Queryable.Single<TSource> Method (IQueryable<TSource>)

Returns the only element of a sequence, and throws an exception if there is not exactly one element in the sequence.

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

Syntax

'Declaration
<ExtensionAttribute> _
Public Shared Function Single(Of TSource) ( _
    source As IQueryable(Of TSource) _
) As TSource
public static TSource Single<TSource>(
    this IQueryable<TSource> source
)

Type Parameters

  • TSource
    The type of the elements of source.

Parameters

Return Value

Type: TSource
The single element of the input sequence.

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 is nulla null reference (Nothing in Visual Basic).

InvalidOperationException

source has more than 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.

Examples

The following code example demonstrates how to use Single<TSource>(IQueryable<TSource>) to select the only element of an array.

      ' 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.
      outputBlock.Text &= "First query: " & result & vbCrLf

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

      ' This code produces the following output:

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

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

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

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

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

         /*
             This code produces the following output:

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

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.