unable to evaluate expression whilst debugging

Question

When debugging asp.net code (running against IIS, and using Visual studio 2013) and in a breakpoint and trying to evaluate a variable using quick watch i quite often get "unable to evaluate expression".

deleting the .suo from the asp.net project folder seems to resolve the issue (After reloading the solution)

Is this a recognised bug? getting this a lot now in Visual studio 2013 on more than one machine.

I've seen this too. In my case, I suspect it has something to do with debugging a process that has multiple AppDomains loaded. Do you have more than one application in your app pool? — jlew, Jan 16 '14 at 16:08
Have to compiled in debug-mode? Is it a single project or are multile projects included? — Tim Schmelter, Jan 16 '14 at 16:09
@jlew only have one application in the app pool. tim-schmelter yes its compiled as debug mode... — Tim, Jan 16 '14 at 16:12
Out of desperation I rebooted my system and it magically started working again... — Ernest, May 15 '15 at 20:31

score 96 · Answer 1 · edited Dec 26 '22 at 08:09

96

I faced it today with VS2013.

Goto Tools --> Options --> Debugging --> General --> Scroll to the bottom for "Use Managed Compatibility Mode" and Select the option.

Screenshot from the blog(url below): enter image description here Restart you debugging.

What Helped Me is below!

Visual Studio 2013 Could not evaluate Expression Debugger Abnormality turned out to be very helpful.

Also, you could see @Dreamers answer Unable to debug managed code using visual studio 2013

edited Dec 26 '22 at 08:09

starball

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answered May 29 '15 at 14:30

Shubh

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Thanks a lot .Great answer. :) – Abhid Feb 02 '17 at 07:58
2

I did this and it makes no difference, I've been debugging C# and C++ for years and for the last few days I cannot view my c++ member variables no matter what options I choose using VS2015. – Josh Aug 11 '17 at 13:53
Followed your steps but still not getting result!! – Unknown_Coder Oct 30 '17 at 09:43
**!** Note that some features don't work along with Managed Compatibility Mode (such as real-time code modifications, performance measurement tools etc.) – Ivan Yurchenko Nov 08 '17 at 21:06
1

For me this option is grayed out – Ryan Glenn Feb 16 '22 at 18:58

score 8 · Answer 2 · answered Jan 16 '14 at 18:13

8

The C# debugging engine relies heavily on the CLR debugger in order to evaluate expressions. That message indicates that the CLR is in a state in which it is unable to perform simple evaluations and the reasons for that can include the following

a local variable is optimized away
the thread is stopped in a GC unsafe point
a previous function call caused the debugger to get into a bad state and hence further evaluations simply aren't possible

answered Jan 16 '14 at 18:13

JaredPar

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this message is something i've only noticed in vs2013 RTM don't remember noticing it (when debugging) in vs2013RC (and beta), 2012 or 2010. – Tim Jan 16 '14 at 22:09

score 6 · Answer 3 · answered Apr 13 '19 at 03:53

6

Some of these options change over time - and top rated solutions in other answers don't seem to all exist any more - searching the options dialog can help.

Right now for a ASPNET Core project I found this, and enabling it seems to be helping:

Suppress JIT optimization on module load (Managed only): Disables the JIT optimization of managed code when a module is loaded and JIT is compiled while the debugger is attached. Disabling optimization may make it easier to debug some problems, although at the expense of performance. If you are using Just My Code, suppressing JIT optimization can cause non-user code to appear as user code ("My Code"). For more information, see JIT optimization and debugging.

If it doesn't seem to help I'd suggest turning it off again.

answered Apr 13 '19 at 03:53

Simon_Weaver

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oh thanks , still works for vs 2019 to evulate linq queries – balron Oct 22 '20 at 22:13
This helped me with my 2019 Visual Studio trying to look a closer look at datasets during debugging at runtime. Before setting that, everything was "unable to evaluate expression". – Jason Shoulders Aug 23 '21 at 13:58
This broke ReSharper's test runner for me. Asked me locate the Program.cs file in some strange temp directory. I had to turn it off again to get it to work. – Jacob Stamm Aug 25 '23 at 16:03

score 0 · Answer 4 · edited Nov 27 '17 at 19:24

0

I have faced this for specific project and the reason is Costura.Fody (embeds all DLLs into the executing assembly).

In this case you can disable Costura.Fody.

Comment out Costura from FodyWever.xml

<Weavers>
  <!--<Costura />-->
</Weavers>

Disable the clean reference target (if included) in *.csproj

<!--<Target Name="CleanReferenceCopyLocalPaths" AfterTargets="AfterBuild">
    <Delete Files="@(ReferenceCopyLocalPaths->'$(OutDir)%
     (DestinationSubDirectory)%(Filename)%(Extension)')" />
    <Exec Command="DeleteEmptyDirectory.bat" />
  </Target>-->

edited Nov 27 '17 at 19:24

CJBS

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answered Jul 19 '17 at 06:55

sudhansu63

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In my case this was the solution so worth the effort. – Hydrogen-4 Jan 29 '19 at 21:42

score -4 · Answer 5 · edited Nov 27 '17 at 19:31

-4

Go to Tools --> Options --> Environment --> General --> check "Automatically adjust visual experience based on client performance" option.

edited Nov 27 '17 at 19:31

CJBS

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answered Apr 19 '16 at 09:24

Aftab Ahmed

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unable to evaluate expression whilst debugging

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