Queryable.SelectMany<TSource, TResult> Method (IQueryable<TSource>, Expression<Func<TSource, Int32, IEnumerable<TResult>>>)


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Projects each element of a sequence to an IEnumerable<T> and combines the resulting sequences into one sequence. The index of each source element is used in the projected form of that element.

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

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


Type: System.Linq.IQueryable<TSource>

A sequence of values to project.

Type: System.Linq.Expressions.Expression<Func<TSource, Int32, IEnumerable<TResult>>>

A projection function to apply to each element; the second parameter of this function represents the index of the source 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.

Type Parameters


The type of the elements of source.


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

Exception Condition

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, Int32, IEnumerable<TResult>>>) method generates a MethodCallExpression that represents calling SelectMany<TSource, TResult>(IQueryable<TSource>, Expression<Func<TSource, Int32, 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, Int32, 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. Each enumerable result incorporates the index of the source element. 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, Int32, IEnumerable<TResult>>>) to perform a one-to-many projection over an array and use the index of each source element.

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

public static void SelectManyEx2()
    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" } },
          new PetOwner { Name="Hines, Patrick", 
              Pets = new List<string>{ "Dusty" } } };

    // For each PetOwner element in the source array,
    // project a sequence of strings where each string
    // consists of the index of the PetOwner element in the
    // source array and the name of each pet in PetOwner.Pets.
    IEnumerable<string> query =
        (petOwner, index) => petOwner.Pets.Select(pet => index + pet)

    foreach (string pet in query)

// This code produces the following output:
// 0Scruffy
// 0Sam
// 1Walker
// 1Sugar
// 2Scratches
// 2Diesel
// 3Dusty

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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