How to: Return a Query from a Method (C# Programming Guide)
This example shows how to return a query from a method as 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 that is returned from a method can still be composed or modified.
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 that is returned, not query results.
class MQ
{
// QueryMethhod1 returns a query as its value.
IEnumerable<string> QueryMethod1(ref int[] ints)
{
var intsToStrings = from i in ints
where i > 4
select i.ToString();
return intsToStrings;
}
// QueryMethod2 returns a query as the value of parameter returnQ.
void QueryMethod2(ref int[] ints, out IEnumerable<string> returnQ)
{
var intsToStrings = from i in ints
where i < 4
select i.ToString();
returnQ = intsToStrings;
}
static void Main()
{
MQ app = new MQ();
int[] nums = { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 };
// QueryMethod1 returns a query as the value of the method.
var myQuery1 = app.QueryMethod1(ref nums);
// Query myQuery1 is executed in the following foreach loop.
Console.WriteLine("Results of executing myQuery1:");
// Rest the mouse pointer over myQuery1 to see its type.
foreach (string s in myQuery1)
{
Console.WriteLine(s);
}
// You also can execute the query returned from QueryMethod1
// directly, without using myQuery1.
Console.WriteLine("\nResults of executing myQuery1 directly:");
// Rest the mouse pointer over the call to QueryMethod1 to see its
// return type.
foreach (string s in app.QueryMethod1(ref nums))
{
Console.WriteLine(s);
}
IEnumerable<string> myQuery2;
// QueryMethod2 returns a query as the value of its out parameter.
app.QueryMethod2(ref nums, out myQuery2);
// Execute the returned query.
Console.WriteLine("\nResults of executing myQuery2:");
foreach (string s in myQuery2)
{
Console.WriteLine(s);
}
// You can modify a query by using query composition. A saved query
// is nested inside a new query definition that revises the results
// of the first query.
myQuery1 = from item in myQuery1
orderby item descending
select item;
// Execute the modified query.
Console.WriteLine("\nResults of executing modified myQuery1:");
foreach (string s in myQuery1)
{
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 or a later version. By default, the project has a reference to System.Core.dll and a using directive for the System.Linq namespace.
Replace the class with the code in the example.
Press F5 to compile and run the program.
Press any key to exit the console window.