "student3" };
public static bool MatchingStudent(StudentType student, int std)
{
  // add check for student being null!!
  // use the appropriate StringComparer based on your needs
  return student.std == std && IDs.Contains(student.StudentID, StringComparer.CurrentCultureIgnoreCase);
}
// if the ID values are in some collection:
public static bool MatchingStudent(StudentType student, int std, IEnumerable<string> studentIDs)
{
  // add checks for student and studentIDs being null!!
  // use the appropriate StringComparer based on your needs
  return student.std == std && studentIDs.Contains(student.StudentID, StringComparer.CurrentCultureIgnoreCase);
}

Then the usage is:

var matching = Student.Where(s => MatchingStudent(s, 8));
// OR, depending on the cases above
var matching = Student.Where(s => MatchingStudent(s, 8, IDsCollection));

If you have the cases where the "std" value can be one of many, then you could extend the above with IEnumerable<int> stds instead of int std, and change the student.std == std to stds.Contains(student.std).

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