如何使用LINQ从列表中返回单个值？ [英] How do I return a single value from a list using LINQ?

问题描述

我是LINQ的新手，我知道这是（应该是？）一个简单的问题，但我正在努力解决简单LINQ查询的语法问题。以下简要说明代码显示了我想要实现的目标：

公共类水果
 {
 public int ID {get;组; } = 0; 
 public name {get;组; } = String.Empty; 
} 
 
公共类水果
 {
公开列表< Fruit> ListOfFruits = new List< Fruit>（）
 {
 new Fruit {ID = 0，Name =Apple}，
 new Fruit {ID = 1，Name =Orange}， 
 new Fruit {ID = 2，Name =Banana}，
} 
 
 public int GetIDFromName（string fruitName）
 {
 //什么去了吗？ 
} 
} 

作为初学者，我会感激任何建议。特别是，我想要一个使用特殊LINQ关键字（'select'，'where'等）以及标准C＃LINQ函数（Select（），Where（）等）的解决方案，所以我可以比较两种方法。



似乎很少有例子将这两种方法并排显示出来进行比较，我也很困惑一种方法何时更好比另一个。为了简单的一致性，我想选择一种方式并坚持下去。



亲切的愿望~Patrick



我尝试过的事情：



我尝试过像......

 ListOfFruits.Select（f = >  f.ID）.Where（...）

...但是我甚至无法编译它，因为它似乎只是在选择中指定了ID我不再能够在 Where 子句中引用名称。

解决方案

 ListOfFruits。 FirstOrDefault（x = >  x.Name ==   Apple）。ID 

您还可以将FirstOrDefault加载到对象中，并在尝试访问ID之前检查null。



根据建议，为了避免NullRef异常，你可以做一个偶然例如：



如果你在Roslyn编译器中使用VS版本：



< pre lang =c＃> ListOfFruits.FirstOrDefault（x = > x.Name == < span class =code-string> Apple）？。ID

否则更详细的方法：

  var  fruit = ListOfFruits.FirstOrDefault（x = >  x.Name ==   Apple）; 
  if （fruit！=  null ）
 {
  return  fruit.ID; 
} 
 
 返回  0 ; 

这不是通往罗马的唯一途径，您也可以使用Single（），SingleOrDefault（）或First（）。 Single希望单个响应必须存在，我发现自己更倾向于倾向于FirstOrDefault。

因为你要求两种语法形式：

< pre lang =c＃> public int GetIDFromName（ string fruitName）
{
// LINQ函数：
Fruit f = ListOfFruits.FirstOrDefault（x = > x.Name == fruitName）;
// LINQ SQL-ish：
Fruit f =（来自 x ListOfFruits 其中 x.Name == fruitName 选择 x）.FirstOrDefault（）;
// （两者都相同）：
return f！= null ？ f.ID：-1; // 一些值表示没有这样的水果
}

这个将一种语法与另一种语法进行比较并不是一个特别好的例子......

我们也可以用户而不是FirstOrDefault.We可以像这样编写上面的代码



public int GetIDFromName（string fruitName）

{

// LINQ函数：

Fruit f = ListOfFruits.First（x => x.Name == fruitName）;

// LINQ SQL-ish：

Fruit f =（来自ListOfFruits中的x，其中x。名称== fruitName选择x）.First（）;

//（两者共通）：

返回f！= null？ f.ID：-1; //某些值表示没有这样的水果

}

I am a newbie to LINQ and I am aware this is (should be?) a simple question, but I am struggling with the syntax of a simple LINQ query. The following cut-down code shows what I am trying to achieve:

public class Fruit
{
    public int ID { get; set; } = 0;
    public Name { get; set; } = String.Empty;
}

public class Fruits
{
    public List<Fruit> ListOfFruits = new List<Fruit>()
    {
        new Fruit { ID = 0, Name = "Apple" },
        new Fruit { ID = 1, Name = "Orange" },
        new Fruit { ID = 2, Name = "Banana" },
    }

    public int GetIDFromName( string fruitName )
    {
        // What goes here?
    }
}

As a beginner at this, I'd appreciate any advice at all. In particular, I would like a solution that uses the special LINQ keywords ('select', 'where', etc.) and also the standard C# LINQ functions ( Select(), Where(), etc.) so I can compare the two approaches.

There seems to be very few examples that show the two approaches side-by-side for comparison, and I am also confused about when one approach is better than the other. For simple consistency, I'd like to pick one way and stick with it.

Kind wishes ~ Patrick

What I have tried:

I have tried expressions like...

ListOfFruits.Select( f => f.ID ).Where( ... )

...but I can't even get this to compile because it seems by specifying just the ID in the Select clause, I stop myself being able to reference the Name in the Where clause.

解决方案

ListOfFruits.FirstOrDefault(x => x.Name == "Apple").ID

You could also load the FirstOrDefault into an object and check for null before trying to access the id.

As suggested, in order to avoid a NullRef exception you can do a couple of things:

If you are using a version of VS with the Roslyn compiler:

ListOfFruits.FirstOrDefault(x => x.Name == "Apple")?.ID

Otherwise the more verbose method:

var fruit = ListOfFruits.FirstOrDefault(x => x.Name == "Apple");
if (fruit != null)
{
     return fruit.ID;
}

return 0;

This is not the only road to Rome, you can also use Single(), SingleOrDefault() or First(). Single expects a single response that has to be there, I find myself gravitating to FirstOrDefault more often than not.

Since you asked for both syntax forms:

public int GetIDFromName( string fruitName )
{
  // LINQ functions:
  Fruit f = ListOfFruits.FirstOrDefault(x => x.Name == fruitName);
  // LINQ SQL-ish:
  Fruit f = (from x in ListOfFruits where x.Name == fruitName select x).FirstOrDefault();
  // (common to both):
  return f != null ? f.ID : -1;   // some value to indicate no such fruit
}

This is not a particularly good case for comparing one syntax with the other...

We can also user First instead of FirstOrDefault.We can write the above code like this

public int GetIDFromName( string fruitName )
{
// LINQ functions:
Fruit f = ListOfFruits.First(x => x.Name == fruitName);
// LINQ SQL-ish:
Fruit f = (from x in ListOfFruits where x.Name == fruitName select x).First();
// (common to both):
return f != null ? f.ID : -1; // some value to indicate no such fruit
}

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