Implementing the ienumerable interface & understanding the yield keyword

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This topic is discussed in this article. Then we will discuss how the ienumerable interface makes the foreach statement usable. Then it will show that if you implement a custom collection class, this collection class implements the ienumerable interface. Yield keywords and traversal sets are also discussed later.

Background

1. Use a set. The traversal set is followed. The best way to traverse a set is to implement the iterator mode-understanding and implementing the iterator pattern in C # And C ++ (this articleArticleC # provides foreach to traverse data in an elegant way.

As long as the set implements the ienumerable interface, you can use foreach to traverse it.

UseCode

First, let's take a look at how the built-in collection class uses foreach to traverse. Arraylist implements the ienumerable interface. Let's take a look.

  //   let's take a look at how to traverse the set that implements the ienumerable interface  arraylist list =  New   arraylist (); list. add (  "  1  "  ); list. add (  2  ); list. add (  "  3  "  ); list. add (  '  4   '  );   foreach  ( Object  S  in   List) {console. writeline (s) ;} 

 

 

Traverse generic collection classes

Arraylist is a general collection class. You can also traverse generic collection classes. Because these generic collection classes implement the ienumerable <t> interface, let's take a look.

 //  Traversal implements the generic class of the ienumerable <t> Interface List < String > Listofstrings = New List < String > (); Listofstrings. Add (  "  One  "  ); Listofstrings. Add (  " Two  "  ); Listofstrings. Add (  "  Three  "  ); Listofstrings. Add (  "  Four  "  );  Foreach ( String S In  Listofstrings) {console. writeline (s );}  

I found it. The custom collection class or generic collection class should implement the ienumerable and ienumerable <t> interfaces. In this way, you can traverse.

 

Understanding yield keywords

Before writing an example of an implementation interface, you should first understand the yield keyword. Yield records the position of the set. Yield can be used to return a value from a function.

The following is a common method. No matter how many calls, only one return is returned.

  static   int   simplereturn () {  return   1  ;   return   2  ;  return   3   ;}   static   void  main ( string   [] ARGs) { ///   let's see   console. writeline (simplereturn (); console. writeline (simplereturn (); console. writeline (simplereturn (); console. writeline (simplereturn () ;} 

 

 

The reason is that normal return statements do not retain the return status of the function. Each call is a new call. Then return the first value.

However, the following statement is different after replacement. When the function is called for the second time. Will continue calling from the last returned location

 Static Ienumerable < Int > Yieldreturn (){  Yield   Return   1  ;  Yield   Return   2  ;  Yield   Return  3  ;}  Static   Void Main ( String  [] ARGs ){  //  See the yield return results      Foreach ( Int I In  Yieldreturn () {console. writeline (I );}} 

 

Obviously, 1, 2, 3 are returned. The only thing to note is that the function must return ienumerable ., Then, call it through foreach.

Implement the ienumerable interface in the custom collection class

Now, if we define a method in our custom set. To iterate all elements. Then return by Using yield. We can achieve this.

Okay. We define the myarraylist class to implement the ienumerable interface, which forces us to implement the getenumerator function. Here we will use yield.

 Class  Myarraylist: ienumerable {  Object [] M_items = Null  ;  Int Freeindex = 0  ;  Public  Myarraylist (){  //  By the way, I can use arrays directly. Instead, I should use linked lists. M_items = New   Object [ 100  ];}  Public   Void Add ( Object  ITEM ){  //  When adding elements M_items [freeindex] = Item; freeindex ++ ;}  //  Ienumerable Function     Public  Ienumerator getenumerator (){  Foreach ( Object O In  M_items ){  //  Check whether the end is reached. Array... Not written              If (O = Null  ){  Break  ;}  // Returns the current element. Next step forward              Yield   Return  O ;}}} 

 

 

 

Then you can use foreach to traverse.

 Static   Void Main ( String  [] ARGs ){  //  Look at the call Section Myarraylist mylist = New  Myarraylist (); mylist. Add (  " 1  "  ); Mylist. Add (  2  ); Mylist. Add (  "  3  "  ); Mylist. Add (  '  4  '  );  Foreach ( Object S In Mylist) {console. writeline (s );}} 

 

 

This class. Not written. Not complete. As long as it is for your understanding .. Simulate it.

Implement the ienumerable <t> interface in a custom generic class

 Class Mylist <t>: ienumerable <t> {T [] m_items = Null  ;  Int Freeindex = 0  ;  Public  Mylist (){  //  For convenience. Use Arrays M_items = New T [ 100  ];}  Public   Void  Add (T item ){  //  Add Element M_items [freeindex] = Item; freeindex ++ ;}  # Region Ienumerable <t> Members Public Ienumerator <t> Getenumerator (){ Foreach (T In  M_items ){  //  Check whether the end is reached. Array... Not written              If (T = Null ) //  If t is not an empty type. Just interrupt  {  Break  ;}  //  Return the current element and proceed              Yield  Return  T ;}}  # Endregion       # Region Ienumerable members System. Collections. ienumerator system. Collections. ienumerable. getenumerator (){  //  The generic version is called here.          Return   This  . Getenumerator ();}  # Endregion  } 

 

 

Then you can use foreach.

 Static  Void Main ( String  [] ARGs ){  //  Example Mylist < String > Mylistofstrings = New Mylist < String > (); Mylistofstrings. Add (  "  One  "  ); Mylistofstrings. Add (  " Two  "  ); Mylistofstrings. Add (  "  Three  "  ); Mylistofstrings. Add (  "  Four  "  );  Foreach ( String S In  Mylistofstrings) {console. writeline (s );}} 

 

 Source codeDownload

Enumerabledemo.7z

Original article address: A-beginners-tutorial-on-implementing-ienumerable-I

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