Enumerable.Cast<TResult>(IEnumerable) 方法


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)



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) 來啟用上的標準查詢運算子 ArrayListThe 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所執行。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. 這個方法所表示的查詢不會執行,直到以 GetEnumerator foreach Visual c # 或 Visual Basic 中的方法來列舉物件為止 For EachThe 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.

方法可提供所需的型別資訊,在 Cast<TResult>(IEnumerable) 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,不 ArrayList 會執行 IEnumerable<T> ,但藉由在 Cast<TResult>(IEnumerable) 物件上呼叫 ArrayList ,則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.

如果元素無法轉換成類型 TResult ,這個方法會擲回 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 ,這表示專案具有的編譯時間靜態型別 objectThe source sequence for this method is IEnumerable, which means the elements have the compile-time static type of object. 這個方法所執行的唯一型別轉換為參考轉換和取消的轉換。The only type conversions that are performed by this method are reference conversions and unboxing conversions. 集合中專案的執行時間型別必須符合目標型別,如果是實值型別,則專案的執行時間型別必須是目標型別之裝箱轉換的結果。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,例如不同數數值型別之間的轉換類型。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的元素 TResult ,請使用方法, OfType 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,明確類型的反覆運算變數會轉譯為的調用 Cast<TResult>(IEnumerable)In a query expression, an explicitly typed iteration variable translates to an invocation of Cast<TResult>(IEnumerable). 此範例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 select 查詢的子句來執行其他轉換類型,例如隱含的數值轉換。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 方法和語句,將 select 一連串的一連串整數轉換成一連串的雙精度浮點數。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)