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Enumerable.SelectMany(TSource, TResult) Method (IEnumerable(TSource), Func(TSource, IEnumerable(TResult)))

Projects each element of a sequence to an IEnumerable(T) and flattens the resulting sequences into one sequence.

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

static member SelectMany : 
        source:IEnumerable<'TSource> * 
        selector:Func<'TSource, IEnumerable<'TResult>> -> IEnumerable<'TResult> 

Type Parameters


The type of the elements of source.


The type of the elements of the sequence returned by selector.


Type: System.Collections.Generic.IEnumerable(TSource)
A sequence of values to project.
Type: System.Func(TSource, IEnumerable(TResult))
A transform function to apply to each element.

Return Value

Type: System.Collections.Generic.IEnumerable(TResult)
An IEnumerable(T) whose elements are the result of invoking the one-to-many transform function on each element of the input sequence.

Usage Note

In Visual Basic and C#, you can call this method as an instance method on any object of type IEnumerable(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 or selector is a null reference (Nothing in Visual Basic).

This method is implemented by using deferred execution. The immediate return value is an object that stores all the information that is required to perform the action. The query represented by this method is not executed until the object is enumerated either by calling its GetEnumerator method directly or by using foreach in Visual C# or For Each in Visual Basic.

The SelectMany(TSource, TResult)(IEnumerable(TSource), Func(TSource, IEnumerable(TResult))) method enumerates the input sequence, uses a transform function to map each element to an IEnumerable(T), and then enumerates and yields the elements of each such IEnumerable(T) object. That is, for each element of source, selector is invoked and a sequence of values is returned. SelectMany(TSource, TResult)(IEnumerable(TSource), Func(TSource, IEnumerable(TResult))) then flattens this two-dimensional collection of collections into a one-dimensional IEnumerable(T) and returns it. For example, if a query uses SelectMany(TSource, TResult)(IEnumerable(TSource), Func(TSource, IEnumerable(TResult))) to obtain the orders (of type Order) for each customer in a database, the result is of type IEnumerable<Order> in C# or IEnumerable(Of Order) in Visual Basic. If instead the query uses Select to obtain the orders, the collection of collections of orders is not combined and the result is of type IEnumerable<List<Order>> in C# or IEnumerable(Of List(Of Order)) in Visual Basic.

In query expression syntax, each from clause (Visual C#) or From clause (Visual Basic) after the initial one translates to an invocation of SelectMany.

The following code example demonstrates how to use SelectMany(TSource, TResult)(IEnumerable(TSource), Func(TSource, IEnumerable(TResult))) to perform a one-to-many projection over an array.

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

.NET Framework

Supported in: 4, 3.5

.NET Framework Client Profile

Supported in: 4, 3.5 SP1

Portable Class Library

Supported in: Portable Class Library

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.