Enumerable.Cast<TResult>(IEnumerable) Method


IEnumerable 的項目轉換成指定的型別。Casts the elements of an IEnumerable to the specified type.

generic <typename TResult>
 static System::Collections::Generic::IEnumerable<TResult> ^ Cast(System::Collections::IEnumerable ^ source);
public static System.Collections.Generic.IEnumerable<TResult> Cast<TResult> (this System.Collections.IEnumerable source);
static member Cast : System.Collections.IEnumerable -> seq<'Result>
Public Function Cast(Of TResult) (source As IEnumerable) As IEnumerable(Of TResult)

Type Parameters


要將 source 之項目轉換成的型別。The type to cast the elements of source to.



IEnumerable,其包含要轉換成型別 TResult 的項目。The IEnumerable that contains the elements to be cast to type TResult.



IEnumerable<T>,其中包含已轉型成指定之型別的每個來源序列項目。An IEnumerable<T> that contains each element of the source sequence cast to the specified type.


sourcenullsource is null.

無法將序列中的項目轉換為型別 TResultAn element in the sequence cannot be cast to type TResult.


下列程式碼範例示範如何使用 Cast<TResult>(IEnumerable),在 ArrayList上啟用標準查詢運算子。The following code example demonstrates how to use Cast<TResult>(IEnumerable) to enable the use of the standard query operators on an ArrayList.

System.Collections.ArrayList fruits = new System.Collections.ArrayList();

IEnumerable<string> query =
    fruits.Cast<string>().OrderBy(fruit => fruit).Select(fruit => fruit);

// The following code, without the cast, doesn't compile.
//IEnumerable<string> query1 =
//    fruits.OrderBy(fruit => fruit).Select(fruit => fruit);

foreach (string fruit in query)

// This code produces the following output:
// apple
// lemon
// mango
' Create an ArrayList and add items to it.
Dim fruits As New System.Collections.ArrayList()

' Call Cast(Of String) to cast the ArrayList elements to strings.
Dim query As IEnumerable(Of String) =
fruits.Cast(Of String)().OrderBy(Function(fruit) fruit).Select(Function(fruit) fruit)

'' The following code, without the cast, doesn't compile.
'Dim query As IEnumerable(Of String) = _
'    fruits.OrderBy(Function(fruit) fruit).Select(Function(fruit) fruit)

Dim output As New System.Text.StringBuilder
For Each fruit As String In query

' Display the output.

' This code produces the following output:
' apple
' lemon
' mango


這個方法是使用延後執行來執行。This method is implemented by using deferred execution. 立即傳回值是一個物件,它會儲存執行動作所需的所有資訊。The immediate return value is an object that stores all the information that is required to perform the action. 在列舉物件之前,不會執行此方法所代表的查詢,方法是直接呼叫其 GetEnumerator 方法,或是在 Visual Basic 中C#使用 Visual 或 For Each 中的 foreachThe query represented by this method is not executed until the object is enumerated either by calling its GetEnumerator method directly or by using foreach in Visual C# or For Each in Visual Basic.

Cast<TResult>(IEnumerable) 方法可提供必要的型別資訊,以在非泛型集合上叫用標準查詢運算子。The Cast<TResult>(IEnumerable) method enables the standard query operators to be invoked on non-generic collections by supplying the necessary type information. 例如,ArrayList 不會執行 IEnumerable<T>,但藉由在 ArrayList 物件上呼叫 Cast<TResult>(IEnumerable),就可以使用標準查詢運算子來查詢順序。For example, ArrayList does not implement IEnumerable<T>, but by calling Cast<TResult>(IEnumerable) on the ArrayList object, the standard query operators can then be used to query the sequence.

如果無法將專案轉換成類型 TResult,這個方法就會擲回 InvalidCastExceptionIf an element cannot be converted to type TResult, this method throws a InvalidCastException.

這個方法的來源序列是 IEnumerable,這表示元素具有 object的編譯時間靜態類型。The source sequence for this method is IEnumerable, which means the elements have the compile-time static type of object. 這個方法唯一執行的類型轉換是參考轉換和取消裝箱轉換。The only type conversions that are performed by this method are reference conversions and unboxing conversions. 集合中元素的執行時間類型必須符合目標型別,或在實數值型別的情況下,專案的執行時間類型必須是目標型別的裝箱轉換結果。The runtime type of the elements in the collection must match the target type, or in the case of value types, the runtime type of elements must be the result of a boxing conversion of the target type. 不允許其他轉換類型,例如在不同的數數值型別之間使用。Other conversion types, such as those between different numeric types, are not allowed.

若只要取得可以轉換成類型 TResult的元素,請使用 OfType 方法,而不是 Cast<TResult>(IEnumerable)To obtain only those elements that can be converted to type TResult, use the OfType method instead of Cast<TResult>(IEnumerable).

在查詢運算式中,明確具類型的反復專案變數會轉譯為 Cast<TResult>(IEnumerable)的調用。In a query expression, an explicitly typed iteration variable translates to an invocation of Cast<TResult>(IEnumerable). 這個範例會顯示明確類型範圍變數的語法。This example shows the syntax for an explicitly typed range variable.

from int i in objects  
From i As Integer In objects  

使用查詢的 select 子句來執行其他轉換類型,例如隱含數值轉換。Use the select clause of a query to perform other conversion types, like the implicit numeric conversions. 下列範例會使用 Cast 方法和 select 語句,將一系列的已裝箱整數轉換成雙精度浮點數的序列。The following example uses both the Cast method and a select statement to convert a sequence of boxed integers to a sequence of doubles.

IEnumerable sequence = Enumerable.Range(0, 10);
var doubles = from int item in sequence
                select (double)item;
Dim sequence As IEnumerable = Enumerable.Range(0, 10)
Dim doubles = From item As Integer In sequence
                Select CType(item, Double)

Applies to

See also