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.

Example

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();
 }
}

Compiling the Code

• 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.

See Also

Concepts

LINQ Query Expressions (C# Programming Guide)