LINQ中的Conversion运算符可用于转换序列(集合)中元素的类型。转换运算符分为三种:As运算符(AsEnumerable和AsQueryable),To运算符(ToArray,ToDictionary,ToList和ToLookup)和转换运算符(Cast和OfType)。
下表列出了所有转换运算符。
方法 | 描述 |
---|---|
AsEnumerable | 将输入序列作为 IEnumerable < T> 返回 |
AsQueryable | 将IEnumerableto转换为IQueryable,以模拟远程查询提供程序 |
Cast | 将非泛型集合转换为泛型集合(IEnumerable到IEnumerable) |
OfType | 基于指定类型筛选集合 |
ToArray | 将集合转换为数组 |
ToDictionary | 根据键选择器函数将元素放入 Dictionary 中 |
ToList | 将集合转换为 List |
ToLookup | 将元素分组到 Lookup<TKey,TElement> |
AsEnumerable和AsQueryable方法分别将源对象转换或转换为IEnumerable <T>或IQueryable <T>。
请看以下示例:
class Program { static void ReportTypeProperties<T>(T obj) { Console.WriteLine("Compile-time type: {0}", typeof(T).Name); Console.WriteLine("Actual type: {0}", obj.GetType().Name); } static void Main(string[] args) { Student[] studentArray = { new Student() { StudentID = 1, StudentName = "John", Age = 18 } , new Student() { StudentID = 2, StudentName = "Steve", Age = 21 } , new Student() { StudentID = 3, StudentName = "Bill", Age = 25 } , new Student() { StudentID = 4, StudentName = "Ram" , Age = 20 } , new Student() { StudentID = 5, StudentName = "Ron" , Age = 31 } , }; ReportTypeProperties( studentArray); ReportTypeProperties(studentArray.AsEnumerable()); ReportTypeProperties(studentArray.AsQueryable()); } }
Compile-time type: Student[] Actual type: Student[] Compile-time type: IEnumerable`1 Actual type: Student[] Compile-time type: IQueryable`1 Actual type: EnumerableQuery`1
如上例所示,AsEnumerable和AsQueryable方法分别将编译时间类型转换为IEnumerable和IQueryable
Cast的作用与AsEnumerable<T>相同。它将源对象强制转换为IEnumerable<T>。
class Program { static void ReportTypeProperties<T>(T obj) { Console.WriteLine("Compile-time type: {0}", typeof(T).Name); Console.WriteLine("Actual type: {0}", obj.GetType().Name); } static void Main(string[] args) { Student[] studentArray = { new Student() { StudentID = 1, StudentName = "John", Age = 18 } , new Student() { StudentID = 2, StudentName = "Steve", Age = 21 } , new Student() { StudentID = 3, StudentName = "Bill", Age = 25 } , new Student() { StudentID = 4, StudentName = "Ram" , Age = 20 } , new Student() { StudentID = 5, StudentName = "Ron" , Age = 31 } , }; ReportTypeProperties( studentArray); ReportTypeProperties(studentArray.Cast<Student>()); } }
Compile-time type: Student[] Actual type: Student[] Compile-time type: IEnumerable`1 Actual type: Student[] Compile-time type: IEnumerable`1 Actual type: Student[] Compile-time type: IEnumerable`1 Actual type: Student[]
studentArray.Cast<Student>() 与 (IEnumerable<Student>)studentArray 相同,但是 Cast<Student>() 可读性更好。
顾名思义,ToArray(),ToList(),ToDictionary()方法的源对象转换分别为一个数组,列表或字典。
To 运算符强制执行查询。它强制远程查询提供者执行查询并从底层数据源(如SQL Server数据库)获取结果。
IList<string> strList = new List<string>() { "One", "Two", "Three", "Four", "Three" }; string[] strArray = strList.ToArray<string>();// 将列表转换为数组 IList<string> list = strArray.ToList<string>(); // converts array into list
ToDictionary - 将泛型列表转换为泛型词典:
IList<Student> studentList = new List<Student>() { new Student() { StudentID = 1, StudentName = "John", age = 18 } , new Student() { StudentID = 2, StudentName = "Steve", age = 21 } , new Student() { StudentID = 3, StudentName = "Bill", age = 18 } , new Student() { StudentID = 4, StudentName = "Ram" , age = 20 } , new Student() { StudentID = 5, StudentName = "Ron" , age = 21 } }; //以下将列表转换成字典,其中StudentId是键 IDictionary<int, Student> studentDict = studentList.ToDictionary<Student, int>(s => s.StudentID); foreach(var key in studentDict.Keys) Console.WriteLine("Key: {0}, Value: {1}", key, (studentDict[key] as Student).StudentName);
Key: 1, Value: John Key: 2, Value: Steve Key: 3, Value: Bill Key: 4, Value: Ram Key: 5, Value: Ron
下图显示了上面示例中的studentDict如何包含一个key-value对,其中key是StudentID,value是Student对象。