Queryable.Single 方法

定义

返回序列中的单个特定元素。Returns a single, specific element of a sequence.

重载

Single<TSource>(IQueryable<TSource>)

返回序列的唯一元素;如果该序列并非恰好包含一个元素,则会引发异常。Returns the only element of a sequence, and throws an exception if there is not exactly one element in the sequence.

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

返回序列中满足指定条件的唯一元素;如果有多个这样的元素存在,则会引发异常。Returns the only element of a sequence that satisfies a specified condition, and throws an exception if more than one such element exists.

Single<TSource>(IQueryable<TSource>)

返回序列的唯一元素;如果该序列并非恰好包含一个元素,则会引发异常。Returns the only element of a sequence, and throws an exception if there is not exactly one element in the sequence.

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

输入序列的单个元素。The single element of the input sequence.

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 Single<TSource>(IQueryable<TSource>) 选择数组的唯一元素。The following code example demonstrates how to use Single<TSource>(IQueryable<TSource>) to select the only element of an array.

// Create two arrays.
string[] fruits1 = { "orange" };
string[] fruits2 = { "orange", "apple" };

// Get the only item in the first array.
string fruit1 = fruits1.AsQueryable().Single();

Console.WriteLine("First query: " + fruit1);

try
{
    // Try to get the only item in the second array.
    string fruit2 = fruits2.AsQueryable().Single();
    Console.WriteLine("Second query: " + fruit2);
}
catch (System.InvalidOperationException)
{
    Console.WriteLine(
        "Second query: The collection does not contain exactly one element."
        );
}

/*
    This code produces the following output:

    First query: orange
    Second query: The collection does not contain exactly one element
*/

' Create two arrays.
Dim fruits1() As String = {"orange"}
Dim fruits2() As String = {"orange", "apple"}

' Get the only item in the first array.
Dim result As String = fruits1.AsQueryable().Single()

' Display the result.
MsgBox("First query: " & result)

Try
    ' Try to get the only item in the second array.
    Dim fruit2 As String = fruits2.AsQueryable().Single()
    MsgBox("Second query: " + fruit2)
Catch
    MsgBox("Second query: The collection does not contain exactly one element.")
End Try

' This code produces the following output:

' First query: orange
' Second query: The collection does not contain exactly one element.

注解

Single<TSource>(IQueryable<TSource>) 方法生成一个 MethodCallExpression,它表示作为构造的泛型方法调用 Single<TSource>(IQueryable<TSource>) 本身。The Single<TSource>(IQueryable<TSource>) method generates a MethodCallExpression that represents calling Single<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 Single<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 Single<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。The expected behavior is that it returns the only element in source.

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

返回序列中满足指定条件的唯一元素;如果有多个这样的元素存在,则会引发异常。Returns the only element of a sequence that satisfies a specified condition, and throws an exception if more than one such element exists.

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

满足 predicate 中条件的输入序列中的单个元素。The single element of the input sequence that satisfies the condition in predicate.

例外

sourcepredicatenullsource or predicate is null.

元素均不满足 predicate 中的条件。No element satisfies the condition in predicate.

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

-or- 源序列为空。The source sequence is empty.

示例

下面的代码示例演示如何使用 Single<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 Single<TSource>(IQueryable<TSource>, Expression<Func<TSource,Boolean>>) to select the only element of an array that satisfies a condition.

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

// Get the only string in the array whose length is greater than 10.
string fruit1 = fruits.AsQueryable().Single(fruit => fruit.Length > 10);

Console.WriteLine("First Query: " + fruit1);

try
{
    // Try to get the only string in the array
    // whose length is greater than 15.
    string fruit2 = fruits.AsQueryable().Single(fruit => fruit.Length > 15);
    Console.WriteLine("Second Query: " + fruit2);
}
catch (System.InvalidOperationException)
{
    Console.Write("Second Query: The collection does not contain ");
    Console.WriteLine("exactly one element whose length is greater than 15.");
}

/*
    This code produces the following output:

    First Query: passionfruit
    Second Query: The collection does not contain exactly one 
    element whose length is greater than 15.
 */

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

' Get the only string in the array whose length is greater than 10.
Dim result As String = _
    fruits.AsQueryable().Single(Function(fruit) fruit.Length > 10)

' Display the result.
MsgBox("First Query: " & result)

Try
    ' Try to get the only string in the array
    ' whose length is greater than 15.
    Dim fruit2 As String = fruits.AsQueryable().Single(Function(fruit) fruit.Length > 15)
    MsgBox("Second Query: " + fruit2)
Catch
    Dim text As String = "Second Query: The collection does not contain "
    text = text & "exactly one element whose length is greater than 15."
    MsgBox(text)
End Try

' This code produces the following output:

' First Query: passionfruit
' Second Query: The collection does not contain exactly one 
'   element whose length is greater than 15.

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>.

Single<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 Single<TSource>(IQueryable<TSource>, Expression<Func<TSource,Boolean>>) 本身。The Single<TSource>(IQueryable<TSource>, Expression<Func<TSource,Boolean>>) method generates a MethodCallExpression that represents calling Single<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 Single<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 Single<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。The expected behavior is that it returns the only element in source that satisfies the condition specified by predicate.

适用于