I had a few tests that failed with error
NSubstitute.Exceptions.AmbiguousArgumentsException: Cannot determine argument specifications to use. Please use specifications for all arguments of the same type.
They are similar to http://stackoverflow.com/questions/26805420/nsubstitute-testfixture-1-causes-ambiguousargumentsexception-in-testfixture-2.
I installed the latest version of NSubstitute, but it didn’t help.
As it was advised by David, the errors were caused by calls to non-substituted methods with Args.* parameters. Arg.Any were passed to actual code methods, that where called without Returns or Received parameters.
The errors did not happened when Microsoft tests were running under MSTest, but started when I switched to VSTest.Console.
To scan my test library I used search with regular expression to find rows with Arg.Any but not Arg.Any following by Returns or preceding by Received
It’s not bullet proof filter( e.g it doesn’t exclude multi line statements), but it helped to reduce number of calls to check.