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Important This document may not represent best practices for current development, links to downloads and other resources may no longer be valid. Current recommended version can be found here.

How to: Return a Query from a Method (C# Programming Guide)

This example shows how to return a query from a method in the return value and as an out parameter.

Any query must have a type of IEnumerable or IEnumerable<T>, or a derived type such as IQueryable<T>. Therefore any return value or out parameter of a method that returns a query must also have that type. If a method materializes a query into a concrete List<T> or Array type, it is considered to be returning the query results instead of the query itself. A query variable returned from a method can still be composed or modified.

For an example of how to execute a query that is passed to a method, see the Object Dumper Sample.

In the following example, the first method returns a query as a return value, and the second method returns a query as an out parameter. Note that in both cases, it is a query and not query results that are being returned.

class MQ
{
    IEnumerable<string> QueryMethod1(ref int[] ints)
    {
        var intsToStrings = from i in ints
                            where i > 4
                            select i.ToString();
        return intsToStrings;
    }

    static void Main()
    {
        MQ app = new MQ();

        int[] nums = { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 };

        var myQuery = app.QueryMethod1(ref nums);


        //execute myQuery 
        foreach (string s in myQuery)
        {
            Console.WriteLine(s);
        }

        //modify myQuery
        myQuery = (from str in myQuery
                   orderby str descending 
                   select str).
                  Take(3);

        // Executing myQuery after more 
        // composition
        Console.WriteLine("After modification:");
        foreach (string s in myQuery)
        {
            Console.WriteLine(s);
        }

        // Keep console window open in debug mode.
        Console.WriteLine("Press any key to exit.");
        Console.ReadKey();
    }
}

  • Create a Visual Studio project that targets the .NET Framework version 3.5. By default, the project has a reference to System.Core.dll and a using directive for the System.Linq namespace.

  • Copy the code into your project.

  • Press F5 to compile and run the program.

  • Press any key to exit the console window.

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