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

May 11, 2014

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)

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

Type Parameters

TSource

The type of the elements of source.

TResult

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

Parameters

source
Type: System.Collections.Generic.IEnumerable<TSource>
A sequence of values to project.
selector
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.

Usage Note

In Visual Basic and C#, you can call this method as an instance method on any object of type IEnumerable<TSource>. When you use instance method syntax to call this method, omit the first parameter.

ExceptionCondition
ArgumentNullException

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 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.

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.


      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.SelectMany(petOwner => petOwner.Pets);

         outputBlock.Text += "Using SelectMany():" + "\n";

         // Only one foreach loop is required to iterate 
         // through the results since it is a
         // one-dimensional collection.
         foreach (string pet in query1)
         {
            outputBlock.Text += pet + "\n";
         }

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

         outputBlock.Text += "\nUsing Select():" + "\n";

         // 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)
            {
               outputBlock.Text += pet + "\n";
            }
            outputBlock.Text += "\n";
         }
      }

      /*
       This code produces the following output:

       Using SelectMany():
       Scruffy
       Sam
       Walker
       Sugar
       Scratches
       Diesel

       Using Select():
       Scruffy
       Sam

       Walker
       Sugar

       Scratches
       Diesel
      */



Windows Phone OS

Supported in: 8.1, 8.0, 7.1, 7.0

Windows Phone

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