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This article mainly introduces the foreach implementation principle in C# in detail, which has certain reference value. Interested friends can refer to it
This article mainly records my learning of the foreach traversal principle in C# experience.
Traversing elements in a collection is an operation often involved in all coding, so most programming languages have written this process into the grammar, such as foreach in C#. It is common to see the following traversal code:
var lstStr = new List<string> { "a", "b" }; foreach (var str in lstStr) { Console.WriteLine(str); }
The actual execution process of this code:
var lstStr = new List<string> {"a", "b"}; IEnumerator<string> enumeratorLst = lstStr.GetEnumerator(); while (enumeratorLst.MoveNext()) { Console.WriteLine(enumeratorLst.Current); }
will find that there is the GetEnumerator() method and the IEnumerator98c455a79ddfebb79781bff588e7b37e type, which involves the concepts of enumerable types and enumerators.
In order to facilitate understanding, the following is a non-generic example:
// 摘要: // 公开枚举器,该枚举器支持在非泛型集合上进行简单迭代。 public interface IEnumerable { // 摘要: // 返回一个循环访问集合的枚举器。 // // 返回结果: // 可用于循环访问集合的 System.Collections.IEnumerator 对象。 IEnumerator GetEnumerator(); }
The class that implements this interface is called an enumerable type, which is A flag that can be traversed using foreach.
The return value of the method GetEnumerator() is an enumerator, which can be understood as a cursor.
// 摘要: // 支持对非泛型集合的简单迭代。 public interface IEnumerator { // 摘要: // 获取集合中的当前元素。 // // 返回结果: // 集合中的当前元素。 // // 异常: // System.InvalidOperationException: // 枚举数定位在该集合的第一个元素之前或最后一个元素之后。 object Current { get; } // 摘要: // 将枚举数推进到集合的下一个元素。 // // 返回结果: // 如果枚举数成功地推进到下一个元素,则为 true;如果枚举数越过集合的结尾,则为 false。 // // 异常: // System.InvalidOperationException: // 在创建了枚举数后集合被修改了。 bool MoveNext(); // // 摘要: // 将枚举数设置为其初始位置,该位置位于集合中第一个元素之前。 // // 异常: // System.InvalidOperationException: // 在创建了枚举数后集合被修改了。 void Reset(); }
The following is an example of customizing an iterator (https://msdn.microsoft.com/en-us/library/system.collections.ienumerator .aspx):
using System; using System.Collections; // Simple business object. public class Person { public Person(string fName, string lName) { this.firstName = fName; this.lastName = lName; } public string firstName; public string lastName; } // Collection of Person objects. This class // implements IEnumerable so that it can be used // with ForEach syntax. public class People : IEnumerable { private Person[] _people; public People(Person[] pArray) { _people = new Person[pArray.Length]; for (int i = 0; i < pArray.Length; i++) { _people[i] = pArray[i]; } } // Implementation for the GetEnumerator method. IEnumerator IEnumerable.GetEnumerator() { return (IEnumerator) GetEnumerator(); } public PeopleEnum GetEnumerator() { return new PeopleEnum(_people); } } // When you implement IEnumerable, you must also implement IEnumerator. public class PeopleEnum : IEnumerator { public Person[] _people; // Enumerators are positioned before the first element // until the first MoveNext() call. int position = -1; public PeopleEnum(Person[] list) { _people = list; } public bool MoveNext() { position++; return (position < _people.Length); } public void Reset() { position = -1; } object IEnumerator.Current { get { return Current; } } public Person Current { get { try { return _people[position]; } catch (IndexOutOfRangeException) { throw new InvalidOperationException(); } } } } class App { static void Main() { Person[] peopleArray = new Person[3] { new Person("John", "Smith"), new Person("Jim", "Johnson"), new Person("Sue", "Rabon"), }; People peopleList = new People(peopleArray); foreach (Person p in peopleList) Console.WriteLine(p.firstName + " " + p.lastName); } } /* This code produces output similar to the following: * * John Smith * Jim Johnson * Sue Rabon * */
After having the yield keyword, we can create an enumerator in this way:
using System; using System.Collections; // Simple business object. public class Person { public Person(string fName, string lName) { this.firstName = fName; this.lastName = lName; } public string firstName; public string lastName; } // Collection of Person objects. This class // implements IEnumerable so that it can be used // with ForEach syntax. public class People : IEnumerable { private Person[] _people; public People(Person[] pArray) { _people = new Person[pArray.Length]; for (int i = 0; i < pArray.Length; i++) { _people[i] = pArray[i]; } } // Implementation for the GetEnumerator method. IEnumerator IEnumerable.GetEnumerator() { for (int i = 0; i < _people.Length; i++) { yield return _people[i]; } } } class App { static void Main() { Person[] peopleArray = new Person[3] { new Person("John", "Smith"), new Person("Jim", "Johnson"), new Person("Sue", "Rabon"), }; People peopleList = new People(peopleArray); foreach (Person p in peopleList) Console.WriteLine(p.firstName + " " + p.lastName); } }
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