Question

I took a second to look why my app had terrible performance. All i did was pause the debugger twice and i found it.

Is there a practical reason why it runs my code everytime? The only way i know to prevent this is to add ToArray() at the end. I guess i need to revise all my code and make sure they return arrays?

Online demo http://ideone.com/EUfJN

using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
class Test
{

    static void Main()
    {
        string[] test = new string[] { "a", "sdj", "bb", "d444"};
        var expensivePrint = false;
        IEnumerable<int> ls = test.Select(s => { if (expensivePrint) { Console.WriteLine("Doing expensive math"); } return s.Length; });
        expensivePrint = true;
        foreach (var v in ls)
        {
            Console.WriteLine(v);
        }
        Console.WriteLine("If you dont think it does it everytime, lets try it again");
        foreach (var v in ls)
        {
            Console.WriteLine(v);
        }
    }
}

Output

Doing expensive math
1
Doing expensive math
3
Doing expensive math
2
Doing expensive math
4
If you dont think it does it everytime, lets try it again
Doing expensive math
1
Doing expensive math
3
Doing expensive math
2
Doing expensive math
4

Answer 1

LINQ has lazy evaluation methods and Select is one of them.

And the thing is you are using foreach two times and it prints the values two times.