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


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Projects each element of a sequence to an IEnumerable<T>, flattens the resulting sequences into one sequence, and invokes a result selector function on each element therein.

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

public static IEnumerable<TResult> SelectMany<TSource, TCollection, TResult>(
	this IEnumerable<TSource> source,
	Func<TSource, IEnumerable<TCollection>> collectionSelector,
	Func<TSource, TCollection, TResult> resultSelector


Type: System.Collections.Generic.IEnumerable<TSource>

A sequence of values to project.

Type: System.Func<TSource, IEnumerable<TCollection>>

A transform function to apply to each element of the input sequence.

Type: System.Func<TSource, TCollection, TResult>

A transform function to apply to each element of the intermediate sequence.

Return Value

Type: System.Collections.Generic.IEnumerable<TResult>

An IEnumerable<T> whose elements are the result of invoking the one-to-many transform function collectionSelector on each element of source and then mapping each of those sequence elements and their corresponding source element to a result element.

Type Parameters


The type of the elements of source.


The type of the intermediate elements collected by collectionSelector.


The type of the elements of the resulting sequence.

Exception Condition

source or collectionSelector or resultSelector 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, TCollection, TResult>(IEnumerable<TSource>, Func<TSource, IEnumerable<TCollection>>, Func<TSource, TCollection, TResult>) method is useful when you have to keep the elements of source in scope for query logic that occurs after the call to SelectMany<TSource, TCollection, TResult>(IEnumerable<TSource>, Func<TSource, IEnumerable<TCollection>>, Func<TSource, TCollection, TResult>). See the Example section for a code example. If there is a bidirectional relationship between objects of type TSource and objects of type TCollection, that is, if an object of type TCollection provides a property to retrieve the TSource object that produced it, you do not need this overload of SelectMany<TSource, TCollection, TResult>(IEnumerable<TSource>, Func<TSource, IEnumerable<TCollection>>, Func<TSource, TCollection, TResult>). Instead, you can use SelectMany<TSource, TResult>(IEnumerable<TSource>, Func<TSource, IEnumerable<TResult>>) and navigate back to the TSource object through the TCollection object.

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, TCollection, TResult>(IEnumerable<TSource>, Func<TSource, IEnumerable<TCollection>>, Func<TSource, TCollection, TResult>) to perform a one-to-many projection over an array and use a result selector function to keep each corresponding element from the source sequence in scope for the final call to Select.

class PetOwner
    public string Name { get; set; }
    public List<string> Pets { get; set; }

public static void SelectManyEx3()
    PetOwner[] petOwners =
        { new PetOwner { Name="Higa", 
              Pets = new List<string>{ "Scruffy", "Sam" } },
          new PetOwner { Name="Ashkenazi", 
              Pets = new List<string>{ "Walker", "Sugar" } },
          new PetOwner { Name="Price", 
              Pets = new List<string>{ "Scratches", "Diesel" } },
          new PetOwner { Name="Hines", 
              Pets = new List<string>{ "Dusty" } } };

    // Project the pet owner's name and the pet's name.
    var query =
        .SelectMany(petOwner => petOwner.Pets, (petOwner, petName) => new { petOwner, petName })
        .Where(ownerAndPet => ownerAndPet.petName.StartsWith("S"))
        .Select(ownerAndPet =>
                    Owner = ownerAndPet.petOwner.Name,
                    Pet = ownerAndPet.petName

    // Print the results.
    foreach (var obj in query)

// This code produces the following output:
// {Owner=Higa, Pet=Scruffy}
// {Owner=Higa, Pet=Sam}
// {Owner=Ashkenazi, Pet=Sugar}
// {Owner=Price, Pet=Scratches}

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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