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

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 Parameters

TSource

The type of the elements of source.

TCollection

The type of the intermediate elements collected by the function represented by collectionSelector.

TResult

The type of the elements of the resulting sequence.

Parameters

source
Type: System.Linq.IQueryable<TSource>
A sequence of values to project.
collectionSelector
Type: System.Linq.Expressions.Expression<Func<TSource, IEnumerable<TCollection>>>
A projection function to apply to each element of the input sequence.
resultSelector
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.

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.

ExceptionCondition
ArgumentNullException

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 =
                petOwners.AsQueryable()
               // 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)
               outputBlock.Text += obj + "\n";
         }

         /* This code produces the following output:

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



Silverlight

Supported in: 5, 4, 3

Silverlight for Windows Phone

Supported in: Windows Phone OS 7.1

For a list of the operating systems and browsers that are supported by Silverlight, see Supported Operating Systems and Browsers.

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