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)

public static TSource Single<TSource>(
	this IQueryable<TSource> source

Type Parameters


The type of the elements of source.


Type: System.Linq.IQueryable<TSource>
An IQueryable<T> to return the single element of.

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.


source is null.


source has more than one element.

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.

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();

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

            // 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


Supported in: 5, 4, 3

Silverlight for Windows Phone

Supported in: Windows Phone OS 7.1

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