yield<\/i> is a new contextual keyword introduced to C# 2.0, vital for lazy evaluation and the performance of queries in LINQ. Being a contextual keyword means that yield<\/i> can be used as a variable name in C# without any problem. When put before return it becomes a keyword.<\/p>\n
yield<\/i> allows one enumerable class to be implemented in terms of another. This enables the delay of execution of queries until the latest possible moment, skipping the generation of intermediate results that would drastically reduce poerformance. The query operators in LINQ operate on sequence. The result of a query is often another sequence. Lazy evaluation means that until you iterate over the result of the query, the source of the query is not iterated.<\/p>\n
To show you how yield<\/i> works, let’s consider the same class I used in my last post<\/a>, Winner.<\/p>\n and create a class WinnerDB, that contains a list of UEFA Champion League winners. This class implements IEnumerable<\/tt> and returns an enumerator, WinnerEnumerator<\/tt>, to be able to iterate over the winners.<\/p>\n The usage of this class would look like this:<\/p>\n and the output of the program<\/p>\n So far so good. But with yield<\/i>, you can let the compiler do all that stuff for you. When you use yield<\/i>, the compiler generates an enumerator that keeps the current state of the iteration.<\/p>\n Running this code will produce the same output as the previous one, except that the implementation is much simpler. Perhaps the example is not the best, but should give you a hint of the use of the yield<\/i> keyword. To see that the source is actually iterated only when the result is iterated, we can modify the WinnersDB<\/tt> method to print a message in the console:<\/p>\n In this case, the output looks like this:<\/p>\n yield is a new contextual keyword introduced to C# 2.0, vital for lazy evaluation and the performance of queries in LINQ. Being a contextual keyword means that yield can be used as a variable name in C# without any problem. When put before return it becomes a keyword. yield allows one enumerable class to be … Read more<\/a><\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_uag_custom_page_level_css":"","advgb_blocks_editor_width":"","advgb_blocks_columns_visual_guide":"","_jetpack_newsletter_access":"","_jetpack_dont_email_post_to_subs":false,"_jetpack_newsletter_tier_id":0,"_jetpack_memberships_contains_paywalled_content":false,"_jetpack_feature_clip_id":0,"_jetpack_memberships_contains_paid_content":false,"footnotes":"","jetpack_publicize_message":"","jetpack_publicize_feature_enabled":true,"jetpack_social_post_already_shared":false,"jetpack_social_options":{"image_generator_settings":{"template":"highway","default_image_id":0,"font":"","enabled":false},"version":2},"jetpack_post_was_ever_published":false},"categories":[11,14],"tags":[454,451,328],"class_list":["post-30","post","type-post","status-publish","format-standard","hentry","category-csharp","category-linq","tag-net","tag-c","tag-linq-2"],"aioseo_notices":[],"aioseo_head":"\n\t\t\n\t\n\t\n\t\n\t\n\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\r\npublic class Winner\r\n{\r\n string _name;\r\n string _country;\r\n int _year;\r\n\r\n public string Name\r\n {\r\n get { return _name; }\r\n set { _name = value; }\r\n }\r\n public string Country\r\n {\r\n get { return _country; }\r\n set { _country = value; }\r\n }\r\n public int Year\r\n {\r\n get { return _year; }\r\n set { _year = value; }\r\n }\r\n public Winner(string name, string country, int year)\r\n {\r\n _name = name;\r\n _country = country;\r\n _year = year;\r\n }\r\n}\r\n<\/pre>\n\r\n public class WinnerEnumerator : IEnumerator\r\n {\r\n int pos = -1;\r\n private Winner[] _winners;\r\n public WinnerEnumerator(Winner[] winners)\r\n {\r\n _winners = winners;\r\n }\r\n\r\n public void Reset()\r\n {\r\n pos = -1;\r\n }\r\n\r\n public bool MoveNext()\r\n {\r\n pos++;\r\n return (pos < _winners.Length);\r\n }\r\n\r\n public object Current\r\n {\r\n get\r\n {\r\n try\r\n {\r\n return _winners[pos];\r\n\r\n }\r\n catch (IndexOutOfRangeException)\r\n {\r\n throw new InvalidOperationException();\r\n }\r\n }\r\n }\r\n }\r\n\r\n public class WinnersDB : IEnumerable\r\n {\r\n private Winner[] _winners;\r\n public WinnersDB()\r\n {\r\n _winners = new Winner[]\r\n {\r\n new Winner(\"Barcelona\", \"Spain\", 2006),\r\n new Winner(\"Liverpool\", \"England\", 2005),\r\n new Winner(\"FC Porto\", \"Portugal\", 2004),\r\n new Winner(\"AC Milan\", \"Italy\", 2003),\r\n new Winner(\"Real Madrid\", \"Spain\", 2002),\r\n new Winner(\"Bayern Munchen\", \"Germany\", 2001),\r\n new Winner(\"Real Madrid\", \"Spain\", 2000),\r\n new Winner(\"Manchester Utd.\", \"England\", 1999),\r\n new Winner(\"Real Madrid\", \"Spain\", 1998),\r\n new Winner(\"Olimpique Marseille\", \"France\", 1993),\r\n };\r\n }\r\n\r\n public IEnumerator GetEnumerator()\r\n {\r\n return new WinnerEnumerator(_winners);\r\n }\r\n }\r\n<\/pre>\n\r\n class Program\r\n {\r\n static void Main(string[] args)\r\n {\r\n WinnersDB db = new WinnersDB();\r\n foreach (Winner w in db)\r\n {\r\n Console.WriteLine(\"{0}\\t{1}, {2}\",\r\n w.Year, w.Name, w.Country);\r\n }\r\n }\r\n }\r\n<\/pre>\n\r\n2006 Barcelona, Spain\r\n2005 Liverpool, England\r\n2004 FC Porto, Portugal\r\n2003 AC Milan, Italy\r\n2002 Real Madrid, Spain\r\n2001 Bayern Munchen, Germany\r\n2000 Real Madrid, Spain\r\n1999 Manchester Utd., England\r\n1998 Real Madrid, Spain\r\n1993 Olimpique Marseille, France\r\n<\/pre>\n
\r\nclass Program\r\n{\r\n public static IEnumerable<Winner> WinnersDB()\r\n {\r\n Winner [] winners = new Winner[]\r\n {\r\n new Winner(\"Barcelona\", \"Spain\", 2006),\r\n new Winner(\"Liverpool\", \"England\", 2005),\r\n new Winner(\"FC Porto\", \"Portugal\", 2004),\r\n new Winner(\"AC Milan\", \"Italy\", 2003),\r\n new Winner(\"Real Madrid\", \"Spain\", 2002),\r\n new Winner(\"Bayern Munchen\", \"Germany\", 2001),\r\n new Winner(\"Real Madrid\", \"Spain\", 2000),\r\n new Winner(\"Manchester Utd.\", \"England\", 1999),\r\n new Winner(\"Real Madrid\", \"Spain\", 1998),\r\n new Winner(\"Olimpique Marseille\", \"France\", 1993),\r\n };\r\n\r\n foreach (Winner w in winners)\r\n {\r\n yield return w;\r\n }\r\n }\r\n\r\n static void Main(string[] args)\r\n {\r\n foreach (Winner w in WinnersDB())\r\n {\r\n Console.WriteLine(\"{0}\\t{1}, {2}\",\r\n w.Year, w.Name, w.Country);\r\n }\r\n }\r\n}\r\n<\/pre>\n\r\nforeach (Winner w in winners)\r\n{\r\n Console.WriteLine(\"yield: {0} {1}, {2}\", w.Year, w.Name, w.Country);\r\n yield return w;\r\n}\r\n<\/pre>\n\r\nyield: 2006 Barcelona, Spain\r\n2006 Barcelona, Spain\r\nyield: 2005 Liverpool, England\r\n2005 Liverpool, England\r\nyield: 2004 FC Porto, Portugal\r\n2004 FC Porto, Portugal\r\nyield: 2003 AC Milan, Italy\r\n2003 AC Milan, Italy\r\nyield: 2002 Real Madrid, Spain\r\n2002 Real Madrid, Spain\r\nyield: 2001 Bayern Munchen, Germany\r\n2001 Bayern Munchen, Germany\r\nyield: 2000 Real Madrid, Spain\r\n2000 Real Madrid, Spain\r\nyield: 1999 Manchester Utd., England\r\n1999 Manchester Utd., England\r\nyield: 1998 Real Madrid, Spain\r\n1998 Real Madrid, Spain\r\nyield: 1993 Olimpique Marseille, France\r\n1993 Olimpique Marseille, France\r\n<\/pre>\n","protected":false},"excerpt":{"rendered":"