C# List Find

红太狼 2022-01-06 05:43 205阅读 0赞

A **List** can be searched imperatively. This often involves a foreach-loop. It can be searched instead with the **Find** method: this often uses a lambda expression. Find makes code clearer in some program contexts. It sometimes makes maintenance easier.

## Example ##

Instead of using a foreach-loop with an if statement, you can use the Find instance method on List. Here we see that it also accepts a Predicate, which you can specify as a lambda expression. It returns the first match.

Program that uses Find method on List [C#]
    
    using System;
    using System.Collections.Generic;
    
    class Program
    {
        static void Main()
        {
    	List<int> list = new List<int>(new int[] { 19, 23, 29 });
    
    	// Finds first element greater than 20
    	int result = list.Find(item => item > 20);
    
    	Console.WriteLine(result);
        }
    }
    
    Output
    
    23

![List type][]

**This code** loops through each int value in the List, starting at the beginning, and tests each one to see if it is greater than 20. The first one that is, 23, is returned. The parameter to the Find method is a lambda expression that is considered a Predicate instance.

[Predicate Type][]

**Tip:**Please see the article on the Predicate type for more specific examples.

![Reverse graphic][]

**To search backwards,** use the FindLast method, which would return 29 in the example above. It will scan the List from the very last element to the first. There are also FindIndex and FindLastIndex methods you can use, which would return the index of the matches, which would correspond to the numbers returned in the example.

## Exists ##

Another useful method on the List type that can be used to search a List is the Exists method. This receives a Predicate parameter and returns a bool value indicating whether the element was found.

[List Exists Method][]

## FindAll ##

![Programming tip][]

The FindAll method on List, which is an instance method that returns a new List with the same element type, is also available. If you want to find all the matching elements based on a Predicate, this is useful.

## Loop with List ##

In situations where the exact behavior of the List code is important, you can use for and foreach loops with List. This can sometimes also be faster than methods that receive delegates.

[List Examples][]

[Delegate Tutorial][]

## Summary ##

[![The C\# programming language][The C_ programming language]][The C_ programming language_The C_ programming language]

In this article, we saw ways to find elements by searching in the List collection from System.Collections.Generic. These methods are convenient and can help you place the emphasis on other logic in your class.

**Note:**I use them extensively in programs with Lists, except in performance-critical situations.

[List Contains Method][]

[List IndexOf Method][]

转载于:https://www.cnblogs.com/zhangchenliang/archive/2012/08/15/2640396.html

[List type]: /images/20211227/72c2c3c760e84644b60f39b6ac7367ef.png
[Predicate Type]: http://www.dotnetperls.com/predicate
[Reverse graphic]: /images/20211227/83e9b4fdb59848afba1b980ad4a581c8.png
[List Exists Method]: http://www.dotnetperls.com/list-exists
[Programming tip]: /images/20211227/bdf3da82b3074f11a7e797637644b9e0.png
[List Examples]: http://www.dotnetperls.com/list
[Delegate Tutorial]: http://www.dotnetperls.com/delegate
[The C_ programming language]: http://www.dotnetperls.com/c.png
[The C_ programming language_The C_ programming language]: /images/20211227/7c6bd1648cb6421caacc096bcdc70c39.png
[List Contains Method]: http://www.dotnetperls.com/list-contains
[List IndexOf Method]: http://www.dotnetperls.com/list-indexof