LINQ 基础教程
LINQ 转换运算符
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
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
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。
To运算符：ToArray()，ToList()，ToDictionary()
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 listToDictionary - 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。
