group 子句（C# 參考） · MSDN C# 編程指南 & 參考手冊 2015

# group 子句（C# 參考） **group** 子句返回一個 [IGrouping<TKey,?TElement>](https://msdn.microsoft.com/zh-CN/library/bb344977.aspx) 對象序列，這些對象包含零個或更多個與該組的鍵值匹配的項。例如，可以按照每個字符串中的第一個字母對字符串序列進行分組。在這種情況下，第一個字母是鍵且具有 [char](https://msdn.microsoft.com/zh-CN/library/x9h8tsay.aspx) 類型，并且存儲在每個 [IGrouping<TKey,?TElement>](https://msdn.microsoft.com/zh-CN/library/bb344977.aspx) 對象的 **Key** 屬性中。編譯器可推斷該鍵的類型。可以用 **group** 子句結束查詢表達式，如下面的示例所示： ``` // Query variable is an IEnumerable<IGrouping<char, Student>> var studentQuery1 = from student in students group student by student.Last[0]; ``` 如果您想要對每個組執行附加查詢操作，則可以使用 [into](https://msdn.microsoft.com/zh-CN/library/bb311045.aspx) 上下文關鍵字指定一個臨時標識符。使用 **into** 時，必須繼續編寫該查詢，并最終用一個 **select** 語句或另一個 **group** 子句結束該查詢，如下面的代碼摘錄所示： ``` // Group students by the first letter of their last name // Query variable is an IEnumerable<IGrouping<char, Student>> var studentQuery2 = from student in students group student by student.Last[0] into g orderby g.Key select g; ``` 本主題中的“示例”部分中提供了使用含有和不含 **into** 的 **group** 的更完整示例。 ## 枚舉組查詢的結果由于 **group** 查詢產生的 [IGrouping<TKey,?TElement>](https://msdn.microsoft.com/zh-CN/library/bb344977.aspx) 對象實質上是列表的列表，因此必須使用嵌套的 [foreach](https://msdn.microsoft.com/zh-CN/library/ttw7t8t6.aspx) 循環來訪問每一組中的各個項。外部循環用于循環訪問組鍵，內部循環用于循環訪問組本身中的每個項。組可能具有鍵，但沒有元素。以下是執行上述代碼示例中的查詢的 **foreach** 循環： ``` // Iterate group items with a nested foreach. This IGrouping encapsulates // a sequence of Student objects, and a Key of type char. // For convenience, var can also be used in the foreach statement. foreach (IGrouping<char, Student> studentGroup in studentQuery2) { Console.WriteLine(studentGroup.Key); // Explicit type for student could also be used here. foreach (var student in studentGroup) { Console.WriteLine(" {0}, {1}", student.Last, student.First); } } ``` ## 鍵類型組鍵可以是任何類型，如字符串、內置數值類型、用戶定義的命名類型或匿名類型。 ### 按字符串進行分組上述代碼示例使用的是 **char**。可以很容易地改為指定字符串鍵，如完整的姓氏： ``` // Same as previous example except we use the entire last name as a key. // Query variable is an IEnumerable<IGrouping<string, Student>> var studentQuery3 = from student in students group student by student.Last; ``` ### 按布爾進行分組下面的示例演示使用布爾值作為鍵將結果劃分成兩個組。請注意，該值是由 **group** 子句中的子表達式產生的。 ``` class GroupSample1 { // The element type of the data source. public class Student { public string First { get; set; } public string Last { get; set; } public int ID { get; set; } public List<int> Scores; } public static List<Student> GetStudents() { // Use a collection initializer to create the data source. Note that each element // in the list contains an inner sequence of scores. List<Student> students = new List<Student> { new Student {First="Svetlana", Last="Omelchenko", ID=111, Scores= new List<int> {97, 72, 81, 60}}, new Student {First="Claire", Last="O'Donnell", ID=112, Scores= new List<int> {75, 84, 91, 39}}, new Student {First="Sven", Last="Mortensen", ID=113, Scores= new List<int> {99, 89, 91, 95}}, new Student {First="Cesar", Last="Garcia", ID=114, Scores= new List<int> {72, 81, 65, 84}}, new Student {First="Debra", Last="Garcia", ID=115, Scores= new List<int> {97, 89, 85, 82}} }; return students; } static void Main() { // Obtain the data source. List<Student> students = GetStudents(); // Group by true or false. // Query variable is an IEnumerable<IGrouping<bool, Student>> var booleanGroupQuery = from student in students