使用LINQ将变量值用作字段 [英] Use variable value as field with LINQ

问题描述

我有这个对象:

public class MyObject
{
    public int Id { get; set; }
    public string FieldA { get; set; }
    public string FieldB { get; set; }
    public string FieldC { get; set; }
    public string FieldD { get; set; }
}

我有一个该对象的IList<>:

I have an IList<> of this object :

IList<MyObject> MyListObject = new List<MyObject>();

我在上面执行Linq查询:

I do a Linq query on it :

var result = (from p in MyListObject where p.FieldC == "Test" select p.FieldA);

在这种情况下，我返回"p.FieldA"，但有时我需要返回"p.FieldB".我想将字段名称(FieldA或FieldB)放在这样的变量中

In this case, I return "p.FieldA" but sometimes I need to return "p.FieldB". I'd like put the name of the field (FieldA or FieldB) in a variable like this

var myvar = "FieldB"
var result = (from p in MyListObject where p.FieldC == "Test" select p.????);

如何在Linq查询中使用myvar内容(FieldB)作为字段名称?

How use myvar content (FieldB) as field name in the Linq query ?

谢谢

推荐答案

如果您的方法本身知道必须在FieldA和FieldB之间进行选择，则可以使用:

If your method itself knows it has to choose between FieldA and FieldB you can use:

var whatField = "FieldA";
var result = (from p in MyListObject 
     where p.FieldA == "Test" 
     select (whatField == FieldA ? p.FieldA : p.FieldB));

如果您有2个以上的选项，我会选择传递lambda；

If you have more as 2 options, I would choose to pass a lambda;

Func<MyObject, object> fieldGetter;

// Option A, switch
switch (whatField) {
   case "FieldA": fieldGetter = o => o.FieldA; break;
   case "FieldB": fieldGetter = o => o.FieldB; break;
   // More options
}

// Option B using reflection:
var t = typeof(MyObject);
var prop = t.GetProperty(whatField);
fieldGetter = o => prop.GetValue(o, null);

// Linq then looks like
var result = (from p in MyListObject 
     where p.FieldA == "Test" 
     select fieldGetter(p));

使用lambda的优点是，您可以拆分逻辑，什么字段以及如何查询.您甚至可以使其适用于不同类型:

The advantage of using the lambda is, you can split your logic, what field and how to query. And you can even make it work for different types:

IEnumerable<T> Query<T>(IQueryable<MyObject> MyListObject, Func<MyObject, T> fieldGetter) {
  return result = (from p in MyListObject 
       where p.FieldA == "Test" 
       select fieldGetter(p));
}

// call using:

var result = Query(MyListObject, o => o.FieldA);

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