Queryable.FirstOrDefault Queryable.FirstOrDefault Queryable.FirstOrDefault Queryable.FirstOrDefault Method

定義

要素が存在しない場合は、シーケンス、または既定値の最初の要素を返します。Returns the first element of a sequence, or a default value if no element is found.

オーバーロード

FirstOrDefault<TSource>(IQueryable<TSource>) FirstOrDefault<TSource>(IQueryable<TSource>) FirstOrDefault<TSource>(IQueryable<TSource>) FirstOrDefault<TSource>(IQueryable<TSource>)

シーケンスの最初の要素を返します。シーケンスに要素が含まれていない場合は既定値を返します。Returns the first element of a sequence, or a default value if the sequence contains no elements.

FirstOrDefault<TSource>(IQueryable<TSource>, Expression<Func<TSource,Boolean>>) FirstOrDefault<TSource>(IQueryable<TSource>, Expression<Func<TSource,Boolean>>) FirstOrDefault<TSource>(IQueryable<TSource>, Expression<Func<TSource,Boolean>>) FirstOrDefault<TSource>(IQueryable<TSource>, Expression<Func<TSource,Boolean>>)

このような要素が見つからない場合に、指定した条件または既定値を満たす、シーケンスの最初の要素を返します。Returns the first element of a sequence that satisfies a specified condition or a default value if no such element is found.

FirstOrDefault<TSource>(IQueryable<TSource>) FirstOrDefault<TSource>(IQueryable<TSource>) FirstOrDefault<TSource>(IQueryable<TSource>) FirstOrDefault<TSource>(IQueryable<TSource>)

シーケンスの最初の要素を返します。シーケンスに要素が含まれていない場合は既定値を返します。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::Linq::IQueryable<TSource> ^ source);
public static TSource FirstOrDefault<TSource> (this System.Linq.IQueryable<TSource> source);
static member FirstOrDefault : System.Linq.IQueryable<'Source> -> 'Source
<Extension()>
Public Function FirstOrDefault(Of TSource) (source As IQueryable(Of TSource)) As TSource

型パラメーター

TSource

source の要素の型。The type of the elements of source.

パラメーター

source
IQueryable<TSource>

IQueryable<T>の最初の要素を返します。The IQueryable<T> to return the first element of.

戻り値

TSource TSource TSource TSource

source が空の場合は default(TSource)。それ以外の場合は source の最初の要素。default(TSource) if source is empty; otherwise, the first element in source.

例外

次のコード例は、使用する方法を示しますFirstOrDefault<TSource>(IQueryable<TSource>)で空のシーケンス。The following code example demonstrates how to use FirstOrDefault<TSource>(IQueryable<TSource>) on an empty sequence.

// Create an empty array.
int[] numbers = { };
// Get the first item in the array, or else the 
// default value for type int (0).
int first = numbers.AsQueryable().FirstOrDefault();

Console.WriteLine(first);

/*
    This code produces the following output:

    0
*/

' Create an empty array.
Dim numbers() As Integer = {}
' Get the first item in the array, or else the 
' default value for type int, which is 0.
Dim first As Integer = numbers.AsQueryable().FirstOrDefault()

MsgBox(first)

' This code produces the following output:

' 0

値もdefault(TSource)コレクションに要素が含まれていない場合は、使用する既定値はありません。Sometimes the value of default(TSource) is not the default value that you want to use if the collection contains no elements. 不要な既定値の結果をチェックし、必要に応じてを変更して、代わりに使用することができます、DefaultIfEmpty<TSource>(IQueryable<TSource>, TSource)コレクションが空の場合に使用する既定値を指定します。Instead of checking the result for the unwanted default value and then changing it if necessary, you can use the DefaultIfEmpty<TSource>(IQueryable<TSource>, TSource) method to specify the default value that you want to use if the collection is empty. 次に呼び出すFirst<TSource>(IQueryable<TSource>)最初の要素を取得します。Then, call First<TSource>(IQueryable<TSource>) to obtain the first element. 次のコード例では、両方の手法を使用して、月を表す数値のコレクションが空の場合は、1 の既定値を取得します。The following code example uses both techniques to obtain a default value of 1 if a collection of numeric months is empty. 整数の既定値は 0 で、任意の月に対応していない、ため、既定値必要があります 1 として代わりに指定します。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. クエリが完了した後、不要な既定値の最初の結果変数がチェックされます。The first result variable is checked for the unwanted default value after the query is completed. 2 番目の結果変数が呼び出すことによって取得DefaultIfEmpty<TSource>(IQueryable<TSource>, TSource)1 の既定値を指定します。The second result variable is obtained by calling DefaultIfEmpty<TSource>(IQueryable<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.AsQueryable().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.AsQueryable().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.AsQueryable().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.AsQueryable().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

