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.
Assembly: System.Core (in System.Core.dll)
The type of the elements of source.
The single element of the input sequence, or default(TSource) if the sequence contains no elements.
Usage NoteIn Visual Basic and C#, you can call this method as an instance method on any object of type IQueryable(). 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).
The method generates a MethodCallExpression that represents calling 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 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 to select the only element of an array. The second query demonstrates that returns a default value when the sequence does not contain exactly one element.
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.
.NET FrameworkSupported in: 4.5, 4, 3.5
.NET Framework Client ProfileSupported in: 4, 3.5 SP1
Portable Class LibrarySupported in: Portable Class Library
.NET for Windows Store appsSupported in: Windows 8
.NET for Windows Phone appsSupported 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.