A detailed explanation of how the foreach implementation works in C #

This article is mainly for you to introduce in detail the principle of foreach in C #, with a certain reference value, interested in small partners can refer to

This article mainly records my experience in learning the principle of foreach traversal in C #.

The traversal of features in a collection is a common practice in all encodings, so most programming languages write this process into syntax, such as foreach in C #. You will often see the following traversal code:

var lststr = new List<string> {"A", "B"};   foreach (Var str in lststr)      {        Console.WriteLine (str);      }

The actual execution of this code is:

var lststr = new List<string> {"A", "B"};   ienumerator<string> enumeratorlst = Lststr.getenumerator ();   while (Enumeratorlst.movenext ())      {        Console.WriteLine (enumeratorlst.current);      }

The GetEnumerator () method and the Ienumerator<string> type are found, which involves the concept of enumerable types and enumerators.

For ease of understanding, the following is a non-generic example:

Summary://   exposes the enumerator, which supports simple iterations on non-generic collections. Public  Interface IEnumerable  {    //Summary:    //   Returns an enumerator that iterates through the collection. /////    Returns the result:    //   The System.Collections.IEnumerator object that can be used to iterate through the collection.    IEnumerator GetEnumerator ();  }

A class that implements this interface is called an enumerable type and is a flag that can be traversed with foreach.

The return value of Method GetEnumerator () is an enumerator that can be understood as a cursor.

Summary://   supports simple iterations for non-generic collections. Public  Interface IEnumerator  {    //Summary:    //   Gets the current element in the collection.    ////    Returns the result:    //   The current element in the collection. /////    exception:    /  /System.InvalidOperationException://The   Enumerator is positioned before the first element of the collection or after the last element.    object current {get;}    Summary:    //   Advances the enumerator to the next element of the collection.    /////    Returns the result:    //   True if the enumerator is successfully pushed to the next element, or False if the enumerator crosses the end of the collection. /////    Exception:    //  System.InvalidOperationException:    //   The collection was modified after the enumerator was created.    bool MoveNext ();    //Summary:    //   Sets the enumerator to its initial position, which precedes the first element in the collection. /////    Exception:    //  System.InvalidOperationException:    //   The collection was modified after the enumerator was created.    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 l    Name) {this.firstname = FName;  This.lastname = LName;  } public string FirstName; public string LastName;} Collection of person objects. This class//implements IEnumerable so the it can be used//with ForEach Syntax.public class people:ienumerable{Priva  Te 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 Pe  Rson[] _people; Enumerators is positioned before the first element//until the firstMoveNext () 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 we have the keyword yield, we can create an enumerator in this way:

Using system;using system.collections;//simple business Object.public class person{public person (string fName, String l    Name) {this.firstname = FName;  This.lastname = LName;  } public string FirstName; public string LastName;} Collection of person objects. This class//implements IEnumerable so the it can be used//with ForEach Syntax.public class people:ienumerable{Priva  Te 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); }}