Try to fix it again

ale_linters/cs/mcsc.vim

@@ -10,7 +10,7 @@ function! s:GetWorkingDirectory(buffer) abort
         return l:working_directory
     endif
 
-    return fnamemodify(bufname(a:buffer), ':p:h')
+    return expand('#' . a:buffer . ':p:h')
 endfunction
 
 function! ale_linters#cs#mcsc#GetCommand(buffer) abort