Enumerable.Cast(IEnumerable) Enumerable.Cast(IEnumerable) Enumerable.Cast(IEnumerable) Enumerable.Cast(IEnumerable) Method

定義

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

public:
generic <typename TResult>
[System::Runtime::CompilerServices::Extension]
 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>
<Extension()>
Public Function Cast(Of TResult) (source As IEnumerable) As IEnumerable(Of TResult)

類型參數

TResult

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

參數

source
IEnumerable IEnumerable IEnumerable IEnumerable

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.

傳回

IEnumerable<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.

例外狀況

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.

範例

下列程式碼範例示範如何使用Cast<TResult>(IEnumerable)來啟用在ArrayList上使用標準查詢運算子。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();
fruits.Add("mango");
fruits.Add("apple");
fruits.Add("lemon");

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)
{
    Console.WriteLine(fruit);
}

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

' 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
    output.AppendLine(fruit)
Next

' Display the output.
MsgBox(output.ToString())

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

備註

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. 這個方法所表示的查詢, 必須等到透過直接GetEnumerator呼叫其方法, 或是在 Visual C#或For Each Visual Basic 中使用foreach來列舉物件之後, 才會執行。The 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)方法可提供必要的類型資訊, 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> Cast<TResult>(IEnumerable) ,ArrayList但是藉由在物件上呼叫, 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.

如果元素無法轉換成類型TResult, 這個方法InvalidCastException會擲回。If an element cannot be converted to type TResult, this method throws a InvalidCastException.

這個方法的來源序列是IEnumerable, 這表示元素具有的編譯時間靜態object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的專案, 請OfType使用方法, 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, 明確類型的反復專案變數會轉譯為的Cast<TResult>(IEnumerable)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select的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select和語句, 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)

適用於

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