DefaultIfEmpty(TSource) Method (IQueryable(TSource))

Queryable.DefaultIfEmpty<TSource> Method (IQueryable<TSource>)

Returns the elements of the specified sequence or the type parameter's default value in a singleton collection if the sequence is empty.

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

public static IQueryable<TSource> DefaultIfEmpty<TSource>(
	this IQueryable<TSource> source

Type Parameters


The type of the elements of source.


Type: System.Linq.IQueryable<TSource>

The IQueryable<T> to return a default value for if empty.

Return Value

Type: System.Linq.IQueryable<TSource>
An IQueryable<T> that contains default(TSource) if source is empty; otherwise, source.

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).


source is null.

The DefaultIfEmpty<TSource>(IQueryable<TSource>) method generates a MethodCallExpression that represents calling DefaultIfEmpty<TSource>(IQueryable<TSource>) itself as a constructed generic method. It then passes the MethodCallExpression to the CreateQuery<TElement>(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 DefaultIfEmpty<TSource>(IQueryable<TSource>) depends on the implementation of the type of the source parameter. The expected behavior is that it returns source if it is not empty. Otherwise, it returns an IQueryable<T> that contains default(TSource).

The following code examples demonstrate how to use DefaultIfEmpty<TSource>(IQueryable<TSource>) to provide a default value in case the source sequence is empty.

class Pet
    public string Name { get; set; }
    public int Age { get; set; }

public static void DefaultIfEmptyEx1()
    // Create a list of Pet objects.
    List<Pet> pets =
        new List<Pet>{ new Pet { Name="Barley", Age=8 },
                       new Pet { Name="Boots", Age=4 },
                       new Pet { Name="Whiskers", Age=1 } };

    // Call DefaultIfEmtpy() on the collection that Select() 
    // returns, so that if the initial list is empty, there 
    // will always be at least one item in the returned array.
    string[] names =
        .Select(pet => pet.Name)

    string first = names[0];

    This code produces the following output:


Windows 7, Windows Vista, Windows XP SP2, Windows Server 2008 R2, Windows Server 2008, Windows Server 2003

The .NET Framework and .NET Compact Framework do not support all versions of every platform. For a list of the supported versions, see .NET Framework System Requirements.

.NET Framework

Supported in: 3.5

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