Articolo
11/18/2011

Queryable.SingleOrDefault<TSource> Method (IQueryable<TSource>, Expression<Func<TSource, Boolean>>)

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Returns the only element of a sequence that satisfies a specified condition or a default value if no such element exists; this method throws an exception if more than one element satisfies the condition.

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

Syntax

'Declaration
<ExtensionAttribute> _
Public Shared Function SingleOrDefault(Of TSource) ( _
    source As IQueryable(Of TSource), _
    predicate As Expression(Of Func(Of TSource, Boolean)) _
) As TSource

public static TSource SingleOrDefault<TSource>(
    this IQueryable<TSource> source,
    Expression<Func<TSource, bool>> predicate
)

Type Parameters

TSource
The type of the elements of source.

Parameters

source
Type: System.Linq.IQueryable<TSource>
An IQueryable<T> to return a single element from.

predicate
Type: System.Linq.Expressions.Expression<Func<TSource, Boolean>>
A function to test an element for a condition.

Return Value

Type: TSource
The single element of the input sequence that satisfies the condition in predicate, or default(TSource) if no such element is found.

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 or predicate is nulla null reference (Nothing in Visual Basic).

Remarks

This method has at least one parameter of type Expression<TDelegate> whose type argument is one of the Func<T, TResult> types. For these parameters, you can pass in a lambda expression and it will be compiled to an Expression<TDelegate>.

The SingleOrDefault<TSource>(IQueryable<TSource>, Expression<Func<TSource, Boolean>>) method generates a MethodCallExpression that represents calling SingleOrDefault<TSource>(IQueryable<TSource>, Expression<Func<TSource, Boolean>>) 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>, Expression<Func<TSource, Boolean>>) depends on the implementation of the type of the source parameter. The expected behavior is that it returns the only element in source that satisfies the condition specified by predicate, or a default value if no such element exists.

Examples

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

      Dim fruits() As String = _
          {"apple", "banana", "mango", "orange", "passionfruit", "grape"}

      ' Get the single string in the array whose length is greater
      ' than 10, or else the default value for type string (null).
      Dim fruit1 As String = _
          fruits.AsQueryable().SingleOrDefault(Function(fruit) fruit.Length > 10)
      ' Display the result.
      outputBlock.Text &= "First Query: " & fruit1 & vbCrLf

      ' Get the single string in the array whose length is greater
      ' than 15, or else the default value for type string (null).
      Dim fruit2 As String = _
          fruits.AsQueryable().SingleOrDefault(Function(fruit) fruit.Length > 15)
      outputBlock.Text &= "Second Query: " & _
          IIf(String.IsNullOrEmpty(fruit2), "No such string!", fruit2) & vbCrLf

      ' This code produces the following output:

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

         string[] fruits = { "apple", "banana", "mango", 
                                  "orange", "passionfruit", "grape" };

         // Get the single string in the array whose length is greater
         // than 10, or else the default value for type string (null).
         string fruit1 =
             fruits.AsQueryable().SingleOrDefault(fruit => fruit.Length > 10);
         outputBlock.Text += "First Query: " + fruit1 + "\n";

         // Get the single string in the array whose length is greater
         // than 15, or else the default value for type string (null).
         string fruit2 =
            fruits.AsQueryable().SingleOrDefault(fruit => fruit.Length > 15);
         outputBlock.Text += "Second Query: " +
             (String.IsNullOrEmpty(fruit2) ? "No such string!" : fruit2) + "\n";

         /*
             This code produces the following output:

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

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.

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