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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)

Syntax

'Declaration
<ExtensionAttribute> _
Public Shared Function SingleOrDefault(Of TSource) ( _
    source As IQueryable(Of TSource) _
) As TSource

public static TSource SingleOrDefault<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, 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.

Exceptions

Exception Condition
ArgumentNullException

source is nulla null reference (Nothing in Visual Basic).

Remarks

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.

Examples

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.
      Dim fruits1() As String = {"orange"}
      Dim fruits2() As String = {}

      ' Get the only item in the first array, or else
      ' the default value for type string (null).
      Dim fruit1 As String = fruits1.AsQueryable().SingleOrDefault()
      outputBlock.Text &= "First Query: " + fruit1 & vbCrLf

      ' Get the only item in the second array, or else
      ' the default value for type string (null). 
      Dim fruit2 As String = fruits2.AsQueryable().SingleOrDefault()
      outputBlock.Text &= "Second Query: " & _
          IIf(String.IsNullOrEmpty(fruit2), "No such string!", fruit2) & vbCrLf

      ' This code produces the following output:

      ' First Query: orange
      ' Second Query: No such string!


         // 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();
         outputBlock.Text += "First Query: " + fruit1 + "\n";

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

         /*
             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.

      Dim pageNumbers() As Integer = {}

      ' Setting the default value to 1 after the query.
      Dim pageNumber1 As Integer = pageNumbers.AsQueryable().SingleOrDefault()
      If pageNumber1 = 0 Then
         pageNumber1 = 1
      End If
      outputBlock.Text &= String.Format("The value of the pageNumber1 variable is {0}", _
                                        pageNumber1) & vbCrLf

      ' Setting the default value to 1 by using DefaultIfEmpty() in the query.
      Dim pageNumber2 As Integer = pageNumbers.AsQueryable().DefaultIfEmpty(1).Single()
      outputBlock.Text &= String.Format("The value of the pageNumber2 variable is {0}", _
                                        pageNumber2) & vbCrLf

      ' This code produces the following output:

      ' The value of the pageNumber1 variable is 1
      ' The value of the pageNumber2 variable is 1


         int[] pageNumbers = { };

         // Setting the default value to 1 after the query.
         int pageNumber1 = pageNumbers.AsQueryable().SingleOrDefault();
         if (pageNumber1 == 0)
         {
            pageNumber1 = 1;
         }
         outputBlock.Text += String.Format("The value of the pageNumber1 variable is {0}", pageNumber1) + "\n";

         // Setting the default value to 1 by using DefaultIfEmpty() in the query.
         int pageNumber2 = pageNumbers.AsQueryable().DefaultIfEmpty(1).Single();
         outputBlock.Text += String.Format("The value of the pageNumber2 variable is {0}", pageNumber2) + "\n";

         /*
          This code produces the following output:

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

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.