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.
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: TSourceThe 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).| Exception | Condition |
|---|---|
| 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.
Windows 7, Windows Vista SP1 or later, Windows XP SP3, Windows Server 2008 (Server Core not supported), Windows Server 2008 R2 (Server Core supported with SP1 or later), Windows Server 2003 SP2
The .NET Framework does not support all versions of every platform. For a list of the supported versions, see .NET Framework System Requirements.