Enumerable.AsEnumerable<TSource>(IEnumerable<TSource>) 方法

定义

返回类型化为 IEnumerable<T> 的输入。Returns the input typed as IEnumerable<T>.

public:
generic <typename TSource>
[System::Runtime::CompilerServices::Extension]
 static System::Collections::Generic::IEnumerable<TSource> ^ AsEnumerable(System::Collections::Generic::IEnumerable<TSource> ^ source);
public static System.Collections.Generic.IEnumerable<TSource> AsEnumerable<TSource> (this System.Collections.Generic.IEnumerable<TSource> source);
static member AsEnumerable : seq<'Source> -> seq<'Source>
<Extension()>
Public Function AsEnumerable(Of TSource) (source As IEnumerable(Of TSource)) As IEnumerable(Of TSource)

类型参数

TSource

source 的元素类型。The type of the elements of source.

参数

source
IEnumerable<TSource>

要作为 IEnumerable<T> 键入的序列。The sequence to type as IEnumerable<T>.

返回

IEnumerable<TSource>

已作为 IEnumerable<T> 键入的输入序列。The input sequence typed as IEnumerable<T>.

示例

下面的代码示例演示当需要标准查询运算符实现时,如何使用 AsEnumerable<TSource>(IEnumerable<TSource>) 隐藏类型的自定义 Where 方法。The following code example demonstrates how to use AsEnumerable<TSource>(IEnumerable<TSource>) to hide a type's custom Where method when the standard query operator implementation is desired.

// Custom class.
class Clump<T> : List<T>
{
    // Custom implementation of Where().
    public IEnumerable<T> Where(Func<T, bool> predicate)
    {
        Console.WriteLine("In Clump's implementation of Where().");
        return Enumerable.Where(this, predicate);
    }
}

static void AsEnumerableEx1()
{
    // Create a new Clump<T> object.
    Clump<string> fruitClump =
        new Clump<string> { "apple", "passionfruit", "banana",
            "mango", "orange", "blueberry", "grape", "strawberry" };

    // First call to Where():
    // Call Clump's Where() method with a predicate.
    IEnumerable<string> query1 =
        fruitClump.Where(fruit => fruit.Contains("o"));

    Console.WriteLine("query1 has been created.\n");

    // Second call to Where():
    // First call AsEnumerable() to hide Clump's Where() method and thereby
    // force System.Linq.Enumerable's Where() method to be called.
    IEnumerable<string> query2 =
        fruitClump.AsEnumerable().Where(fruit => fruit.Contains("o"));

    // Display the output.
    Console.WriteLine("query2 has been created.");
}

// This code produces the following output:
//
// In Clump's implementation of Where().
// query1 has been created.
//
// query2 has been created.
Dim output As New System.Text.StringBuilder

' A custom class.
Class Clump(Of T)
    Inherits List(Of T)

    ' Constructor.
    Public Sub New(ByVal collection As IEnumerable(Of T))
        MyBase.New(collection)
    End Sub

    ' Custom implementation of Where().
    Function Where(ByVal predicate As Func(Of T, Boolean)) As IEnumerable(Of T)
        output.AppendLine("In Clump's implementation of Where().")
        Return Enumerable.Where(Me, predicate)
    End Function
End Class

Sub AsEnumerableEx1()
    ' Create a new Clump(Of T) object.
    Dim fruitClump As New Clump(Of String)(New String() _
                                       {"apple", "passionfruit", "banana",
                                        "mango", "orange", "blueberry",
                                        "grape", "strawberry"})

    ' First call to Where():
    ' Call Clump's Where() method with a predicate.
    Dim query1 As IEnumerable(Of String) =
    fruitClump.Where(Function(fruit) fruit.Contains("o"))
    output.AppendLine("query1 has been created." & vbCrLf)

    ' Second call to Where():
    ' First call AsEnumerable() to hide Clump's Where() method and thereby
    ' force System.Linq.Enumerable's Where() method to be called.
    Dim query2 As IEnumerable(Of String) =
    fruitClump.AsEnumerable().Where(Function(fruit) fruit.Contains("o"))
    output.AppendLine("query2 has been created.")

    ' Display the output.
    Console.WriteLine(output.ToString())
End Sub

' This code produces the following output:
'
' In Clump's implementation of Where().
' query1 has been created.
'
' query2 has been created.

注解

除了将 source 的编译时类型从实现 IEnumerable<T> 的类型更改为 IEnumerable<T> 自身之外,AsEnumerable<TSource>(IEnumerable<TSource>) 方法不会产生任何影响。The AsEnumerable<TSource>(IEnumerable<TSource>) method has no effect other than to change the compile-time type of source from a type that implements IEnumerable<T> to IEnumerable<T> itself.

当序列实现 IEnumerable<T> 但还具有一组不同的公共查询方法可用时,AsEnumerable<TSource>(IEnumerable<TSource>) 可用于在查询实现之间进行选择。AsEnumerable<TSource>(IEnumerable<TSource>) can be used to choose between query implementations when a sequence implements IEnumerable<T> but also has a different set of public query methods available. 例如,给定一个实现 IEnumerable<T> 的泛型类 Table,并有自己的方法(如 WhereSelectSelectMany),对 Where 的调用将调用 Where 的公共 Table方法。For example, given a generic class Table that implements IEnumerable<T> and has its own methods such as Where, Select, and SelectMany, a call to Where would invoke the public Where method of Table. 表示数据库表的 Table 类型可以有一个 Where 方法,该方法采用谓词参数作为表达式树,并将树转换为 SQL 以进行远程执行。A Table type that represents a database table could have a Where method that takes the predicate argument as an expression tree and converts the tree to SQL for remote execution. 如果不需要远程执行(例如因为谓词调用了本地方法),则可以使用 AsEnumerable 方法隐藏自定义方法,并改为提供标准查询运算符。If remote execution is not desired, for example because the predicate invokes a local method, the AsEnumerable method can be used to hide the custom methods and instead make the standard query operators available.

适用于