“Internal error in the expression evaluator”

Question

I\'ve encountered a problem in expression evaluator of visual studio 2015 that says \"Internal error in the expression evaluator\", after some investigations I found that th

Answer 1

For me checking "Use Managed Compatibility Mode" option worked. I was also seeing question marks when hovering over variables, instead of properties and values

Answer 2

Just extending on the solution provided by Patrick Nelson. For Visual Studio 2015+ as inquired, the steps are as follows.

If you're debugging at the moment, this option will be unavailable. Stop the debugger.

Go to Tools -> Options

and here under the Options select Debug -> General and scroll down to find the ...legacy C# expression.. option:

More information is provided here:

Switching to Managed Compatibility Mode

Note: There are also some serious drawbacks that occur from switching to the legacy mode. Especially Runtime Reflection of the implemented Interfaces becomes almost an impossibility or extremely inconvenient. There are also other Reflection methods which will throw errors.

Answer 3

I resolved this issue by simply resetting my visual studio settings by going to: to Tools->Import and Export Settings and selecting to reset to default settings

Answer 4

In my case, I had 2 same dll files in 2 different folders (seems, one dll was not correct). Deleting the .dll and rebuilding solution solved my issue.

Answer 5

Check your use of the [DebuggerBrowsable] attribute; I found a minimal case in VisualStudio 2017 15.5 and posted it here.

In this particular case, the expression evaluator (EE) crash appears related to the [DebuggerBrowsable] attribute applied to a property overriding a field of the same name. This will account for some percentage of the cases that people are experiencing out there, but there's no way of knowing how many are due to this specific issue until it gets fixed.

The full and complete demonstration example is shown in the image (and included below for good measure)

Machine-readable copy of the code in the image:

using System;
using System.Diagnostics;

class Program { static void Main() => new _derived(); }

abstract class _base
{
    [DebuggerBrowsable(DebuggerBrowsableState.Never)]
    public Object trace;
};

class _derived : _base
{
    public _derived() => Debugger.Break();      // <-- vs2017 EE crash when stopped here

    [DebuggerBrowsable(DebuggerBrowsableState.Never)]
    new public Object trace => base.trace;
}

Answer 6

I had the same issue with VS2019. I ended up deleting my Documents/Visual Studio 2019 folder. Hope this might help someone, one day. Cost me a day.

PS. Probably not required to delete all, and of course not your projects (if they're in there), but in my case, everything in there was autogenerated by VS.

I of course tried all solutions mentioned here, and even reinstalling VS didn't work. Refactoring the class to another name was the 'trigger' which made me think there must be some cache, despite cleaning symbols and such didnt work.