Enumerable.FirstOrDefault 方法

傳回序列中符合條件的第一個元素；如果找不到這類元素，則傳回預設值。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

類型參數

參數

傳回

例外狀況

範例

下列程式碼範例示範如何藉由傳入述詞來使用 FirstOrDefault<TSource>(IEnumerable<TSource>, Func<TSource,Boolean>)。The following code example demonstrates how to use FirstOrDefault<TSource>(IEnumerable<TSource>, Func<TSource,Boolean>) by passing in a predicate. 在第二次呼叫方法時，陣列中沒有符合條件的元素。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.

備註

Reference 和 nullable 類型的預設值為 null。The default value for reference and nullable types is null.

傳回序列的第一個元素；如果序列中沒有包含任何元素，則傳回預設值。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

類型參數

參數

傳回

例外狀況

範例

下列程式碼範例示範如何在空陣列上使用 FirstOrDefault<TSource>(IEnumerable<TSource>)。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

有時候，如果集合未包含任何專案，則 default(TSource) 的值不是您想要使用的預設值。Sometimes the value of default(TSource) is not the default value that you want to use if the collection contains no elements. 您可以使用 DefaultIfEmpty<TSource>(IEnumerable<TSource>, TSource) 方法來指定您想要在集合是空的時使用的預設值，而不是檢查不必要的預設值的結果，然後在必要時加以變更。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. 然後，呼叫 First<TSource>(IEnumerable<TSource>) 以取得第一個元素。Then, call First<TSource>(IEnumerable<TSource>) to obtain the first element. 如果數值月份的集合是空的，下列程式碼範例會使用這兩種技術來取得預設值1。The following code example uses both techniques to obtain a default value of 1 if a collection of numeric months is empty. 因為整數的預設值是0（未對應至任何月份），所以預設值必須改為指定為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. 查詢完成執行之後，第一個結果變數會檢查是否有不必要的預設值。The first result variable is checked for the unwanted default value after the query has finished executing. 第二個結果變數的取得方式是使用 DefaultIfEmpty<TSource>(IEnumerable<TSource>, TSource) 來指定預設值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

備註

Reference 和 nullable 類型的預設值為 null。The default value for reference and nullable types is null.

@No__t-0 方法不會提供指定預設值的方式。The FirstOrDefault method does not provide a way to specify a default value. 如果您想要指定 default(TSource) 以外的預設值，請使用如範例一節中所述的 DefaultIfEmpty<TSource>(IEnumerable<TSource>, TSource) 方法。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.