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

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)

[ExtensionAttribute]
public:
generic<typename TSource>
static TSource SingleOrDefault(
	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. For more information, see Extension Methods (Visual Basic) or Extension Methods (C# Programming Guide).

ExceptionCondition
ArgumentNullException

source or predicate is nullptr.

InvalidOperationException

More than one element satisfies the condition in predicate.

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.

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.

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

.NET Framework

Supported in: 4.6, 4.5, 4, 3.5

.NET Framework Client Profile

Supported in: 4, 3.5 SP1

Portable Class Library

Supported in: Portable Class Library

Supported in: Windows Phone 8.1

Supported in: Windows Phone Silverlight 8.1

Supported in: Windows Phone Silverlight 8
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