Queryable.SingleOrDefault 方法

定义

返回序列中单个特定元素,如果未找到这样的元素,则返回默认值。Returns a single, specific element of a sequence, or a default value if no such element is found.

重载

SingleOrDefault<TSource>(IQueryable<TSource>)

返回序列中的唯一元素;如果该序列为空,则返回默认值;如果该序列包含多个元素,此方法将引发异常。Returns the only element of a sequence, or a default value if the sequence is empty; this method throws an exception if there is more than one element in the sequence.

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

返回序列中满足指定条件的唯一元素;如果这类元素不存在,则返回默认值;如果有多个元素满足该条件,此方法将引发异常。Returns the only element of a sequence that satisfies a specified condition or a default value if no such element exists; this method throws an exception if more than one element satisfies the condition.

SingleOrDefault<TSource>(IQueryable<TSource>)

返回序列中的唯一元素;如果该序列为空,则返回默认值;如果该序列包含多个元素,此方法将引发异常。Returns the only element of a sequence, or a default value if the sequence is empty; this method throws an exception if there is more than one element in the sequence.

public:
generic <typename TSource>
[System::Runtime::CompilerServices::Extension]
 static TSource SingleOrDefault(System::Linq::IQueryable<TSource> ^ source);
public static TSource SingleOrDefault<TSource> (this System.Linq.IQueryable<TSource> source);
static member SingleOrDefault : System.Linq.IQueryable<'Source> -> 'Source
<Extension()>
Public Function SingleOrDefault(Of TSource) (source As IQueryable(Of TSource)) As TSource

类型参数

TSource

source 的元素类型。The type of the elements of source.

参数

source
IQueryable<TSource>

要返回其单个元素的 IQueryable<T>An IQueryable<T> to return the single element of.

返回

TSource

输入序列中的单个元素;如果序列不包含任何元素,则为 default(TSource)。The single element of the input sequence, or default(TSource) if the sequence contains no elements.

例外

sourcenullsource is null.

source 具有多个元素。source has more than one element.

示例

下面的代码示例演示如何使用 SingleOrDefault<TSource>(IQueryable<TSource>) 选择数组的唯一元素。The following code example demonstrates how to use SingleOrDefault<TSource>(IQueryable<TSource>) to select the only element of an array. 第二个查询演示当序列不只包含一个元素时,SingleOrDefault<TSource>(IQueryable<TSource>) 返回默认值。The second query demonstrates that SingleOrDefault<TSource>(IQueryable<TSource>) returns a default value when the sequence does not contain exactly one element.

// Create two arrays. The second is empty.
string[] fruits1 = { "orange" };
string[] fruits2 = { };

// Get the only item in the first array, or else
// the default value for type string (null).
string fruit1 = fruits1.AsQueryable().SingleOrDefault();
Console.WriteLine("First Query: " + fruit1);

// Get the only item in the second array, or else
// the default value for type string (null). 
string fruit2 = fruits2.AsQueryable().SingleOrDefault();
Console.WriteLine("Second Query: " +
    (String.IsNullOrEmpty(fruit2) ? "No such string!" : fruit2));

/*
    This code produces the following output:

    First Query: orange
    Second Query: No such string!
*/

' Create two arrays. The second is empty.
Dim fruits1() As String = {"orange"}
Dim fruits2() As String = {}

' Get the only item in the first array, or else
' the default value for type string (null).
Dim fruit1 As String = fruits1.AsQueryable().SingleOrDefault()
MsgBox("First Query: " + fruit1)

' Get the only item in the second array, or else
' the default value for type string (null). 
Dim fruit2 As String = fruits2.AsQueryable().SingleOrDefault()
MsgBox("Second Query: " & _
    IIf(String.IsNullOrEmpty(fruit2), "No such string!", fruit2))

' This code produces the following output:

' First Query: orange
' Second Query: No such string!

