Does “foreach” cause repeated Linq execution?

Question

I\'ve been working for the first time with the Entity Framework in .NET, and have been writing LINQ queries in order to get information from my model. I would like to progr

Answer 1

try this on LinqPad

void Main()
{
    var testList = Enumerable.Range(1,10);
    var query = testList.Where(x => 
    {
        Console.WriteLine(string.Format("Doing where on {0}", x));
        return x % 2 == 0;
    });
    Console.WriteLine("First foreach starting");
    foreach(var i in query)
    {
        Console.WriteLine(string.Format("Foreached where on {0}", i));
    }

    Console.WriteLine("First foreach ending");
    Console.WriteLine("Second foreach starting");
    foreach(var i in query)
    {
        Console.WriteLine(string.Format("Foreached where on {0} for the second time.", i));
    }
    Console.WriteLine("Second foreach ending");
}

Each time the where delegate is being run we shall see a console output, hence we can see the Linq query being run each time. Now by looking at the console output we see the second foreach loop still causes the "Doing where on" to print, thus showing that the second usage of foreach does in fact cause the where clause to run again...potentially causing a slow down.

First foreach starting
Doing where on 1
Doing where on 2
Foreached where on 2
Doing where on 3
Doing where on 4
Foreached where on 4
Doing where on 5
Doing where on 6
Foreached where on 6
Doing where on 7
Doing where on 8
Foreached where on 8
Doing where on 9
Doing where on 10
Foreached where on 10
First foreach ending
Second foreach starting
Doing where on 1
Doing where on 2
Foreached where on 2 for the second time.
Doing where on 3
Doing where on 4
Foreached where on 4 for the second time.
Doing where on 5
Doing where on 6
Foreached where on 6 for the second time.
Doing where on 7
Doing where on 8
Foreached where on 8 for the second time.
Doing where on 9
Doing where on 10
Foreached where on 10 for the second time.
Second foreach ending