Article
11/18/2011

Queryable.Single<TSource> Method (IQueryable<TSource>, Expression<Func<TSource, Boolean>>)

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Returns the only element of a sequence that satisfies a specified condition, and throws an exception if more than one such element exists.

Namespace: System.Linq
Assembly: System.Core (in System.Core.dll)

Syntax

'Declaration
<ExtensionAttribute> _
Public Shared Function Single(Of TSource) ( _
    source As IQueryable(Of TSource), _
    predicate As Expression(Of Func(Of TSource, Boolean)) _
) As TSource

public static TSource Single<TSource>(
    this IQueryable<TSource> source,
    Expression<Func<TSource, bool>> predicate
)

Type Parameters

TSource
The type of the elements of source.

Parameters

source
Type: System.Linq.IQueryable<TSource>
An IQueryable<T> to return a single element from.

predicate
Type: System.Linq.Expressions.Expression<Func<TSource, Boolean>>
A function to test an element for a condition.

Return Value

Type: TSource
The single element of the input sequence that satisfies the condition in predicate.

Usage Note

In Visual Basic and C#, you can call this method as an instance method on any object of type IQueryable<TSource>. When you use instance method syntax to call this method, omit the first parameter.

Exceptions

Exception	Condition
ArgumentNullException	source or predicate is nulla null reference (Nothing in Visual Basic).
InvalidOperationException	No element satisfies the condition in predicate. -or- More than one element satisfies the condition in predicate. -or- The source sequence is empty.

ArgumentNullException

source or predicate is nulla null reference (Nothing in Visual Basic).

InvalidOperationException

No element satisfies the condition in predicate.

-or-

More than one element satisfies the condition in predicate.

-or-

The source sequence is empty.

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.

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.

      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.
      outputBlock.Text &= "First Query: " & result & vbCrLf

      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)
         outputBlock.Text &= "Second Query: " + fruit2 & vbCrLf
      Catch
         Dim text As String = "Second Query: The collection does not contain "
         text = text & "exactly one element whose length is greater than 15."
         outputBlock.Text &= text & vbCrLf
      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.

         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);

         outputBlock.Text += "First Query: " + fruit1 + "\n";

         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);
            outputBlock.Text += "Second Query: " + fruit2 + "\n";
         }
         catch (System.InvalidOperationException)
         {
            outputBlock.Text += "Second Query: The collection does not contain ";
            outputBlock.Text += "exactly one element whose length is greater than 15." + "\n";
         }

         /*
             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.
          */

Version Information

Silverlight

Supported in: 5, 4, 3

Silverlight for Windows Phone

Supported in: Windows Phone OS 7.1

Platforms

For a list of the operating systems and browsers that are supported by Silverlight, see Supported Operating Systems and Browsers.

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