Debugging in Visual Studio

Every programmer writes bugs. Good programmers find them quickly. Adding Console.WriteLine everywhere works for tiny programs, but a debugger lets you pause a running program at any line and look inside it: every variable, every call.

Three kinds of errors

Logic errors are the hardest because nothing tells you about them. The debugger is the tool for exactly this. Try it: the program below should print an average of 133.33, but it doesn’t.

1int[] prices = { 120, 80, 200 };2int total = 0;3 4for (int i = 1; i < prices.Length; i++)5{6    total += prices[i];7}8 9double average = total / prices.Length;10Console.WriteLine($"Average: {average}");

Breakpoints

A breakpoint tells the debugger: “pause here, before running this line”. In Visual Studio click in the grey margin left of a line, or press F9. A red dot appears. Then start with F5 (Start Debugging).

The keys you will use every day

Look inside while paused

• Hover over any variable to see its value in a tooltip (DataTip). You can even pin it.
• Locals window: all variables in the current method.
• Autos window: variables used on the current and previous lines.
• Watch window: type any variable or expression (like total / 3.0) and see its value update as you step.
• Call Stack window: which methods called which to get here (the stack frames you saw in the methods lesson). Double-click a frame to see its variables.
• Immediate window: run small expressions while paused, e.g. prices.Length.

Conditional breakpoints

If a bug happens only on the 500th loop iteration, stopping 500 times is painful. Right-click the red dot → Conditions and enter something like i == 499 or total < 0. The debugger then pauses only when the condition is true.

When an exception is thrown

While debugging, an unhandled exception pauses execution on the exact line that failed and shows the Exception Helper: the exception type, its message and a “View Details” link with the stack trace. Read the message carefully. It usually names the problem exactly.

1int[] data = { 1, 2, 3 };2for (int i = 0; i <= data.Length; i++)   // <= is the bug3{4    Console.WriteLine(data[i]);          // debugger stops here when i == 35}

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System.IndexOutOfRangeException: Index was outside the bounds of the array.

A debugging routine that works

1. Reproduce the bug reliably: find the exact input that causes it.
2. Form a guess: where could the wrong value come from?
3. Pause before that point with a breakpoint and check your guess in Locals or Watch.
4. Step forward until a value differs from what you expected. The bug is in the line you just ran.
5. Fix one thing, then run again to confirm, including the inputs that worked before.

1static int Largest(int[] a)2{3    int max = a[0];              // start from a real element, not 04    foreach (int x in a)5        if (x > max) max = x;6    return max;7}8 9Console.WriteLine(Largest(new[] { -5, -2, -9 }));

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