Queryable.Single Method
Definition
Important
Some information relates to prerelease product that may be substantially modified before it’s released. Microsoft makes no warranties, express or implied, with respect to the information provided here.
Returns a single, specific element of a sequence.
Overloads
Single<TSource>(IQueryable<TSource>) |
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>>) |
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>)
- Source:
- Queryable.cs
- Source:
- Queryable.cs
- Source:
- Queryable.cs
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
Type Parameters
- TSource
The type of the elements of source
.
Parameters
- source
- IQueryable<TSource>
An IQueryable<T> to return the single element of.
Returns
The single element of the input sequence.
Exceptions
source
is null
.
Examples
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.
Remarks
The Single<TSource>(IQueryable<TSource>) method generates a MethodCallExpression that represents calling Single<TSource>(IQueryable<TSource>) itself as a constructed generic method. It then passes the MethodCallExpression to the Execute<TResult>(Expression) method of the IQueryProvider represented by the Provider property of the source
parameter.
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. The expected behavior is that it returns the only element in source
.
Applies to
Single<TSource>(IQueryable<TSource>, Expression<Func<TSource,Boolean>>)
- Source:
- Queryable.cs
- Source:
- Queryable.cs
- Source:
- Queryable.cs
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
Type Parameters
- TSource
The type of the elements of source
.
Parameters
- source
- IQueryable<TSource>
An IQueryable<T> to return a single element from.
- predicate
- Expression<Func<TSource,Boolean>>
A function to test an element for a condition.
Returns
The single element of the input sequence that satisfies the condition in predicate
.
Exceptions
source
or predicate
is null
.
No element satisfies the condition in predicate
.
-or-
More than one element satisfies the condition in predicate
.
-or-
The source sequence is empty.
Examples
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.
Remarks
This method has at least one parameter of type Expression<TDelegate> whose type argument is one of the Func<T,TResult> types. For these parameters, you can pass in a lambda expression and it will be compiled to an Expression<TDelegate>.
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. It then passes the MethodCallExpression to the Execute<TResult>(Expression) method of the IQueryProvider represented by the Provider property of the source
parameter.
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. The expected behavior is that it returns the only element in source
that satisfies the condition specified by predicate
.