Question

As per this link :http://msdn.microsoft.com/en-us/library/bb397906.aspx

namespace Linq
{
    class IntroToLINQ
    {
        static void Main()
        {
            // The Three Parts of a LINQ Query: 
            //  1. Data source. 
            int[] numbers = new int[7] { 0, 1, 2, 3, 4, 5, 6 };

            // 2. Query creation. 
            // numQuery is an IEnumerable<int> 
            var numQuery =
                from num in numbers
                where (num % 2) == 0
                select num;

            // 3. Query execution. 
            foreach (int num in numQuery)
            {
                Console.Write("{0,1} ", num);
            }
        }
    }

}

It states that query will not be exeucted , until data is iterated through foreach. But when I debugged , the data memeber of var(resultviews) contains the result values before the execution of foreach. Why this is happening?

Answer 1

Because your debugger is executing the query for you.

Linq is using something that is called Deferred Execution. Here is a good blog post that explains it: LINQ and Deferred Execution.

When you first execute the iterator, the query is processed and executed (in memory, to a database or in another way). Your debugger will do this for you.

Take for example the following code (you can paste this in a Console Application):

using System;
using System.Diagnostics;
using System.Linq;

namespace Stackoverflow
{
    class Program
    {
        static void Main()
        {
            var numbers = Enumerable.Range(0, 100).ToList();

            var filteredNumbers = from n in numbers
                                  where n > 50
                                  select n;

            Debugger.Break();

            foreach(int f in filteredNumbers)
            {
                Console.WriteLine(f);
            }

        }
    }
}

When you look at filteredNumbers on the Debugger.Break() statement you will see the following:

The Results View options has the Value: “Expanding the Results View will enumerate the IEnumerable”. And that’s what’s happening in the Debugger.