Visual Studio quirk- it works on my box

I encountered an interesting Visual Studio thing today. Someone sent me a bug with a repro. I ran it on my machine and started stepping through the internals of the framework to see what the issue is. It worked fine. Hmm, that was interesting, I wonder why? So I run it again and this time I don't step through it and it fails. Ok, that's strange. When I just execute the code, it fails, but if I step into it and don't do ANYTHING except step through the code, it succeeds. WTF?

Well, I had forgotten about something: Autos and locals. In Visual Studio, when debugging, the debugger creates watches on local variables as well as a few things it just watches automatically. In order to get the values of these, it has to evaluate them, and therein lies the problem: If evaluating any of those variables causes any side effects that don't occur during the normal running of the application, it can cause unexpected behavior. Here's an example:

Let's say that I have some state property on my object that is initialized to null. I have a method that depends on this state property being set. That state property is set when you access another property somewhere. Assuming that property is not accessed in the code path that I'm executing, the state property will not be set. HOWEVER, if I trace through the code and evaluate the property that sets the state property, it will end up setting my state, thus changing the way my code executes. Let's look at a concrete example:

class testthing
{
private string s = null;

public string PropString
{
get { if (s == null)s = "new"; return "new";}
set { s = value; }
}

public bool forceit = false;

public bool DoSomething()
{

if (forceit)
Console.WriteLine(PropString);

return s == null;
}

}

class Program
{
static void Main(string[] args)
{
Console.WriteLine("Starting run");
testthing t = new testthing();
bool x = t.DoSomething();

Console.WriteLine("result was: " + (string)(x ? "true" : "false"));
Console.ReadKey();

}
}

So the class testthing has a property PropString that doesn't set the internal value of the private field s until the Get is called. Therefore, if you never call PropString.Get, it never sets the value of s, and DoSomething() will return true because s defaults to null. Run the example code and observe this, it's pretty straightforward.

Now, run it a second time, except this time put a breakpoint on the first line in DoSomething(). When it breaks, hover over the Console.WriteLine(PropString) so that it forces the debugger to evaluate PropString. Now, execute the rest of the code (f5) and observe the output is false, because the debugger has executed the getter of PropString which had a side-effect.

So, the next time you debug an application in Visual Studio and it works in the debugger but not in the code, look at your variables within the method throwing the exception and see if any of them could possibly be changed through evaluation. If so, then you may have found the problem.

One final word: If you have a unit test for something like this, your unit test will fail since it won't evaluate the property when it's run. It would be far easier to write a failing unit test around the method that has the bug and then figure out why it's failing than to step through the method and hope that you can see where it's going wrong.