🔍 C# Interview Challenge: What Does This Loop Really Do?

In a recent C# test, I came across this thought-provoking code snippet that’s deceptively simple — until you look a little closer.

public static void Main()
{
    int sum = 0;
    for (int i = 0; i < 10; i++)
    {
        if (i % 3 == 0)
        {
            DoIncrementStuff(ref i);
        }
        else
        {
            DoIncrementOtherStuff(i);
        }
        sum = sum + i;
    }

    // Console.WriteLine(sum);
}

public static void DoIncrementStuff(ref int a)
{
    a++;
}

public static void DoIncrementOtherStuff(int a)
{
    a++;
    a++;
}

🧠 What Would Be Printed?

The question is: What would be the value of sum when printed?
At first glance, it might look straightforward — but here’s where it gets interesting.

💡 Key Concepts Being Tested

This snippet tests your understanding of three important C# concepts:

1. Passing by Reference (ref) vs. Passing by Value

   • ref allows a method to modify the original variable.

   • Passing a value (without ref) gives the method only a copy of the variable.

2. How modifying the loop variable affects loop execution

   • Changing i with ref inside the loop affects the actual loop counter.

3. The order of loop operations

   • In for (int i = 0; i < 10; i++), the increment i++ happens after the loop body.

🔎 Step-by-Step Breakdown

✅ Final value of sum: 37

🧪 Lessons You Can Take Away

1. Ref Parameters Are Powerful — and Dangerous
   Using ref can change control variables in loops. If not used carefully, this can lead to hard-to-find bugs.

2. Value Parameters Don't Affect the Caller
   DoIncrementOtherStuff(i) just plays with a local copy — no side effects on the original i.

3. Loop Counter Manipulation Can Break Expectations
   Modifying i inside the loop body affects the flow in ways that might surprise you.

4. Be Mindful in Interviews
   If you're in an interview setting, explaining why this code behaves the way it does will demonstrate your deep understanding — way beyond just giving the final output.

🧵 Conclusion

This short snippet is a goldmine for learning. It reinforces core concepts like parameter passing, loop behavior, and code predictability — all essential skills for a clean and bug-free C# development journey.

Have you seen similar questions in interviews? Or do you use ref in any smart ways in your codebase? Let's discuss 👇