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)

Syntax

'Declaration
<ExtensionAttribute> _
Public Shared Function SelectMany(Of TSource, TResult) ( _
    source As IQueryable(Of TSource), _
    selector As Expression(Of Func(Of TSource, IEnumerable(Of TResult))) _
) As IQueryable(Of TResult)
public static IQueryable<TResult> SelectMany<TSource, TResult>(
    this IQueryable<TSource> source,
    Expression<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 the function represented by selector.

Parameters

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.

Exceptions

Exception Condition
ArgumentNullException

source or selector is nulla null reference (Nothing in Visual Basic).

Remarks

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.

Examples

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.

   Structure PetOwner
      Public Name As String
      Public Pets() As String
   End Structure

   Shared Sub SelectManyEx1()
      Dim petOwners() As PetOwner = _
          {New PetOwner With _
           {.Name = "Higa, Sidney", .Pets = New String() {"Scruffy", "Sam"}}, _
           New PetOwner With _
           {.Name = "Ashkenazi, Ronen", .Pets = New String() {"Walker", "Sugar"}}, _
           New PetOwner With _
           {.Name = "Price, Vernette", .Pets = New String() {"Scratches", "Diesel"}}}

      ' Query using SelectMany().
      Dim query1 As IEnumerable(Of String) = _
                  petOwners.AsQueryable().SelectMany(Function(petOwner) petOwner.Pets)

      Dim output As New System.Text.StringBuilder("Using SelectMany():" & vbCrLf)
      ' Only one foreach loop is required to iterate through 
      ' the results because it is a one-dimensional collection.
      For Each pet As String In query1
         output.AppendLine(pet)
      Next

      ' This code shows how to use Select() instead of SelectMany().
      Dim query2 As IEnumerable(Of String()) = _
                  petOwners.AsQueryable().Select(Function(petOwner) petOwner.Pets)

      output.AppendLine(vbCrLf & "Using Select():")
      ' Notice that two foreach loops are required to iterate through 
      ' the results because the query returns a collection of arrays.
      For Each petArray() As String In query2
         For Each pet As String In petArray
            output.AppendLine(pet)
         Next
      Next

      ' Display the output.
      outputBlock.Text &= output.ToString() & vbCrLf
   End Sub

   ' This code produces the following output:

   ' Using SelectMany():
   ' Scruffy
   ' Sam
   ' Walker
   ' Sugar
   ' Scratches
   ' Diesel

   ' Using Select():
   ' Scruffy
   ' Sam
   ' Walker
   ' Sugar
   ' Scratches
   ' Diesel

         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);

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

            // Only one foreach loop is required to iterate through the
            // results because 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.AsQueryable().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
         */

Version Information

Silverlight

Supported in: 5, 4, 3

Silverlight for Windows Phone

Supported in: Windows Phone OS 7.1

Platforms

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