Enumerable.FirstOrDefault Enumerable.FirstOrDefault Enumerable.FirstOrDefault Enumerable.FirstOrDefault Method

Restituisce il primo elemento della sequenza che soddisfa una condizione specificata o un valore predefinito se tale elemento non viene trovato.Returns the first element of the sequence that satisfies a condition or a default value if no such element is found.

public:
generic <typename TSource>
[System::Runtime::CompilerServices::Extension]
 static TSource FirstOrDefault(System::Collections::Generic::IEnumerable<TSource> ^ source, Func<TSource, bool> ^ predicate);

public static TSource FirstOrDefault<TSource> (this System.Collections.Generic.IEnumerable<TSource> source, Func<TSource,bool> predicate);

static member FirstOrDefault : seq<'Source> * Func<'Source, bool> -> 'Source

<Extension()>
Public Function FirstOrDefault(Of TSource) (source As IEnumerable(Of TSource), predicate As Func(Of TSource, Boolean)) As TSource

Parametri di tipo

Parametri

Restituisce

Eccezioni

Esempi

Esempio di codice seguente viene illustrato come utilizzare FirstOrDefault<TSource>(IEnumerable<TSource>, Func<TSource,Boolean>) passando in un predicato.The following code example demonstrates how to use FirstOrDefault<TSource>(IEnumerable<TSource>, Func<TSource,Boolean>) by passing in a predicate. Nella seconda chiamata al metodo, non è presente alcun elemento nella matrice che soddisfa la condizione.In the second call to the method, there is no element in the array that satisfies the condition.

string[] names = { "Hartono, Tommy", "Adams, Terry", 
                     "Andersen, Henriette Thaulow", 
                     "Hedlund, Magnus", "Ito, Shu" };

string firstLongName = names.FirstOrDefault(name => name.Length > 20);

Console.WriteLine("The first long name is '{0}'.", firstLongName);

string firstVeryLongName = names.FirstOrDefault(name => name.Length > 30);

Console.WriteLine(
    "There is {0} name longer than 30 characters.",
    string.IsNullOrEmpty(firstVeryLongName) ? "not a" : "a");

/*
 This code produces the following output:

 The first long name is 'Andersen, Henriette Thaulow'.
 There is not a name longer than 30 characters.
*/

' Create an array of strings.
Dim names() As String =
{"Hartono, Tommy", "Adams, Terry", "Andersen, Henriette Thaulow", "Hedlund, Magnus", "Ito, Shu"}

' Select the first string in the array whose length is greater than 20.
Dim firstLongName As String =
names.FirstOrDefault(Function(name) name.Length > 20)

' Display the output.
MsgBox("The first long name is " & firstLongName)

' Select the first string in the array whose length is greater than 30,
' or a default value if there are no such strings in the array.
Dim firstVeryLongName As String =
names.FirstOrDefault(Function(name) name.Length > 30)

Dim text As String = IIf(String.IsNullOrEmpty(firstVeryLongName), "not a", "a")

MsgBox("There is " & text & " name longer than 30 characters.")

' This code produces the following output:
'
' The first long name is Andersen, Henriette Thaulow
'
' There is not a name longer than 30 characters.

Commenti

Il valore predefinito per riferimento e i tipi nullable è null.The default value for reference and nullable types is null.

Restituisce il primo elemento di una sequenza o un valore predefinito se la sequenza non contiene elementi.Returns the first element of a sequence, or a default value if the sequence contains no elements.

public:
generic <typename TSource>
[System::Runtime::CompilerServices::Extension]
 static TSource FirstOrDefault(System::Collections::Generic::IEnumerable<TSource> ^ source);

public static TSource FirstOrDefault<TSource> (this System.Collections.Generic.IEnumerable<TSource> source);

static member FirstOrDefault : seq<'Source> -> 'Source

<Extension()>
Public Function FirstOrDefault(Of TSource) (source As IEnumerable(Of TSource)) As TSource

Parametri di tipo

Parametri

Restituisce

Eccezioni

Esempi

Esempio di codice seguente viene illustrato come utilizzare FirstOrDefault<TSource>(IEnumerable<TSource>) in una matrice vuota.The following code example demonstrates how to use FirstOrDefault<TSource>(IEnumerable<TSource>) on an empty array.

