Is Debug.WriteLine() thread safe?

Question

Is Debug.WriteLine() thread safe?

According to this, it is thread safe. But, in my multithreaded program I am getting some strange output.

For example:

Code

// these statements are found throughout the program
Debug.WriteLine("Polled database. {0} batch items retrieved.", items.Count());
Debug.WriteLine("Queued batch item: {0}", bm.BatchName);
Debug.WriteLine("Discarded batch item: {0} already queued.", bm.BatchName);
Debug.WriteLine("Creating task for batch item: {0}", bm.BatchName);
Debug.WriteLine("Removed batch item from processing items collection: {0}", bm.BatchName);
Debug.WriteLine("Could not remove batch item from processing items collection: {0}", bm.BatchName);
Debug.WriteLine("Begin Processing: {0}", bm.BatchName);
Debug.WriteLine("End Processing: {0}", bm.BatchName);

Output

"Polled database. 0 batch items retrieved."
"Polled database. 0 batch items retrieved."
"Polled database. 0 batch items retrieved."
"Polled database. 0 batch items retrieved."
"Polled database. 1 batch items retrieved."
"ronnie's batch: Queued batch item: {0}"
"ronnie's batch: Creating task for batch item: {0}"
"Begin Processing: ronnie's batch"
"Polled database. 1 batch items retrieved."
"ronnie's batch: Discarded batch item: {0} already queued."
"End Processing: ronnie's batch"
"ronnie's batch: Removed batch item from processing items collection: {0}"
"Polled database. 0 batch items retrieved."

You can see that things start going off the rails with ronnie's batch: Queued batch item: {0} If I use string.Format() first, I don't have the problem. What is going on?

Answer 1

The problem is that you're not calling the overload you think you are. You're calling Debug.WriteLine(string, string) which uses the first parameter as the message and the second as a category, not a format argument.

The simplest way to fix this is to cast your argument to object to force it to use the Debug.WriteLine(string, params object[]) overload:

Debug.WriteLine("Queued batch item: {0}", (object) bm.BatchName);

A slightly longer-winded approach, but one which is perhaps more object, is to explicitly create the array:

Debug.WriteLine("Queued batch item: {0}", new object[] { bm.BatchName });

Or (just to keep supplying options :) call string.Format explicitly to call the Debug.WriteLine(string) overload:

Debug.WriteLine(string.Format("Queued batch item: {0}", bm.BatchName));

or when you're just including the argument directly at the end:

Debug.WriteLine("Queued batch item: " + bm.BatchName);

Alternatively, you might want to create your own convenience method which doesn't have the extra, unhelpful (in your case) overload.

Answer 2

I know this is an old thread and it's not super relevant to the question, but with newer versions of .NET you can also use the interpolation:

Debug.WriteLine($"Queued batch item: {bm.BatchName}");

Also if you end up here, yes it is threadsafe. This is not, which I found out the hard way:

Console.WriteLine($"Queued batch item: {bm.BatchName}");