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

Projects each element of a sequence to an IEnumerable<T> and combines the resulting sequences into one sequence.

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

public static IQueryable<TResult> SelectMany<TSource, TResult>(
	this IQueryable<TSource> source,
	Expression<Func<TSource, IEnumerable<TResult>>> selector

Type Parameters


The type of the elements of source.


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


Type: System.Linq.IQueryable<TSource>
A sequence of values to project.
Type: System.Linq.Expressions.Expression<Func<TSource, IEnumerable<TResult>>>
A projection function to apply to each element.

Return Value

Type: System.Linq.IQueryable<TResult>
An IQueryable<T> whose elements are the result of invoking a one-to-many projection 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 IQueryable<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 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, TResult>(IQueryable<TSource>, Expression<Func<TSource, IEnumerable<TResult>>>) method generates a MethodCallExpression that represents calling SelectMany<TSource, TResult>(IQueryable<TSource>, Expression<Func<TSource, IEnumerable<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, TResult>(IQueryable<TSource>, Expression<Func<TSource, IEnumerable<TResult>>>) depends on the implementation of the type of the source parameter. The expected behavior is that it invokes selector on each element of source to project it into an enumerable form. It then concatenates the enumerable results into a single, one-dimensional sequence.

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

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

            public static void SelectManyEx1()
                PetOwner[] petOwners = 
                    { new PetOwner { Name="Higa, Sidney", 
                          Pets = new List<string>{ "Scruffy", "Sam" } },
                      new PetOwner { Name="Ashkenazi, Ronen", 
                          Pets = new List<string>{ "Walker", "Sugar" } },
                      new PetOwner { Name="Price, Vernette", 
                          Pets = new List<string>{ "Scratches", "Diesel" } } };

                // Query using SelectMany().
                IEnumerable<string> query1 =
                    petOwners.AsQueryable().SelectMany(petOwner => petOwner.Pets);

                Console.WriteLine("Using SelectMany():");

                // Only one foreach loop is required to iterate through the
                // results because it is a one-dimensional collection.
                foreach (string pet in query1)

                // This code shows how to use Select() instead of SelectMany().
                IEnumerable<List<String>> query2 =
                    petOwners.AsQueryable().Select(petOwner => petOwner.Pets);

                Console.WriteLine("\nUsing Select():");

                // Notice that two foreach loops are required to iterate through
                // the results because the query returns a collection of arrays.
                foreach (List<String> petList in query2)
                    foreach (string pet in petList)

                This code produces the following output:

                Using SelectMany():

                Using Select():



.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.