注釈

FirstOrDefault<TSource>(IQueryable<TSource>)メソッドを生成、MethodCallExpression呼び出しを表すFirstOrDefault<TSource>(IQueryable<TSource>)として構築されたジェネリック メソッド自体。The FirstOrDefault<TSource>(IQueryable<TSource>) method generates a MethodCallExpression that represents calling FirstOrDefault<TSource>(IQueryable<TSource>) itself as a constructed generic method. これは、後、渡します、MethodCallExpressionExecute<TResult>(Expression)のメソッド、IQueryProviderによって表される、Providerのプロパティ、sourceパラメーター。It then passes the MethodCallExpression to the Execute<TResult>(Expression) method of the IQueryProvider represented by the Provider property of the source parameter.

呼び出し元を表す式ツリーを実行した結果として発生するクエリの動作FirstOrDefault<TSource>(IQueryable<TSource>)の型の実装によって異なります、sourceパラメーター。The query behavior that occurs as a result of executing an expression tree that represents calling FirstOrDefault<TSource>(IQueryable<TSource>) depends on the implementation of the type of the source parameter. 想定される動作では最初の要素を返しますsource、既定値の場合は値またはsourceが空です。The expected behavior is that it returns the first element in source, or a default value if source is empty.

FirstOrDefaultメソッドの場合に返す既定値を指定する方法は示しませんsourceが空です。The FirstOrDefault method does not provide a way to specify the default value to return if source is empty. 以外の既定値を指定したい場合default(TSource)を使用して、DefaultIfEmpty<TSource>(IQueryable<TSource>, TSource)メソッドの使用例」で説明します。If you want to specify a default value other than default(TSource), use the DefaultIfEmpty<TSource>(IQueryable<TSource>, TSource) method as described in the Example section.

FirstOrDefault<TSource>(IQueryable<TSource>, Expression<Func<TSource,Boolean>>) FirstOrDefault<TSource>(IQueryable<TSource>, Expression<Func<TSource,Boolean>>) FirstOrDefault<TSource>(IQueryable<TSource>, Expression<Func<TSource,Boolean>>) FirstOrDefault<TSource>(IQueryable<TSource>, Expression<Func<TSource,Boolean>>)

このような要素が見つからない場合に、指定した条件または既定値を満たす、シーケンスの最初の要素を返します。Returns the first element of a sequence that satisfies a specified condition or a default value if no such element is found.

public:
generic <typename TSource>
[System::Runtime::CompilerServices::Extension]
 static TSource FirstOrDefault(System::Linq::IQueryable<TSource> ^ source, System::Linq::Expressions::Expression<Func<TSource, bool> ^> ^ predicate);
public static TSource FirstOrDefault<TSource> (this System.Linq.IQueryable<TSource> source, System.Linq.Expressions.Expression<Func<TSource,bool>> predicate);
static member FirstOrDefault : System.Linq.IQueryable<'Source> * System.Linq.Expressions.Expression<Func<'Source, bool>> -> 'Source
<Extension()>
Public Function FirstOrDefault(Of TSource) (source As IQueryable(Of TSource), predicate As Expression(Of Func(Of TSource, Boolean))) As TSource

型パラメーター

TSource

source の要素の型。The type of the elements of source.

パラメーター

source
IQueryable<TSource>

IQueryable<T>から要素を返します。An IQueryable<T> to return an element from.

predicate
Expression<Func<TSource,Boolean>>

各要素が条件を満たしているかどうかをテストする関数。A function to test each element for a condition.

