Queryable.SelectMany<TSource, TCollection, TResult> Method (IQueryable<TSource>, Expression<Func<TSource, IEnumerable<TCollection>>>, Expression<Func<TSource, TCollection, TResult>>)


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Projects each element of a sequence to an IEnumerable<T> and invokes a result selector function on each element therein. The resulting values from each intermediate sequence are combined into a single, one-dimensional sequence and returned.

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

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


Type: System.Linq.IQueryable<TSource>

A sequence of values to project.

Type: System.Linq.Expressions.Expression<Func<TSource, IEnumerable<TCollection>>>

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

Type: System.Linq.Expressions.Expression<Func<TSource, TCollection, TResult>>

A projection function to apply to each element of each intermediate sequence.

Return Value

Type: System.Linq.IQueryable<TResult>

An IQueryable<T> whose elements are the result of invoking the one-to-many projection 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 the function represented by collectionSelector.


The type of the elements of the resulting sequence.

Exception Condition

source or collectionSelector or resultSelector is null.

This method has at least one parameter of type Expression<TDelegate> whose type argument is one of the Func<T, TResult> types. For these parameters, you can pass in a lambda expression and it will be compiled to an Expression<TDelegate>.

The SelectMany<TSource, TCollection, TResult>(IQueryable<TSource>, Expression<Func<TSource, IEnumerable<TCollection>>>, Expression<Func<TSource, TCollection, TResult>>) method generates a MethodCallExpression that represents calling SelectMany<TSource, TCollection, TResult>(IQueryable<TSource>, Expression<Func<TSource, IEnumerable<TCollection>>>, Expression<Func<TSource, TCollection, TResult>>) itself as a constructed generic method. It then passes the MethodCallExpression to the CreateQuery(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 SelectMany<TSource, TCollection, TResult>(IQueryable<TSource>, Expression<Func<TSource, IEnumerable<TCollection>>>, Expression<Func<TSource, TCollection, TResult>>) depends on the implementation of the type of the source parameter. The expected behavior is that it invokes collectionSelector on each element of source to project it into an enumerable form. Then the function represented by resultSelector is invoked on each element in each intermediate sequence. The resulting values are concatenated into a single, one-dimensional sequence.

The following code example demonstrates how to use SelectMany<TSource, TCollection, TResult>(IQueryable<TSource>, Expression<Func<TSource, IEnumerable<TCollection>>>, Expression<Func<TSource, TCollection, TResult>>) to perform a one-to-many projection over an array. This example uses a result selector function to keep the source element that corresponds to each intermediate sequence in scope for the final call to Select.

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

class Pet
    public string Name { get; set; }
    public string Breed { get; set; }

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

    // This query demonstrates how to obtain a sequence of
    // the names of all the pets whose breed is "Collie", while
    // keeping an association with the owner that owns the pet.
    var query =
        // Create a sequence of ALL the Pet objects. Then
        // project an anonymous type that consists of each
        // Pet in the new sequence and the PetOwner object
        // from the initial array that corresponds to that pet.
       .SelectMany(owner => owner.Pets,
                   (owner, pet) => new { owner, pet })
        // Filter the sequence of anonymous types to only
        // keep pets whose breed is "Collie".
        .Where(ownerAndPet => ownerAndPet.pet.Breed == "Collie")
        // Project an anonymous type that consists
        // of the pet owner's name and the pet's name.
        .Select(ownerAndPet => new
            Owner = ownerAndPet.owner.Name,
            Pet = ownerAndPet.pet.Name

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

/* This code produces the following output:

    { Owner = Ashkenazi, Pet = Walker }
    { Owner = Price, Pet = Diesel }
    { Owner = Hines, Pet = Dusty }

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 7.1
Windows Phone
Available since 8.1
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