有时,如果集合不包含任何元素,则 default(TSource) 的值不是要使用的默认值。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>(IQueryable<TSource>, TSource) 方法来指定要在集合为空时使用的默认值,而不是检查不需要的默认值的结果,然后根据需要进行更改。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. 然后,调用 Single<TSource>(IQueryable<TSource>) 获取元素。Then, call Single<TSource>(IQueryable<TSource>) to obtain the element. 下面的代码示例使用这两种方法来获取默认值1(如果页码的集合为空)。The following code example uses both techniques to obtain a default value of 1 if a collection of page numbers is empty. 由于整数的默认值为0(这通常不是有效的页码),因此必须将默认值指定为1。Because the default value for an integer is 0, which is not usually a valid page number, the default value must be specified as 1 instead. 查询完成后,将检查第一个结果变量中是否有不需要的默认值。The first result variable is checked for the unwanted default value after the query is completed. 第二个结果变量是通过调用 DefaultIfEmpty<TSource>(IQueryable<TSource>, TSource) 指定默认值1获取的。The second result variable is obtained by calling DefaultIfEmpty<TSource>(IQueryable<TSource>, TSource) to specify a default value of 1.

int[] pageNumbers = { };

// Setting the default value to 1 after the query.
int pageNumber1 = pageNumbers.AsQueryable().SingleOrDefault();
if (pageNumber1 == 0)
{
    pageNumber1 = 1;
}
Console.WriteLine("The value of the pageNumber1 variable is {0}", pageNumber1);

// Setting the default value to 1 by using DefaultIfEmpty() in the query.
int pageNumber2 = pageNumbers.AsQueryable().DefaultIfEmpty(1).Single();
Console.WriteLine("The value of the pageNumber2 variable is {0}", pageNumber2);

/*
 This code produces the following output:

 The value of the pageNumber1 variable is 1
 The value of the pageNumber2 variable is 1
*/

Dim pageNumbers() As Integer = {}

' Setting the default value to 1 after the query.
Dim pageNumber1 As Integer = pageNumbers.AsQueryable().SingleOrDefault()
If pageNumber1 = 0 Then
    pageNumber1 = 1
End If
MsgBox(String.Format("The value of the pageNumber1 variable is {0}", pageNumber1))

' Setting the default value to 1 by using DefaultIfEmpty() in the query.
Dim pageNumber2 As Integer = pageNumbers.AsQueryable().DefaultIfEmpty(1).Single()
MsgBox(String.Format("The value of the pageNumber2 variable is {0}", pageNumber2))

' This code produces the following output:

' The value of the pageNumber1 variable is 1
' The value of the pageNumber2 variable is 1

注解

SingleOrDefault<TSource>(IQueryable<TSource>) 方法生成一个 MethodCallExpression,它表示作为构造的泛型方法调用 SingleOrDefault<TSource>(IQueryable<TSource>) 本身。The SingleOrDefault<TSource>(IQueryable<TSource>) method generates a MethodCallExpression that represents calling SingleOrDefault<TSource>(IQueryable<TSource>) itself as a constructed generic method. 然后,它将 MethodCallExpression 传递到 IQueryProviderExecute<TResult>(Expression) 方法(由 source 参数的 Provider 属性表示)。It then passes the MethodCallExpression to the Execute<TResult>(Expression) method of the IQueryProvider represented by the Provider property of the source parameter.

因执行表示调用 SingleOrDefault<TSource>(IQueryable<TSource>) 的表达式树而发生的查询行为取决于 source 参数类型的实现。The query behavior that occurs as a result of executing an expression tree that represents calling SingleOrDefault<TSource>(IQueryable<TSource>) depends on the implementation of the type of the source parameter. 预期的行为是返回 source中的唯一元素; 如果 source 为空,则返回默认值。The expected behavior is that it returns the only element in source, or a default value if source is empty.

SingleOrDefault 方法不提供指定默认值的方法。The SingleOrDefault method does not provide a way to specify a default value. 如果要指定 default(TSource)以外的默认值,请使用 "示例" 部分中所述的 DefaultIfEmpty<TSource>(IQueryable<TSource>, TSource) 方法。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.

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

返回序列中满足指定条件的唯一元素;如果这类元素不存在,则返回默认值;如果有多个元素满足该条件,此方法将引发异常。Returns the only element of a sequence that satisfies a specified condition or a default value if no such element exists; this method throws an exception if more than one element satisfies the condition.

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

类型参数

TSource

source 的元素类型。The type of the elements of source.

参数

source
IQueryable<TSource>

要从中返回单个元素的 IQueryable<T>An IQueryable<T> to return a single element from.

predicate
Expression<Func<TSource,Boolean>>

用于测试元素是否满足条件的函数。A function to test an element for a condition.

返回

TSource

输入序列中满足 predicate 中条件的单个元素;如果未找到这类元素,则为 default(TSource)。The single element of the input sequence that satisfies the condition in predicate, or default(TSource) if no such element is found.

例外

sourcepredicatenullsource or predicate is null.

