DeleteFiles task exclude single file from match list

Question

I'm trying to setup the DeleteFiles task to delete all DLL files that start with 'ab' except for a specific one.

For example if I have:

abfile.one.dll
abfile.two.dll
abotherfile.dll
System.Stuff.dll (there are 20-30 of these that need to be included)
abdontdeleteme.dll

I expect the resulting files:

System.Stuff.dll
abdontdeleteme.dll

I've tried a bunch of different configs but none have worked. The config that I thought made the most sense was:

  - task: DeleteFiles@1
    displayName: 'Cleanup Assemblies'
    inputs:
      sourceFolder: ${{parameters.theDirectoryToCleanup}}
      contents: ab*.dll !abdontdeleteme.dll

This config does nothing. Setting System.Debug to true and looking at the output I see:

##[debug]pattern: 'D:/a/1/b/api/bin/ab*.dll !abdontdeleteme.dll'
##[debug]applying include pattern against original list
##[debug]0 matches
##[debug]0 final results

I also tried with just ab*.dll which worked but that deletes the one DLL I care about. Is there a way to exclude a specific file from the match list?

Edit

As per a suggestion in the comments I tried the following to the same results (no files were deleted):

- task: DeleteFiles@1
    displayName: 'Cleanup Assemblies'
    inputs:
      sourceFolder: ${{parameters.theDirectoryToCleanup}}
      contents:
        ab*.dll 
        !abdontdeleteme.dll

Edit 2

I tried the following:

- task: DeleteFiles@1
    displayName: 'Cleanup Assemblies'
    inputs:
      sourceFolder: ${{parameters.theDirectoryToCleanup}}
      contents: |
        ab*.dll 
        !(abdontdeleteme.dll)

This deletes everything except 'abdontdeleteme.dl'

- task: DeleteFiles@1
    displayName: 'Cleanup Assemblies'
    inputs:
      sourceFolder: ${{parameters.theDirectoryToCleanup}}
      contents:
        ab*.dll 
        !(abdontdeleteme.dll)

This again matches to nothing and nothing gets deleted.

Answer 1

After a lot of frustration I gave up and used Powershell because it can do what I want. It's not the best solution but it works as some of the other steps require a Windows image anyways. The code that worked is:

- task: PowerShell@2
      displayName: "Cleanup Web Services"
      inputs:
        targetType: 'inline'
        script: Remove-Item "path\to\my\stuff\*" -Include "ab*.dll" -Exclude "abdontdeleteme.dll"