Parts of code only execute when there is a Debug.Log() statement in them. Why?

Hi.

Throughout my work with Unity I sometimes run into a bug, and a code that isn’t executing (I know because a debug log inside doesn’t show anything).

To fix I go back in the code and use other Debug.log() statements to get info on my variables, but when I add these suddenly the code that wasn’t executed is now executing.

This happens a lot, and I’m at a loss why. Logically maybe the debug.log statement is taking enough time to make the code run correctly, but this indicates a bigger problem. Any help on what I should be looking for when this happens? Thanks.

Here is an example. The editor doesn’t show “a” until the previous debug.log() statements are uncommented.

			else if (nameOCE != "Floor" && !nameOCE.StartsWith("Wheel") && !otherOCE.CompareTag("bullet"))
			{
				
//				Debug.Log(nameOCE);
//				Debug.Log(otherOCE.tag);
//				Debug.Log(gameObject.layer);
//				Debug.Log(otherOCE.gameObject.layer);

				if (!otherOCE.CompareTag("Dangerous"))
				{
					if (otherOCE.CompareTag("future"))
					{
						if (mymen.misslev == 9)
						{
							if (!mymen.missdo2)
							{
								mymen.finmiss(1);
							}
						}
					}
					else if (otherOCE.CompareTag("peace"))
					{
						Debug.Log("a");
						if (mymen.misslev == 10)
						{
							if (!mymen.missdo2)
							{
								mymen.finmiss(1);
							}
						}
					}

There’s no reason why the inclusion of Debug.Log would make a difference, unless the tiniest timing delay of printing to the console had meant that the value of one of the conditions had changed (which seems incredibly unlikely).
However, your code has several logical inconsistencies and redundancy. I’d set about correcting those and see if it makes a difference:

As far as I can tell, the entire logic can be reduced to:

else if (nameOCE != "Floor" && !nameOCE.StartsWith("Wheel") && !mymen.missdo2) {
  if (otherOCE.CompareTag("future") && mymen.misslev == 9) {
    mymen.finmiss(1);
  }
  else if (otherOCE.CompareTag("peace") && mymen.misslev == 10) {
    mymen.finmiss(1);
  }
}

How is this being called? In an OnCollisionEnter? It might help to see the whole function.

Did you find a solution to this problem? I’m experiencing the same thing with if statements and while loops!

I sometimes have the same issue. I’ve had it occur with multiple projects in basically of Unity’s versions. With different IDE’s: Jetbrains Rider & Visual Studio.
I can’t really explain it, it’s very frustrating.

This is what ChatGPT had to say about it:
The phenomenon you’re describing—where inserting a Debug.Log seems to change the behavior of your code—is indeed puzzling, but not entirely unheard of. This behavior can be attributed to various reasons, such as compiler optimizations, Unity’s built-in JIT/AOT compilation nuances, multithreading issues, or IDE-to-Unity syncing quirks, among others.

Here are some approaches you might consider to help diagnose the issue:

  1. Full Rebuild: Start by cleaning your project and forcing a full rebuild. Sometimes intermediate build artifacts can cause issues.
  2. Scripting Backend: Unity offers different scripting backends, and sometimes switching between them can solve such issues. If you’re using IL2CPP, try Mono, and vice versa, to check if the issue persists. Remember to test the game in both the editor and the build, as there can be differences.
  3. Disable Compiler Optimizations: Depending on the configuration and the scripting backend, the compiler might be optimizing out certain sections of code, thinking they aren’t necessary. This is less likely in this context but is a potential cause.
  4. Debugging:
  • Use breakpoints instead of Debug.Log. This will pause the execution, and you can step through to see what’s being executed and what’s not.
  1. External Libraries: Ensure there are no external libraries or packages that could be interfering. A good way to diagnose this is to create a fresh project and gradually introduce parts of your project into it to see when the behavior starts.
  2. Version Control: If you’re using a version control system (like Git), check for any changes made to relevant parts of the codebase. Maybe the behavior changed due to an unseen change or merge.
  3. IDE-to-Unity Syncing: As you suspected, sometimes the IDE (like Visual Studio or Rider) might not sync properly with Unity.
  • Ensure the scripting define symbols and API compatibility level in Unity match what’s expected.
  • Try closing and reopening your IDE.
  • Ensure your Unity and IDE plugins (like the Visual Studio Tools for Unity) are up to date.
  1. Thread Safety: If there’s any multithreading or asynchronous operation happening, adding a Debug.Log might introduce just enough of a delay to make certain race conditions visible or invisible.