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.