Enumerable.Empty<TResult> 方法

public:
generic <typename TResult>
 static System::Collections::Generic::IEnumerable<TResult> ^ Empty();

public static System.Collections.Generic.IEnumerable<TResult> Empty<TResult>();

static member Empty : unit -> seq<'Result>

Public Function Empty(Of TResult) () As IEnumerable(Of TResult)

類型參數

傳回

一個型態參數為 IEnumerable<T>的空 TResult 。

範例

以下程式碼範例示範如何使用 Empty<TResult>() 來產生空 IEnumerable<T>。

IEnumerable<decimal> empty = Enumerable.Empty<decimal>();

' Create an empty sequence.
Dim empty As IEnumerable(Of Decimal) = Enumerable.Empty(Of Decimal)()

以下程式碼範例展示了此 Empty<TResult>() 方法的可能應用。 此 Aggregate 方法應用於一組字串陣列。 集合中每個陣列的元素只有在該陣列包含四個或以上元素時才會被加入結果 IEnumerable<T> 中。 Empty 用於產生 的 Aggregate 種子值，因為如果集合中沒有四個或以上元素的陣列，則只會回傳空序列。

string[] names1 = { "Hartono, Tommy" };
string[] names2 = { "Adams, Terry", "Andersen, Henriette Thaulow",
                      "Hedlund, Magnus", "Ito, Shu" };
string[] names3 = { "Solanki, Ajay", "Hoeing, Helge",
                      "Andersen, Henriette Thaulow",
                      "Potra, Cristina", "Iallo, Lucio" };

List<string[]> namesList =
    new List<string[]> { names1, names2, names3 };

// Only include arrays that have four or more elements
IEnumerable<string> allNames =
    namesList.Aggregate(Enumerable.Empty<string>(),
    (current, next) => next.Length > 3 ? current.Union(next) : current);

foreach (string name in allNames)
{
    Console.WriteLine(name);
}

/*
 This code produces the following output:

 Adams, Terry
 Andersen, Henriette Thaulow
 Hedlund, Magnus
 Ito, Shu
 Solanki, Ajay
 Hoeing, Helge
 Potra, Cristina
 Iallo, Lucio
*/

' Create three string arrays.
Dim names1() As String =
{"Hartono, Tommy"}
Dim names2() As String =
{"Adams, Terry", "Andersen, Henriette Thaulow", "Hedlund, Magnus", "Ito, Shu"}
Dim names3() As String =
{"Solanki, Ajay", "Hoeing, Helge", "Andersen, Henriette Thaulow", "Potra, Cristina", "Iallo, Lucio"}

' Create a List that contains 3 elements, where
' each element is an array of strings.
Dim namesList As New List(Of String())(New String()() {names1, names2, names3})

' Select arrays that have four or more elements and union
' them into one collection, using Empty() to generate the
' empty collection for the seed value.
Dim allNames As IEnumerable(Of String) =
namesList.Aggregate(Enumerable.Empty(Of String)(),
                    Function(current, nextOne) _
                        IIf(nextOne.Length > 3, current.Union(nextOne), current))

Dim output As New System.Text.StringBuilder
For Each name As String In allNames
    output.AppendLine(name)
Next

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

' This code produces the following output:
'
' Adams, Terry
' Andersen, Henriette Thaulow
' Hedlund, Magnus
' Ito, Shu
' Solanki, Ajay
' Hoeing, Helge
' Potra, Cristina
' Iallo, Lucio

備註

此 Empty<TResult>() 方法會快取一個類型的 TResult空序列。 當它回傳的物件被枚舉時，則不會產生任何元素。

在某些情況下，此方法用於將空序列傳給使用者定義的方法，該方法取 IEnumerable<T>。 它也可以用來產生中性元素，適用於如 Union。 請參閱範例章節，了解此種使用 Empty<TResult>()範例。