戻り値

TSource TSource TSource TSource

source が空の場合または predicate で指定されたテストに合格する要素がない場合は default(TSource)。それ以外の場合は、predicate で指定されたテストに合格する、source の最初の要素。default(TSource) if source is empty or if no element passes the test specified by predicate; otherwise, the first element in source that passes the test specified by predicate.

例外

source または predicatenull です。source or predicate is null.

次のコード例は、使用する方法を示しますFirstOrDefault<TSource>(IQueryable<TSource>, Expression<Func<TSource,Boolean>>)述語内で渡すことで。The following code example demonstrates how to use FirstOrDefault<TSource>(IQueryable<TSource>, Expression<Func<TSource,Boolean>>) by passing in a predicate. 2 番目のクエリでは、要素が存在しない条件を満たす、シーケンスにします。In the second query, there is no element in the sequence that satisfies the condition.

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

// Get the first string in the array that is longer
// than 20 characters, or the default value for type
// string (null) if none exists.
string firstLongName =
    names.AsQueryable().FirstOrDefault(name => name.Length > 20);

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

// Get the first string in the array that is longer
// than 30 characters, or the default value for type
// string (null) if none exists.
string firstVeryLongName =
    names.AsQueryable().FirstOrDefault(name => name.Length > 30);

Console.WriteLine(
    "There is {0} name that is 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 that is longer than 30 characters.
*/

Dim names() As String = {"Hartono, Tommy", "Adams, Terry", _
                     "Andersen, Henriette Thaulow", _
                     "Hedlund, Magnus", "Ito, Shu"}

' Get the first string in the array that is longer
' than 20 characters, or the default value for type
' string (null) if none exists.
Dim firstLongName As String = _
            names.AsQueryable().FirstOrDefault(Function(name) name.Length > 20)

MsgBox(String.Format("The first long name is '{0}'.", firstLongName))

' Get the first string in the array that is longer
' than 30 characters, or the default value for type
' string (null) if none exists.
Dim firstVeryLongName As String = _
    names.AsQueryable().FirstOrDefault(Function(name) name.Length > 30)

MsgBox(String.Format( _
    "There is {0} name that is longer than 30 characters.", _
    IIf(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 that is longer than 30 characters.

注釈

このメソッドは、少なくとも 1 つのパラメーター型のExpression<TDelegate>型引数は、のいずれか、Func<T,TResult>型。This method has at least one parameter of type Expression<TDelegate> whose type argument is one of the Func<T,TResult> types. ラムダ式で、これらのパラメーターを渡すことができますにコンパイルされると、Expression<TDelegate>します。For these parameters, you can pass in a lambda expression and it will be compiled to an Expression<TDelegate>.

FirstOrDefault<TSource>(IQueryable<TSource>, Expression<Func<TSource,Boolean>>)メソッドを生成、MethodCallExpression呼び出しを表すFirstOrDefault<TSource>(IQueryable<TSource>, Expression<Func<TSource,Boolean>>)として構築されたジェネリック メソッド自体。The FirstOrDefault<TSource>(IQueryable<TSource>, Expression<Func<TSource,Boolean>>) method generates a MethodCallExpression that represents calling FirstOrDefault<TSource>(IQueryable<TSource>, Expression<Func<TSource,Boolean>>) itself as a constructed generic method. これは、後、渡します、MethodCallExpressionExecute<TResult>(Expression)のメソッド、IQueryProviderによって表される、Providerのプロパティ、sourceパラメーター。It then passes the MethodCallExpression to the Execute<TResult>(Expression) method of the IQueryProvider represented by the Provider property of the source parameter.

呼び出し元を表す式ツリーを実行した結果として発生するクエリの動作FirstOrDefault<TSource>(IQueryable<TSource>, Expression<Func<TSource,Boolean>>)の型の実装によって異なります、sourceパラメーター。The query behavior that occurs as a result of executing an expression tree that represents calling FirstOrDefault<TSource>(IQueryable<TSource>, Expression<Func<TSource,Boolean>>) depends on the implementation of the type of the source parameter. 想定される動作は、最初の要素が返されるsourceの条件を満たすpredicate、または条件を満たす要素がない場合は、既定値。The expected behavior is that it returns the first element in source that satisfies the condition in predicate, or a default value if no element satisfies the condition.

適用対象