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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?

Peter Mortensen
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Xaqron
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    @Paolo: This web application works as the heart of a large web farm and the buggy situation is impossible to mimic in testing scenarios. – Xaqron Mar 14 '11 at 21:39
  • For VS 2019 maybe you should try this: https://stackoverflow.com/a/61868591/8579563 – Gozo May 18 '20 at 11:35

12 Answers12

402

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.

Peter Mortensen
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Matt Faus
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    This doesn't work for me in a context where I have about 8 tasks running in different threads. I freeze all other threads and "step over" but the IDE freezes for a while, then jumps to another thread anyway. – Meta-Knight Dec 05 '12 at 14:02
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    @Diego: I'm not working on the project anymore but if it is going to freeze all threads except one, it will change the scenario of which caused the bug, since threads were complementary in buggy situation. Although this is an elegant solution, it seems this feature should be embedded into VS. – Xaqron Dec 25 '12 at 21:01
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    @Meta-Knight You should freeze all threads **except** Main Thread. If you will do like this IDE won't be freezed – Alex Zhukovskiy Aug 26 '13 at 14:17
181

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.

Isak Savo
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hzdbyte
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    Although it is not exactly what it should be (still ThreadId is needed as a filter) but the most correct solution with existing options. – Xaqron May 15 '14 at 11:11
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    Does this prevent the "Step" debugger command from entering other threads? That was a big problem I had. I'm stepping through my thread and all of a sudden I'm in a completely unrelated portion of code. I no longer develop in Visual Studio, so I can't test. – Matt Faus May 29 '14 at 23:33
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    Right clicking in the breakpoints window has no "filter" command... and how do you find out the current thread Id anyway? -- Are you going to the immediate window and typing `System.Threading.Thread.CurrentThread.ManagedThreadId` or something? – BrainSlugs83 Dec 10 '15 at 01:00
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    @BrainSlugs83 From the VS menu select DEBUG | Windows | Threads. That will display a panel with a list of threads. – John MacIntyre Jan 27 '16 at 17:51
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    "Filter" is "Condition" in my VS, and another way to see thread id is ticking Tools -> Customize -> Debug Location. It will show the thread id by your debug toolbar. – Chad Hedgcock Apr 19 '16 at 16:13
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    In my VS (2015, Community Edition) it is not possible to modify the Settings of multiple breakpoints at once. Thus the Filter can be set only one by one. – robert4 Aug 10 '16 at 12:19
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    I've had this problem forever, and I could swear at my last job I found a setting that made visual studio work like eclipse where you'd stick to the thread you were working with, but I can't find it or any reference to it. I'm beginning to wonder if I dreamed it. – stu Aug 17 '16 at 13:07
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    -1 because this only allows for breakpoints, but not to actually debug: step over/step into do not work this way for debugging a single thread. – Serge Rogatch Oct 11 '17 at 12:03
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    While this answer is also useful, saying that freeze/thaw technique is incorrect in all cases is not fair, in a lot of cases it's much more convenient. – Predelnik Mar 11 '19 at 08:49
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    This answer has nothing to do with the question asked. It doesn't let you debug only a single thread but shows how to break on a single thread. – Cesar Jul 24 '19 at 09:07
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    This does not work in VS2019 as the settings is greyed out when selecting multiple break points. – Sigex Sep 02 '19 at 16:18
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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.
Mikaël Mayer
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20

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.

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Erwin Mayer
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  • Which version of VS are you using? – Erwin Mayer Nov 11 '13 at 18:20
  • I installed the extension, but did not manage to make it work (for my current project, I use VS2010). [By the way what I thought was a problem with threads becoming active just happens when threads are created, it is probably standard behavior so I removed my previous comment]. – wip Nov 12 '13 at 02:36
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    I'm going to check this out. You may be the only person on the planet who has made any attempt to solve one of microsoft's biggest debugging failings. – stu Feb 19 '15 at 19:53
  • alas, it won't install on vs2012. Do you have a newer version or are you will to share the source code so I can build it myself? – stu Feb 19 '15 at 20:01
  • @stu The source code is on Codeplex here: http://singlethread.codeplex.com/ it should work on VS 2012 and VS 2013 but I have not updated it (no one has requested it and I have not had the need myself). If you can make it work easily with VS 2012+ and make a commit on Codeplex then I can push it on the Gallery as well. – Erwin Mayer Feb 20 '15 at 18:14
  • @stu, I was finally able to make a commit with the changes needed to make it compatible with VS 2012 and 2013. Hopefully it works (not thoroughly tested). The update is on the Gallery and on Codeplex. – Erwin Mayer Feb 20 '15 at 19:17
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.

Matt
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    +1. "This assumes you have time...before a second thread hits your breakpoint". I wrap a semicolon in a lock and put a breakpoint on the semicolon. When the breakpoint is hit the first time, I disable the breakpoint. No other threads can come in due to the lock. `lock(m_someObject) { ; }` – bluedog May 04 '15 at 05:04
  • To save you a Google, the threads window is found under Debug > Windows > Threads. – Gabe Nov 28 '18 at 13:39
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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

James Sheridan
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  • As of VS 2022, this is the only way I've found to break on a particular thread access. ThreadId == 'ManagedId' and tid == 'ManagedId' (suggested elsewhere) generate a warning and do not work. – d ei Mar 01 '23 at 13:43
3

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

kenny mccormick
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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.

Patelos
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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.

Peter Mortensen
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steinar
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  • That's what I have tried. The problem is this instance is not working on the same domain (running on IP or another domain) and this leads to many certificate problems (SSL, WCF, ...) and also under low load the buggy situation never happens ! – Xaqron Mar 14 '11 at 21:55
  • Sorry, I'm not sure which of these you've tried, have you tried the conditional breakpoint? – steinar Mar 14 '11 at 21:58
  • Yes, I named them based on their managed Id to ensure uniqueness. Then it's hard to guess which Id is assigned and set the condition based on a guess. Sometimes the guess is close and sometimes it takes a long time to catch the thread. – Xaqron Mar 14 '11 at 22:02
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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.

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kamaradclimber
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0

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.

Homr Zodyssey
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