Queryable.FirstOrDefault(TSource) Method (IQueryable(TSource))
Returns the first element of a sequence, or a default value if the sequence contains no elements.
Assembly: System.Core (in System.Core.dll)
The type of the elements of source.
default(TSource) if source is empty; otherwise, the first element in source.
Usage NoteIn Visual Basic and C#, you can call this method as an instance method on any object of type IQueryable(). 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).
source is a null reference (Nothing in Visual Basic).
The method generates a MethodCallExpression that represents calling 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 depends on the implementation of the type of the source parameter. The expected behavior is that it returns the first element in source, or a default value if source is empty.
The FirstOrDefault method does not provide a way to specify the default value to return if source is empty. 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 on an empty sequence.
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 First(TSource)(IQueryable(TSource)) to obtain the first element. The following code example uses both techniques to obtain a default value of 1 if a collection of numeric months is empty. Because the default value for an integer is 0, which does not correspond to any month, 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.
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.