group student by student.Scores.Average() >= 80; //pass or fail! // Execute the query and access items in each group foreach (var studentGroup in booleanGroupQuery) { Console.WriteLine(studentGroup.Key == true ? "High averages" : "Low averages"); foreach (var student in studentGroup) { Console.WriteLine(" {0}, {1}:{2}", student.Last, student.First, student.Scores.Average()); } } // Keep the console window open in debug mode. Console.WriteLine("Press any key to exit."); Console.ReadKey(); } } /* Output: Low averages Omelchenko, Svetlana:77.5 O'Donnell, Claire:72.25 Garcia, Cesar:75.5 High averages Mortensen, Sven:93.5 Garcia, Debra:88.25 */ ``` ### 按數值范圍進行分組下一個示例使用表達式創建表示百分比范圍的數值組鍵。請注意，該示例使用 [let](https://msdn.microsoft.com/zh-CN/library/bb383976.aspx) 作為方法調用結果的方便存儲位置，從而無需在 **group** 子句中調用該方法兩次。另請注意，在 **group** 子句中，為了避免發生“被零除”異常，代碼進行了相應檢查以確保學生的平均成績不為零。有關如何在查詢表達式中安全使用方法的更多信息，請參見[如何：在查詢表達式中處理異常（C# 編程指南）](https://msdn.microsoft.com/zh-CN/library/bb513730.aspx)。 ``` class GroupSample2 { // The element type of the data source. public class Student { public string First { get; set; } public string Last { get; set; } public int ID { get; set; } public List<int> Scores; } public static List<Student> GetStudents() { // Use a collection initializer to create the data source. Note that each element // in the list contains an inner sequence of scores. List<Student> students = new List<Student> { new Student {First="Svetlana", Last="Omelchenko", ID=111, Scores= new List<int> {97, 72, 81, 60}}, new Student {First="Claire", Last="O'Donnell", ID=112, Scores= new List<int> {75, 84, 91, 39}}, new Student {First="Sven", Last="Mortensen", ID=113, Scores= new List<int> {99, 89, 91, 95}}, new Student {First="Cesar", Last="Garcia", ID=114, Scores= new List<int> {72, 81, 65, 84}}, new Student {First="Debra", Last="Garcia", ID=115, Scores= new List<int> {97, 89, 85, 82}} }; return students; } // This method groups students into percentile ranges based on their // grade average. The Average method returns a double, so to produce a whole // number it is necessary to cast to int before dividing by 10\. static void Main() { // Obtain the data source. List<Student> students = GetStudents(); // Write the query. var studentQuery = from student in students let avg = (int)student.Scores.Average() group student by (avg == 0 ? 0 : avg / 10) into g orderby g.Key select g; // Execute the query. foreach (var studentGroup in studentQuery) { int temp = studentGroup.Key * 10; Console.WriteLine("Students with an average between {0} and {1}", temp, temp + 10); foreach (var student in studentGroup) { Console.WriteLine(" {0}, {1}:{2}", student.Last, student.First, student.Scores.Average()); } } // Keep the console window open in debug mode. Console.WriteLine("Press any key to exit."); Console.ReadKey(); } } /* Output: Students with an average between 70 and 80 Omelchenko, Svetlana:77.5 O'Donnell, Claire:72.25 Garcia, Cesar:75.5 Students with an average between 80 and 90 Garcia, Debra:88.25 Students with an average between 90 and 100 Mortensen, Sven:93.5 */ ``` ### 按復合鍵進行分組當您想要按照多個鍵對元素進行分組時，可使用復合鍵。通過使用匿名類型或命名類型來存儲鍵元素，可以創建復合鍵。在下面的示例中，假定已經使用名為 surname 和 city 的兩個成員聲明了類 Person。 **group** 子句使得為每組具有相同姓氏和相同城市的人員創建一個單獨的組。 ``` group person by new {name = person.surname, city = person.city}; ``` 如果必須將查詢變量傳遞給其他方法，請使用命名類型。