Can't import: MyPostProcessor: No export was found for the contract 'IPostProcessor "MyPostProcessor"'

[michfuchs]

DocFX version: 2.70.3
Project built with .NET Core SDK 7.0.401 (also tried .NET 6.0, same behaviour)

Reproducing the issue:

1. Follow the instructions in the Documentation.
2. Copy the generated assembly to templates/my-template/plugins
3. Build the site

Observed behavior:

• Exception message (see below), PostProcessor does not work

Expected behavior:

• PostProcessor is picked up as the documentation would suggest

Others seem to struggle with the issue as well:

• https://stackoverflow.com/questions/60062901/how-to-run-docfx-plugins-post-processors-under-docfx-console
• Same issue but marked as fixed: [BUG] Custom post processors are completely ignored by docfx #8740

Discussed in #9239

^{Originally posted by michfuchs September 25, 2023}
One appraoch to solve #9238 seems to be implementing a custom PostProcessor, as described in the Documentation.

However, I don't seem to be able to load my post processor.

My docfx.json configuration for the post processors looks like this:

"postProcessors": [ "ExtractSearchIndex", "MyPostProcessor" ]

Since ExtractSearchIndex works, I'm confident it's in the right spot.

Step 4 in the documentation specifies where the built dll of the assembly containing the preprocessor should be copied to:

I tried several folder options, none seem to work:

I am consistently getting the following error:

1>Searching custom plugins in directory ****\802_Documentation\templates\my-template\plugins...
1>6 plug-in(s) loaded.
1>Post processor ExtractSearchIndex loaded.
1>EXEC : warning : Can't import: MyPostProcessor, System.Composition.Hosting.CompositionFailedException: No export was found for the contract 'IPostProcessor "MyPostProcessor"'.
1>   at System.Composition.CompositionContext.GetExport(CompositionContract contract)
1>   at System.Composition.CompositionContext.GetExport(Type exportType, String contractName)
1>   at Docfx.Common.CompositionContainer.GetExport(CompositionHost container, Type type, String name) in /_/src/Docfx.Build.Engine/CompositionContainer.cs:line 41
1>EXEC : warning : Can't find the post processor: MyPostProcessor

It was always 6 plug-ins, even before I added the processor, so the plug-in is definitely not picked up.

[yufeih]

Is there a DLL that contains MyPostProcessor you could share to help us diagnose the issue?

[michfuchs]

A version mismatch was responsible for the issue (see answer in #9239).

Maybe a warning could be shown if versions are not matched. Thanks for your help.

[alricsans]

Have you ever had a similar issues in docker containers ?

On my windows machine, everything works perfectly, but inside a container based on mcr.microsoft.com/dotnet/sdk:6.0 (or even 7.0). I get the same error :

warning: Can't import: TagsProcessor, System.Composition.Hosting.CompositionFailedException: No export was found for the contract 'IPostProcessor "TagsProcessor"'.
   at System.Composition.CompositionContext.GetExport(CompositionContract contract)
   at System.Composition.CompositionContext.GetExport(Type exportType, String contractName)
   at Docfx.Common.CompositionContainer.GetExport(CompositionHost container, Type type, String name) in /_/src/Docfx.Build/CompositionContainer.cs:line 41
warning: Can't find the post processor: TagsProcessor

It is exactly the same code, using docfx version 2.75.3 and Docfx.Plugins and Docfx.Common version 2.75.3 also.
I tried upgrading and downgrading all of those versions accordingly from 2.75.0 to 2.76.0 but nothing woked.

Did anyone encounter the same issue ? If there is something to do for unix-based OS so that the Export attribute works, please let me know.

[filzrev]

@alricsans
Plugin DLL loading failure is reported as Verbose level message.
Is there any log output when running docfx with the --verbose parameter?

[alricsans]

I compared the one that works with the one that does not (with the --verbose parameter), and I have no difference until the warning that appears during the Loading plug-ins and post-processors... step :

• Local setup (works fine):

Loading plug-ins and post-processors...
Searching custom plugins in directory C:\<project-path>\docs\<my-template>\plugins...
Scanning assembly file C:\<project-path>\docs\<my-template>\plugins\Docfx.TagsGenerator.PostProcessor.dll...
Post processor ExtractSearchIndex loaded.
Post processor TagsProcessor loaded.
Post processor SitemapGenerator loaded.
7 plug-in(s) loaded.

• Docker container (failing):

Loading plug-ins and post-processors...
warning: Can't import: TagsProcessor, System.Composition.Hosting.CompositionFailedException: No export was found for the contract 'IPostProcessor "TagsProcessor"'.
   at System.Composition.CompositionContext.GetExport(CompositionContract contract)
   at System.Composition.CompositionContext.GetExport(Type exportType, String contractName)
   at Docfx.Common.CompositionContainer.GetExport(CompositionHost container, Type type, String name) in /_/src/Docfx.Build/CompositionContainer.cs:line 41
warning: Can't find the post processor: TagsProcessor
Post processor SitemapGenerator loaded.

Unfortunately I don't see any differences in the first lines (Skipping assembly, Scanning assembly file and so on)

[filzrev]

It seems that the DLL in the <my-template>\plugins directory is not searched on docker environment.
Because following lines are not exists on docker logs.

Searching custom plugins in directory C:\<project-path>\docs\<my-template>\plugins...
Scanning assembly file C:\<project-path>\docs\<my-template>\plugins\Docfx.TagsGenerator.PostProcessor.dll...

Is it able to confirm <project-path>/docs/<my-template>/plugins directory is exists on docker build environment?

---

I've also tried to run docfx on docker by using this Dockerfile
And it seems to work as expected. (Custom template's plugin directory search log outputted).

Searching custom plugins in directory /root/.dotnet/tools/.store/docfx/2.76.0/docfx/2.76.0/tools/net6.0/any/...
Searching custom plugins in directory /opt/prj/docs/templates/custom/plugins...
Post processor ExtractSearchIndex loaded.
...

[alricsans]

Thank you so much @filzrev your comment helped me troubleshoot my problem.

What seemed to cause the problem was that the dll from my postprocessor were copied to : ./docs/my-template/Plugins instead of ./docs/my-template/plugins, and Windows is probably not case sensitive, but linux is ...

Many thanks again for your help @filzrev !

Edit :
In case other have this issue, I saw it had to be named with an uppercase in this section : https://dotnet.github.io/docfx/tutorial/howto_add_a_customized_post_processor.html#step4-build-your-project-and-copy-the-output-dll-files-to