使用 LINQ 从 C# 中的泛型列表中过滤值的最简单方法
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Simplest way to filter value from generic List in C# using LINQ
提问by Eduardo Matos
I have two classes. The first one is Person, and the second one is Student(which inherits from Person). I want to filter a generic Listand find all Studentswhich grades are higher than 7. I came up with the following solution:
我有两节课。第一个是Person，第二个是Student（从 Person 继承）。我想过滤一个通用列表并找到所有成绩高于 7 的学生。我想出了以下解决方案：
class Person
{
public string Name {get; set;}
}
class Student : Person
{
public decimal Grade {get; set;}
}
class Program
{
static void Main(string[] args)
{
List<Person> people = new List<Person>();
people.Add(new Person() {Name="John"});
people.Add(new Student() {Name="Joe", Grade=6});
people.Add(new Student() {Name="Jane", Grade=8});
people.Where(delegate (Person person) {
var student = person as Student;
return student != null && student.Grade > 7;
});
}
}
Is there a simpler way to filter this list?
有没有更简单的方法来过滤这个列表？
采纳答案by Hogan
The only improvement I see is using OfType, like this
我看到的唯一改进是使用OfType，像这样
var result = people.OfType<Student>().Where(s => s.Grade > 7);
...and my syntax is simpler... but that is in the eye of the beholder.
......我的语法更简单......但这是旁观者的眼睛。
回答by steve
Here's a few different ways of doing it, with some relative performance numbers:
这里有几种不同的方法，以及一些相对的性能数据：
Initial
最初的
people.Where(delegate(Person person)
{
var student = person as Student;
return student != null && student.Grade > 7m;
});
Initial Modified(same speed as Initial)
初始修改（与初始速度相同）
people.Where(p =>
{
var student = p as Student;
return student != null && student.Grade > 7m;
});
OfType(40-52% SLOWER than Initial)
OfType（比初始慢40-52％）
people.OfType<Student>().Where(s => s.Grade > 7m)
Foreach(9-16% faster than Initial)
Foreach（比初始速度快 9-16%）
var results = new List<Student>();
foreach (var person in people)
{
var student = person as Student;
if (student != null && student.Grade > 7m)
{
results.Add(student);
}
}
For(12-18% faster than initial)
对于（比初始速度快 12-18%）
var results = new List<Student>();
for (var idxPerson = 0; idxPerson < people.Count; idxPerson++)
{
var student = people[idxPerson] as Student;
if (student != null && student.Grade > 7m)
{
results.Add(student);
}
}
To get the performance numbers, I:
为了获得性能数据，我：
- Ran a control test that did nothing
- Timed each of the functions 100 times over a List with 10 to 1,000,000 elements
- Subtracted the control time from each tests time (to make the results more accurate)
- Each function used the same random data set and ToList was used to force the enumerator to run
- 进行了一个什么都没做的对照测试
- 对具有 10 到 1,000,000 个元素的 List 对每个函数进行 100 次计时
- 从每次测试时间中减去控制时间（使结果更准确）
- 每个函数使用相同的随机数据集，并使用 ToList 强制枚举器运行
Of course, these are just performance numbers on my machine, you'll have to test on real-world data to get actual results as the distribution of Students vs. People, the average grade of the student, etc. will cause a lot of variation in the timings.
当然，这些只是我机器上的表现数字，你必须在真实世界的数据上进行测试才能得到实际结果，因为学生与人的分布、学生的平均成绩等会导致很多时间的变化。
回答by Jannie Theunissen
people.RemoveAll(p => p.Grade <= 7);
