如何：查詢對象集合（C# 編程指南） · MSDN C# 編程指南 & 參考手冊 2015

# 如何：查詢對象集合（C# 編程指南）此示例演示如何對一個 Student 對象列表執行簡單查詢。每個 Student 對象都包含一些有關該學生的基本信息，以及一個表示該學生的四次考試得分的列表。此應用程序充當此部分中其他很多示例的框架，這些示例都使用相同的 students 數據源。下面的查詢返回那些在其第一次考試中得分為 90 分或更高的學生。 ``` public class StudentClass { #region data protected enum GradeLevel { FirstYear = 1, SecondYear, ThirdYear, FourthYear }; protected class Student { public string FirstName { get; set; } public string LastName { get; set; } public int ID { get; set; } public GradeLevel Year; public List<int> ExamScores; } protected static List<Student> students = new List<Student> { new Student {FirstName = "Terry", LastName = "Adams", ID = 120, Year = GradeLevel.SecondYear, ExamScores = new List<int>{ 99, 82, 81, 79}}, new Student {FirstName = "Fadi", LastName = "Fakhouri", ID = 116, Year = GradeLevel.ThirdYear, ExamScores = new List<int>{ 99, 86, 90, 94}}, new Student {FirstName = "Hanying", LastName = "Feng", ID = 117, Year = GradeLevel.FirstYear, ExamScores = new List<int>{ 93, 92, 80, 87}}, new Student {FirstName = "Cesar", LastName = "Garcia", ID = 114, Year = GradeLevel.FourthYear, ExamScores = new List<int>{ 97, 89, 85, 82}}, new Student {FirstName = "Debra", LastName = "Garcia", ID = 115, Year = GradeLevel.ThirdYear, ExamScores = new List<int>{ 35, 72, 91, 70}}, new Student {FirstName = "Hugo", LastName = "Garcia", ID = 118, Year = GradeLevel.SecondYear, ExamScores = new List<int>{ 92, 90, 83, 78}}, new Student {FirstName = "Sven", LastName = "Mortensen", ID = 113, Year = GradeLevel.FirstYear, ExamScores = new List<int>{ 88, 94, 65, 91}}, new Student {FirstName = "Claire", LastName = "O'Donnell", ID = 112, Year = GradeLevel.FourthYear, ExamScores = new List<int>{ 75, 84, 91, 39}}, new Student {FirstName = "Svetlana", LastName = "Omelchenko", ID = 111, Year = GradeLevel.SecondYear, ExamScores = new List<int>{ 97, 92, 81, 60}}, new Student {FirstName = "Lance", LastName = "Tucker", ID = 119, Year = GradeLevel.ThirdYear, ExamScores = new List<int>{ 68, 79, 88, 92}}, new Student {FirstName = "Michael", LastName = "Tucker", ID = 122, Year = GradeLevel.FirstYear, ExamScores = new List<int>{ 94, 92, 91, 91}}, new Student {FirstName = "Eugene", LastName = "Zabokritski", ID = 121, Year = GradeLevel.FourthYear, ExamScores = new List<int>{ 96, 85, 91, 60}} }; #endregion //Helper method, used in GroupByRange. protected static int GetPercentile(Student s) { double avg = s.ExamScores.Average(); return avg > 0 ? (int)avg / 10 : 0; } public void QueryHighScores(int exam, int score) { var highScores = from student in students where student.ExamScores[exam] > score select new {Name = student.FirstName, Score = student.ExamScores[exam]}; foreach (var item in highScores) { Console.WriteLine("{0,-15}{1}", item.Name, item.Score); } } } public class Program { public static void Main() { StudentClass sc = new StudentClass(); sc.QueryHighScores(1, 90); // Keep the console window open in debug mode. Console.WriteLine("Press any key to exit"); Console.ReadKey(); } } ``` 我們特意將此查詢編寫得非常簡單，以使您能夠進行試驗。例如，您可以在 **where** 子句中嘗試使用更多的謂詞，也可以使用 **orderby** 子句對結果進行排序。 ## 編譯代碼 * 創建面向 .NET Framework 3.5 版的 Visual Studio 項目。默認情況下，該項目具有一個對 System.Core.dll 的引用以及一條針對 System.Linq 命名空間的 **using** 指令。 * 將代碼復制到項目中。 * 按 F5 編譯并運行程序。 * 按任意鍵退出控制臺窗口。 ## 請參閱 [LINQ 查詢表達式（C# 編程指南）](https://msdn.microsoft.com/zh-cn/library/bb397676.aspx)