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

Returns the only element of a sequence, or a default value if the sequence is empty; this method throws an exception if there is more than one element in the sequence.

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

public static TSource SingleOrDefault<TSource>(
	this IQueryable<TSource> source
)

Type Parameters

TSource

The type of the elements of source.

Parameters

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, or default(TSource) if the sequence contains no elements.

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. For more information, see Extension Methods (Visual Basic) or Extension Methods (C# Programming Guide).

ExceptionCondition
ArgumentNullException

source is null.

InvalidOperationException

source has more than one element.

The SingleOrDefault<TSource>(IQueryable<TSource>) method generates a MethodCallExpression that represents calling SingleOrDefault<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 SingleOrDefault<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, or a default value if source is empty.

The SingleOrDefault method does not provide a way to specify a default value. If you want to specify a default value other than default(TSource), use the DefaultIfEmpty<TSource>(IQueryable<TSource>, TSource) method as described in the Example section.

The following code example demonstrates how to use SingleOrDefault<TSource>(IQueryable<TSource>) to select the only element of an array. The second query demonstrates that SingleOrDefault<TSource>(IQueryable<TSource>) returns a default value when the sequence does not contain exactly one element.

            // Create two arrays. The second is empty. 
            string[] fruits1 = { "orange" };
            string[] fruits2 = { };

            // Get the only item in the first array, or else 
            // the default value for type string (null). 
            string fruit1 = fruits1.AsQueryable().SingleOrDefault();
            Console.WriteLine("First Query: " + fruit1);

            // Get the only item in the second array, or else 
            // the default value for type string (null).  
            string fruit2 = fruits2.AsQueryable().SingleOrDefault();
            Console.WriteLine("Second Query: " +
                (String.IsNullOrEmpty(fruit2) ? "No such string!" : fruit2));

            /*
                This code produces the following output:

                First Query: orange
                Second Query: No such string!
            */

Sometimes the value of default(TSource) is not the default value that you want to use if the collection contains no elements. Instead of checking the result for the unwanted default value and then changing it if necessary, you can use the DefaultIfEmpty<TSource>(IQueryable<TSource>, TSource) method to specify the default value that you want to use if the collection is empty. Then, call Single<TSource>(IQueryable<TSource>) to obtain the element. The following code example uses both techniques to obtain a default value of 1 if a collection of page numbers is empty. Because the default value for an integer is 0, which is not usually a valid page number, the default value must be specified as 1 instead. The first result variable is checked for the unwanted default value after the query is completed. The second result variable is obtained by calling DefaultIfEmpty<TSource>(IQueryable<TSource>, TSource) to specify a default value of 1.

            int[] pageNumbers = { };

            // Setting the default value to 1 after the query. 
            int pageNumber1 = pageNumbers.AsQueryable().SingleOrDefault();
            if (pageNumber1 == 0)
            {
                pageNumber1 = 1;
            }
            Console.WriteLine("The value of the pageNumber1 variable is {0}", pageNumber1);

            // Setting the default value to 1 by using DefaultIfEmpty() in the query. 
            int pageNumber2 = pageNumbers.AsQueryable().DefaultIfEmpty(1).Single();
            Console.WriteLine("The value of the pageNumber2 variable is {0}", pageNumber2);

            /*
             This code produces the following output:

             The value of the pageNumber1 variable is 1
             The value of the pageNumber2 variable is 1
            */

.NET Framework

Supported in: 4.5, 4, 3.5

.NET Framework Client Profile

Supported in: 4, 3.5 SP1

Portable Class Library

Supported in: Portable Class Library

.NET for Windows Store apps

Supported in: Windows 8

.NET for Windows Phone apps

Supported in: Windows Phone 8.1, Windows Phone Silverlight 8.1, Windows Phone Silverlight 8

Windows Phone 8.1, Windows Phone 8, Windows 8.1, Windows Server 2012 R2, Windows 8, Windows Server 2012, Windows 7, Windows Vista SP2, Windows Server 2008 (Server Core Role not supported), Windows Server 2008 R2 (Server Core Role supported with SP1 or later; Itanium not supported)

The .NET Framework does not support all versions of every platform. For a list of the supported versions, see .NET Framework System Requirements.

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