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My .Net Framework WinForms app builds without errors and runs without errors.

The VS2017 Professional 15.9.2 designer crashes when I double-click to open user controls I've created. These user controls all inherit from a class named MyUserControl. I can open MyUserControl in the designer without an error.

When I run VS2017 alone (no second copy with the debugger attached), I see the following exception when I double-click the target user control:

'Could not load file or assembly 'MyEventArgs, Version=1.0.6896.23135, Culture=neutral, PublicKeyToken=null' or one of its dependencies. The system cannot find the file specified.'

The target user control does not reference MyEventArgs. Adding a reference in the target project to MyEventArgs doesn't help. All references to MyEventArgs in the solution are references to a DLL in a Libraries folder, not project references.

I opened each of the .csproj files in the solution in Notepad. None of them refer to a specific version of the MyEventArgs DLL.

I have cleared all the obj and bin directories.

I have cleared the ProjectAssemblies directory. This is a WinForms app, so no need to clear the temporary ASP.Net files directory.

I started a second copy of VS2017 and attached the debugger to the VS2017 instance where I try to open the user control in the designer. I checked the parent check box to choose to break on all CLR exceptions.

I double-click to open the target user control in the VS designer of the first VS instance.

First I get an exception

The parameter is incorrect. (Exception from HRESULT: 0x80070057 (E_INVALIDARG))

with no info on what parameter is incorrect. I looked in all the properties of the exception object. None of my DLLs have the "downloaded from the Internt" bit set.

I click Continue in the debugger, then get this exception

'Could not load file or assembly 'MyEventArgs, Version=1.0.6896.23135,
Culture=neutral, PublicKeyToken=null' or one of its dependencies. The
system cannot find the file specified.'

=== Pre-bind state information === LOG: DisplayName = MyEventArgs, Version=1.0.6896.23135, Culture=neutral, PublicKeyToken=null
(Fully-specified) LOG: Appbase = file:///C:/Program Files
(x86)/Microsoft Visual Studio/2017/Professional/Common7/IDE/ LOG:
Initial PrivatePath = NULL Calling assembly : (Unknown).
=== LOG: This bind starts in default load context. LOG: Using application configuration file:
C:UsersAdamAppDataLocalMicrosoftVisualStudio15.0_fdc9bc52devenv.exe.config
LOG: Using host configuration file: LOG: Using machine configuration
file from
C:WindowsMicrosoft.NETFrameworkv4.0.30319configmachine.config.
LOG: Policy not being applied to reference at this time (private,
custom, partial, or location-based assembly bind). LOG: The same bind
was seen before, and was failed with hr = 0x80070002.

However, I don't have this version of MyEvents anywhere. All references are to the DLL in the Libraries folder. This DLL has a newer version.

How do I get VS2017 to stop looking for this old version of the DLL?

I ran two copies of AstroGrep (free Windows utility) to search for that version number (1.0.6896.23135) in all files on my C drive as well as the D drive, where the solution is stored. It did not find any instances of this version number in any file (csproj, etc.).

The target user control project does not reference MyEventArgs. I checked each csproj in the solution. All reference a copy of MyEventArgs.dll in a library folder. This copy has a newer version than the one named in the exception.

MyEventArgs is not in the GAC on the target machine.

Where is VS getting this old version number from? If I knew where VS was reading this version number, I could clear it or change it. It can't have made up this number, but I can't find where it is stored.

TLDR: This was my fault, not Visual Studio's. That said, a more human-readable exception would have saved me a lot of debugging time.

The exception was caused by the fact that I recently revised my solution by changing many project references to refer to DLLs in a Libraries folder rather than to other projects in the same solution.

For example, I have many projects which contain simple interfaces (IDoStuff). I figured these don't change often, so maybe referring to DLLs would speed up the compile process a bit.

What I believe happened was that I compiled MyEventArgs, creating version 1.0.6896.23135, then compiled other classes which depend on MyEventArgs, then put those DLLs in my Libraries folder. Then I must have re-compiled MyEventArgs, and stored a newer version in the Libraries folder.

I assumed if I had all my project references set to not require specific versions, it would all just work.

I spent hours looking for the "1.0.6896.23135" reference in project files, clearing bin folders, etc.

I now believe that the reference to that specific version of MyEventArgs was embedded in another DLL which I had copied to the Libraries folder.

I revised my solution to go back to using project references and the VS designer no longer crashes when opening the target user controls.