LINQ 菜鳥教程
LINQ 轉(zhuǎn)換運(yùn)算符
LINQ中的Conversion運(yùn)算符可用于轉(zhuǎn)換序列（集合）中元素的類型。轉(zhuǎn)換運(yùn)算符分為三種：As運(yùn)算符（AsEnumerable和AsQueryable），To運(yùn)算符（ToArray，ToDictionary，ToList和ToLookup）和轉(zhuǎn)換運(yùn)算符（Cast和OfType）。
下表列出了所有轉(zhuǎn)換運(yùn)算符。
| 方法 | 描述 |
|---|---|
| AsEnumerable | 將輸入序列作為 IEnumerable < T> 返回 |
| AsQueryable | 將IEnumerableto轉(zhuǎn)換為IQueryable，以模擬遠(yuǎn)程查詢提供程序 |
| Cast | 將非泛型集合轉(zhuǎn)換為泛型集合（IEnumerable到IEnumerable） |
| OfType | 基于指定類型篩選集合 |
| ToArray | 將集合轉(zhuǎn)換為數(shù)組 |
| ToDictionary | 根據(jù)鍵選擇器函數(shù)將元素放入 Dictionary 中 |
| ToList | 將集合轉(zhuǎn)換為 List |
| ToLookup | 將元素分組到 Lookup<TKey,TElement> |
AsEnumerable和AsQueryable方法
AsEnumerable和AsQueryable方法分別將源對象轉(zhuǎn)換或轉(zhuǎn)換為IEnumerable <T>或IQueryable <T>。
請看以下示例：
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方法分別將編譯時間類型轉(zhuǎn)換為IEnumerable和IQueryable
Cast
Cast的作用與AsEnumerable<T>相同。它將源對象強(qiáng)制轉(zhuǎn)換為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>() 與 (IEnumerable<Student>)studentArray 相同，但是 Cast<Student>() 可讀性更好。
To運(yùn)算符：ToArray()，ToList()，ToDictionary()
顧名思義，ToArray()，ToList()，ToDictionary()方法的源對象轉(zhuǎn)換分別為一個數(shù)組，列表或字典。
To 運(yùn)算符強(qiáng)制執(zhí)行查詢。它強(qiáng)制遠(yuǎn)程查詢提供者執(zhí)行查詢并從底層數(shù)據(jù)源(如SQL Server數(shù)據(jù)庫)獲取結(jié)果。
IList<string> strList = new List<string>() {
"One",
"Two",
"Three",
"Four",
"Three"
};
string[] strArray = strList.ToArray<string>();// 將列表轉(zhuǎn)換為數(shù)組
IList<string> list = strArray.ToList<string>(); // converts array into listToDictionary - 將泛型列表轉(zhuǎn)換為泛型詞典：
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 }
};
//以下將列表轉(zhuǎn)換成字典，其中StudentId是鍵
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
下圖顯示了上面示例中的studentDict如何包含一個key-value對，其中key是StudentID，value是Student對象。
