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


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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)

static member SingleOrDefault<'TSource> : 
        source:IQueryable<'TSource> -> 'TSource


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.

Type Parameters


The type of the elements of source.

Exception Condition

source is null.


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.

No code example is currently available or this language may not be supported.

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.

No code example is currently available or this language may not be supported.

Universal Windows Platform
Available since 8
.NET Framework
Available since 3.5
Portable Class Library
Supported in: portable .NET platforms
Available since 2.0
Windows Phone Silverlight
Available since 7.1
Windows Phone
Available since 8.1
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