使用自動實現的屬性作為鍵來創建一個特殊類，然后重寫 [Equals](https://msdn.microsoft.com/zh-CN/library/bsc2ak47.aspx) 和 [GetHashCode](https://msdn.microsoft.com/zh-CN/library/system.object.gethashcode.aspx) 方法。還可以使用結構；在此情況下，并不絕對需要重寫這些方法。有關更多信息，請參見[如何：使用自動實現的屬性實現輕量類（C# 編程指南）](https://msdn.microsoft.com/zh-CN/library/bb383979.aspx)和[How to: Query for Duplicate Files in a Directory Tree (LINQ)](https://msdn.microsoft.com/zh-CN/library/bb546133.aspx)。后一個主題包含一個代碼示例，該示例演示如何將復合鍵與命名類型結合使用。下面的示例演示在沒有向組應用附加查詢邏輯時將源數據排序放入不同組中的標準模式。這稱為不帶延續的分組。字符串數組中的元素按照它們的第一個字母進行分組。查詢結果是一個 [IGrouping<TKey,?TElement>](https://msdn.microsoft.com/zh-CN/library/bb344977.aspx) 類型，其中包含一個 **char** 類型的公共 Key 屬性以及一個包含分組中每個項的 [IEnumerable<T>](https://msdn.microsoft.com/zh-CN/library/9eekhta0.aspx) 集合。 **group** 子句的結果是序列的序列。因此，若要訪問所返回的每個組中的單個元素，請在循環訪問組鍵的循環內使用嵌套的 **foreach** 循環，如下面的示例所示。 ``` class GroupExample1 { static void Main() { // Create a data source. string[] words = { "blueberry", "chimpanzee", "abacus", "banana", "apple", "cheese" }; // Create the query. var wordGroups = from w in words group w by w[0]; // Execute the query. foreach (var wordGroup in wordGroups) { Console.WriteLine("Words that start with the letter '{0}':", wordGroup.Key); foreach (var word in wordGroup) { Console.WriteLine(word); } } // Keep the console window open in debug mode Console.WriteLine("Press any key to exit."); Console.ReadKey(); } } /* Output: Words that start with the letter 'b': blueberry banana Words that start with the letter 'c': chimpanzee cheese Words that start with the letter 'a': abacus apple */ ``` 此示例演示在創建組之后，如何使用通過 **into** 實現的延續對這些組執行附加邏輯。有關更多信息，請參見 [into（C# 參考）](https://msdn.microsoft.com/zh-CN/library/bb311045.aspx)。下面的示例查詢每個組以僅選擇那些鍵值為元音的元素。 ``` class GroupClauseExample2 { static void Main() { // Create the data source. string[] words2 = { "blueberry", "chimpanzee", "abacus", "banana", "apple", "cheese", "elephant", "umbrella", "anteater" }; // Create the query. var wordGroups2 = from w in words2 group w by w[0] into grps where (grps.Key == 'a' || grps.Key == 'e' || grps.Key == 'i' || grps.Key == 'o' || grps.Key == 'u') select grps; // Execute the query. foreach (var wordGroup in wordGroups2) { Console.WriteLine("Groups that start with a vowel: {0}", wordGroup.Key); foreach (var word in wordGroup) { Console.WriteLine(" {0}", word); } } // Keep the console window open in debug mode Console.WriteLine("Press any key to exit."); Console.ReadKey(); } } /* Output: Groups that start with a vowel: a abacus apple anteater Groups that start with a vowel: e elephant Groups that start with a vowel: u umbrella */ ``` ## 備注編譯時，**group** 子句被轉換為對 [GroupBy<TSource,?TKey>](https://msdn.microsoft.com/zh-CN/library/bb534334.aspx) 方法的調用。 ## 請參閱 [IGrouping<TKey,?TElement>](https://msdn.microsoft.com/zh-CN/library/bb344977.aspx) [GroupBy<TSource,?TKey>](https://msdn.microsoft.com/zh-CN/library/bb534501.aspx) [ThenBy<TSource,?TKey>](https://msdn.microsoft.com/zh-CN/library/bb534743.aspx) [ThenByDescending<TSource,?TKey>](https://msdn.microsoft.com/zh-CN/library/bb534736.aspx) [查詢關鍵字（C# 參考）](https://msdn.microsoft.com/zh-CN/library/bb310804.aspx) [LINQ 查詢表達式（C# 編程指南）](https://msdn.microsoft.com/zh-CN/library/bb397676.aspx) [如何：創建嵌套組（C# 編程指南）](https://msdn.microsoft.com/zh-CN/library/bb545974.aspx) [如何：對查詢結果進行分組（C# 編程指南）](https://msdn.microsoft.com/zh-CN/library/bb545971.aspx) [如何：對分組操作執行子查詢（C# 編程指南）](https://msdn.microsoft.com/zh-CN/library/bb311041.aspx)