How to debug a single thread in Visual Studio?

Question

I have a solution with some projects. There are several break-points in different projects. I want to trace the first thread hit one of these break-points and continue tracing that single thread despite of other threads entering the same code-blocks.

I know this is possible through defining a condition on the break-point, that is, thread name = ... or thread Id = ... but my case is a heavy loaded ASP.NET application and as soon as I attach to w3wp.exe many threads will hit the break-points. I need some thing like a ThreadLocal<break-point>.

Is it possible? If so, how?

Answer 1

The select all, right click, freeze approach no longer works in VS-2022 - right click resets the selection to one list item.

Regarding the "better" solutions, they are use case dependent and not always "best."

In my case, the threads are running identical code and "should be" running in parallel with no cross thread effects. There is something unique/non deterministic in at least one thread and I need to single step through that single thread, with all other threads frozen, to find what's causing the nondeterminism.

Isolating only one thread causes the problem to go away.

There are odd use cases where freezing all other threads is the best choice in that use case - but I see no way to do it.

I can get the job done, but it's very tedious and time consuming. "Freezing All Other Threads" would be handy right now.

Answer 2

You can use this new extension which might help.

https://marketplace.visualstudio.com/items?itemName=vsdbgplat.StickToThread

This extension adds a "Stick to Thread" mode to Visual Studio 2022+. When the "Stick to Thread" button is clicked initially, it toggles the mode to "on" and all threads except the current thread are frozen. Which makes it easy to debug single thread.

Answer 3

in VS2019, the conditional breakpoint is tid == xxxxx this way the breakpoint will be hit only on that thread

Answer 4

TL;DR; Just press F5 when it jumps to the wrong thread.

I'm debugging a multithreaded queue processor in Visual Studio Pro 2022 Preview, I placed a breakpoint at a point where each queue item being processed would hit it. The IDE pauses because it hit the breakpoint. I press F10 (Step Over) a few times to step through the code. Then the another thread hit the breakpoint, and the IDE popped me over to that thread. I pressed F5 (Continue). The second thread continued processing, and the yellow debug-line returned to my original thread.

Answer 5

In VS 2019:

1. Set a breakpoint somewhere.
2. Hit F5 (Continue) until your thread comes.
3. Click on breakpoint to remove it.
4. You can step the thread with F10 or F11.

Answer 6

Set a Breakpoint Condition by right clicking the side bar of the line. Select "Condition" and enter the following with the name of your thread in quotes:

System.Threading.Thread.CurrentThread.Name=="name_of_your_thread"

Alternatively you can do the same by getting the thread's "Managed ID" from the "Threads" Window and use:

System.Threading.Thread.CurrentThread.ManagedThreadId==your_managed_thread_id

Answer 7

Freeze/Thaw threads is an incorrect way because other threads don't execute any code.

The most correct and usable way is to:

1. Hit Ctrl+A in the breakpoints window (select all breakpoints).
2. Right click and select "Filter...".
3. Enter "ThreadId=(current thread id)".

In Visual Studio 2015 and newer, the process is similar:

1. Hit Ctrl+A in the breakpoints window (select all breakpoints).
2. Right click and select "Settings...".
3. Check "Conditions" and select "Filter" in the dropdown
4. Enter "ThreadId=(current thread id)".

So all threads are executed, but the debugger hits on the current thread only.

Answer 8

I have just released a Visual Studio 2010+ extension that does exactly what you are looking for. And it's free :).

Presentation

This Visual Studio extension adds two shortcuts and toolbar buttons to allow developers to easily focus on single threads while debugging multi-threaded applications.

It dramatically reduces the need to manually go into the Threads window to freeze/thaw all threads but the one that needs to be followed, and therefore helps improve productivity.

Features

Restrict further execution to the current thread only. Will freeze all other threads. Shortcut: CTRL+T+T or Snowflake button. Switch to the next single thread (based on ID). Will change current thread and freeze all other threads. Shortcut: CTRL+T+J or Next button.

Check it out here on the Gallery, on the official page or the Github repository.

Answer 9

I think this is slightly different in Visual Studio 2015. They've changed a few things in the breakpoints, but here's how to apply the accepted answer from hzdbyte (above):

On the breakpoint in the coding margin, Right-Click > Conditions > Change from 'Conditional Expression' to 'Filter'. This then allows you to filter by ThreadId.

Alternatively on the breakpoint in the Breakpoints window, Right-Click > Settings > tick the Conditions box and do the above.

Answer 10

If multiple threads are being spawned as for a web application, @MattFaus answer's will not work. what I did instead is the following

• Set up a breakpoint to interrupt the thread in the function I want.
• Once the thread gets to the breakpoint and is paused, I remove the breakpoint and continue debugging using F8,F10 and F11, so that the others threads can run.

Answer 11

A slightly different approach which I've used:

1. Create a normal breakpoint and let it get hit
2. Look in your threads window for the managed thread ID that you're current debugging
3. Right click your breakpoint in the breakpoints window and selecter filter
4. Enter ThreadId=xxx where xxx is the thread ID from 2
5. You can now debug without stopping other threads and without them hitting your breakpoint

This assumes you have time to do the above before a second thread hits your breakpoint. If not and other threads hit your breakpoint before you've done the above you can right click them in the threads window and choose freeze.

Answer 12

If you don't want to stop all other threads (maybe you are attaching Visual Studio debugger to a running application that needs to answer to requests), you can use a macro that create and remove breakpoints automatically.

This is suggested in an answer to Stack Overflow question "Step over" when debugging multithreaded programs in Visual Studio.

However, the link only explain how to debug line by line. I suggest you modify the macro (if you're comfortable with it) to make it modify all breakpoints (in a given range of line for instance) to stop only on the current thread.

Answer 13

Here's what I did:

1. Set a conditional break point that I knew would only hit on the thread that I was looking for.

2. Once the breakpoint hits and you are in the thread you want, in the Visual Studio Threads window (while debugging, Debug -> Windows -> Threads), Ctrl + A (to select all threads), and then Ctrl + click the thread you are currently on. You should have all threads except the one you want to debug selected.

3. Right-click, and choose "Freeze".

Now, Visual Studio will only step through the thawed thread. It seems to be much slower when doing this, presumably because it has to loop through all of the frozen threads, but it brought some sanity to my multi-threaded debugging.

Answer 14

I would suggest adding another instance of the application on the live server, either on the same hardware or a new machine (cluster it) and then debug only that instance. I wouldn't add a breakpoint in code users are triggering. If that's not an option, I'd add more tracing.

However, if this is absolutely necessary and you need a solution stat, I'm sure you could add a breakpoint that breaks only if the request is coming from your IP address. You would do this by adding a conditional breakpoint that inspects HttpContext.Request.UserHostAddress. Note however that this slows down your application considerably.