多个元素满足 predicate 中的条件。More than one element satisfies the condition in predicate.

示例

下面的代码示例演示如何使用 SingleOrDefault<TSource>(IQueryable<TSource>, Expression<Func<TSource,Boolean>>) 来选择满足某个条件的数组的唯一元素。The following code example demonstrates how to use SingleOrDefault<TSource>(IQueryable<TSource>, Expression<Func<TSource,Boolean>>) to select the only element of an array that satisfies a condition. 第二个查询演示当序列不只包含一个满足条件的元素时,SingleOrDefault<TSource>(IQueryable<TSource>, Expression<Func<TSource,Boolean>>) 返回默认值。The second query demonstrates that SingleOrDefault<TSource>(IQueryable<TSource>, Expression<Func<TSource,Boolean>>) returns a default value when the sequence does not contain exactly one element that satisfies the condition.

string[] fruits = { "apple", "banana", "mango", 
                      "orange", "passionfruit", "grape" };

// Get the single string in the array whose length is greater
// than 10, or else the default value for type string (null).
string fruit1 =
    fruits.AsQueryable().SingleOrDefault(fruit => fruit.Length > 10);
Console.WriteLine("First Query: " + fruit1);

// Get the single string in the array whose length is greater
// than 15, or else the default value for type string (null).
string fruit2 =
   fruits.AsQueryable().SingleOrDefault(fruit => fruit.Length > 15);
Console.WriteLine("Second Query: " +
    (String.IsNullOrEmpty(fruit2) ? "No such string!" : fruit2));

/*
    This code produces the following output:

    First Query: passionfruit
    Second Query: No such string!
*/

Dim fruits() As String = _
    {"apple", "banana", "mango", "orange", "passionfruit", "grape"}

' Get the single string in the array whose length is greater
' than 10, or else the default value for type string (null).
Dim fruit1 As String = _
    fruits.AsQueryable().SingleOrDefault(Function(fruit) fruit.Length > 10)
' Display the result.
MsgBox("First Query: " & fruit1)

' Get the single string in the array whose length is greater
' than 15, or else the default value for type string (null).
Dim fruit2 As String = _
    fruits.AsQueryable().SingleOrDefault(Function(fruit) fruit.Length > 15)
MsgBox("Second Query: " & _
    IIf(String.IsNullOrEmpty(fruit2), "No such string!", fruit2))

' This code produces the following output:

' First Query: passionfruit
' Second Query: No such string!

注解

此方法至少具有一个类型 Expression<TDelegate> 的参数,其类型参数为 Func<T,TResult> 类型之一。This method has at least one parameter of type Expression<TDelegate> whose type argument is one of the Func<T,TResult> types. 对于这些参数,可以传入 lambda 表达式,并将其编译为 Expression<TDelegate>For these parameters, you can pass in a lambda expression and it will be compiled to an Expression<TDelegate>.

SingleOrDefault<TSource>(IQueryable<TSource>, Expression<Func<TSource,Boolean>>) 方法生成一个 MethodCallExpression,它表示作为构造的泛型方法调用 SingleOrDefault<TSource>(IQueryable<TSource>, Expression<Func<TSource,Boolean>>) 本身。The SingleOrDefault<TSource>(IQueryable<TSource>, Expression<Func<TSource,Boolean>>) method generates a MethodCallExpression that represents calling SingleOrDefault<TSource>(IQueryable<TSource>, Expression<Func<TSource,Boolean>>) itself as a constructed generic method. 然后,它将 MethodCallExpression 传递到 IQueryProviderExecute<TResult>(Expression) 方法(由 source 参数的 Provider 属性表示)。It then passes the MethodCallExpression to the Execute<TResult>(Expression) method of the IQueryProvider represented by the Provider property of the source parameter.

因执行表示调用 SingleOrDefault<TSource>(IQueryable<TSource>, Expression<Func<TSource,Boolean>>) 的表达式树而发生的查询行为取决于 source 参数类型的实现。The query behavior that occurs as a result of executing an expression tree that represents calling SingleOrDefault<TSource>(IQueryable<TSource>, Expression<Func<TSource,Boolean>>) depends on the implementation of the type of the source parameter. 预期的行为是返回 source 中满足 predicate指定的条件的唯一元素; 如果此类元素不存在,则返回默认值。The expected behavior is that it returns the only element in source that satisfies the condition specified by predicate, or a default value if no such element exists.

适用于