Enumerable.SelectMany<'TSource, 'TResult> Method (IEnumerable<'TSource>, Func<'TSource, IEnumerable<'TResult>>)


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


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.

Type Parameters


The type of the elements of source.


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

Exception Condition

source or selector is null.

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<'TSource, 'TResult> 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<'TSource, 'TResult>.

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.

Universal Windows Platform
Available since 8
.NET Framework
Available since 3.5
Portable Class Library
Supported in: portable .NET platforms
Available since 2.0
Windows Phone Silverlight
Available since 7.0
Windows Phone
Available since 8.1
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