Enumerable.Empty<TResult> Method
Definition
Important
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Returns an empty IEnumerable<T> that has the specified type argument.
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)Type Parameters
- TResult
The type to assign to the type parameter of the returned generic IEnumerable<T>.
Returns
An empty IEnumerable<T> whose type argument is TResult.
Examples
The following code example demonstrates how to use Empty<TResult>() to generate an empty IEnumerable<T>.
IEnumerable<decimal> empty = Enumerable.Empty<decimal>();
' Create an empty sequence.
Dim empty As IEnumerable(Of Decimal) = Enumerable.Empty(Of Decimal)()
The following code example demonstrates a possible application of the Empty<TResult>() method. The Aggregate method is applied to a collection of string arrays. The elements of each array in the collection are added to the resulting IEnumerable<T> only if that array contains four or more elements. Empty is used to generate the seed value for Aggregate because if no array in the collection has four or more elements, only the empty sequence is returned.
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
Remarks
The Empty<TResult>() method caches an empty sequence of type TResult. When the object it returns is enumerated, it yields no elements.
In some cases, this method is useful for passing an empty sequence to a user-defined method that takes an IEnumerable<T>. It can also be used to generate a neutral element for methods such as Union. See the Example section for an example of this use of Empty<TResult>().