int[] numbers = { };
int first = numbers.FirstOrDefault();
Console.WriteLine(first);

/*
 This code produces the following output:

 0
*/

' Create an empty array.
Dim numbers() As Integer = {}

' Select the first element in the array, or a default value
' if there are not elements in the array.
Dim first As Integer = numbers.FirstOrDefault()

' Display the output.
MsgBox(first)

' This code produces the following output:
'
' 0

In alcuni casi il valore di default(TSource) non è il valore predefinito da usare se la raccolta non contiene elementi.Sometimes the value of default(TSource) is not the default value that you want to use if the collection contains no elements. Invece di controllare il risultato per il valore predefinito non desiderati e quindi modificarlo se necessario, è possibile usare il DefaultIfEmpty<TSource>(IEnumerable<TSource>, TSource) metodo per specificare il valore predefinito da usare se la raccolta è vuota.Instead of checking the result for the unwanted default value and then changing it if necessary, you can use the DefaultIfEmpty<TSource>(IEnumerable<TSource>, TSource) method to specify the default value that you want to use if the collection is empty. Chiamare quindi First<TSource>(IEnumerable<TSource>) per ottenere il primo elemento.Then, call First<TSource>(IEnumerable<TSource>) to obtain the first element. Esempio di codice seguente usa entrambe le tecniche per ottenere un valore predefinito pari a 1, se un insieme di mesi numerici è vuoto.The following code example uses both techniques to obtain a default value of 1 if a collection of numeric months is empty. Poiché il valore predefinito per un tipo integer è 0, che non corrisponde a un mese, il valore predefinito deve essere invece specificato come 1.Because the default value for an integer is 0, which does not correspond to any month, the default value must be specified as 1 instead. La prima variabile di risultato viene verificata la il valore predefinito non desiderato dopo che la query è stata eseguita.The first result variable is checked for the unwanted default value after the query has finished executing. La seconda variabile di risultato viene ottenuta tramite DefaultIfEmpty<TSource>(IEnumerable<TSource>, TSource) per specificare un valore predefinito 1.The second result variable is obtained by using DefaultIfEmpty<TSource>(IEnumerable<TSource>, TSource) to specify a default value of 1.

List<int> months = new List<int> { };

// Setting the default value to 1 after the query.
int firstMonth1 = months.FirstOrDefault();
if (firstMonth1 == 0)
{
    firstMonth1 = 1;
}
Console.WriteLine("The value of the firstMonth1 variable is {0}", firstMonth1);

// Setting the default value to 1 by using DefaultIfEmpty() in the query.
int firstMonth2 = months.DefaultIfEmpty(1).First();
Console.WriteLine("The value of the firstMonth2 variable is {0}", firstMonth2);

/*
 This code produces the following output:

 The value of the firstMonth1 variable is 1
 The value of the firstMonth2 variable is 1
*/

Dim months As New List(Of Integer)(New Integer() {})

' Setting the default value to 1 after the query.
Dim firstMonth1 As Integer = months.FirstOrDefault()
If firstMonth1 = 0 Then
    firstMonth1 = 1
End If
MsgBox(String.Format("The value of the firstMonth1 variable is {0}", firstMonth1))

' Setting the default value to 1 by using DefaultIfEmpty() in the query.
Dim firstMonth2 As Integer = months.DefaultIfEmpty(1).First()
MsgBox(String.Format("The value of the firstMonth2 variable is {0}", firstMonth2))

' This code produces the following output:
'
' The value of the firstMonth1 variable is 1
' The value of the firstMonth2 variable is 1

Commenti

Il valore predefinito per riferimento e i tipi nullable è null.The default value for reference and nullable types is null.

Il FirstOrDefault metodo non fornisce un modo per specificare un valore predefinito.The FirstOrDefault method does not provide a way to specify a default value. Se si desidera specificare un valore predefinito diverso da default(TSource), usare il DefaultIfEmpty<TSource>(IEnumerable<TSource>, TSource) metodo come descritto nella sezione esempio.If you want to specify a default value other than default(TSource), use the DefaultIfEmpty<TSource>(IEnumerable<TSource>, TSource) method as described in the Example section.