Steps to Reproduce

This was found attempting to get the C# semantic tests running on Mono

1. Clone https://github.com/jaredpar/roslyn
2. Run ./build.sh --restore --build
3. Run cd src/Compilers/CSharp/Test/Symbol
4. Run dotnet msbuild /t:Test /p:TestRuntime=Mono

Actual Behavior

This will run the Symbol unit tests and eventually Mono will fail executing the test FileThatCannotBeDecoded.

Expected Behavior

The test passes

On which platforms did you notice this

This was discovered on Ubuntu 18.04. I'm using the latest nightly mono as described by the Download page

https://www.mono-project.com/download/nightly/#download-lin

2019-01-24T17:00:15.7166214Z Copyright (C) 2002-2014 Novell, Inc, Xamarin Inc and Contributors. www.mono-project.com
2019-01-24T17:00:15.7166659Z 	TLS:           __thread
2019-01-24T17:00:15.7166780Z 	SIGSEGV:       altstack
2019-01-24T17:00:15.7166881Z 	Notifications: epoll
2019-01-24T17:00:15.7166949Z 	Architecture:  amd64
2019-01-24T17:00:15.7166998Z 	Disabled:      none
2019-01-24T17:00:15.7167048Z 	Misc:          softdebug 
2019-01-24T17:00:15.7167112Z 	Interpreter:   yes
2019-01-24T17:00:15.7167163Z 	LLVM:          yes(600)
2019-01-24T17:00:15.7167211Z 	Suspend:       hybrid
2019-01-24T17:00:15.7167261Z 	GC:            sgen (concurrent by default)

Note: the mono version is no longer being printed out anymore where it used to be. Hence I'm unsure exactly which nightly package is being used but it was done the morning of 1/24/2019

Full Details

This is a test of the C# compilers ability to detect binary files and not attempt to fully parse them. The binary file in this case is created with the content new byte[] { 0xd8, 0x00, 0x00, 0x00 }.

When reading the string the compiler the compiler uses a StreamReader instance. The first attempt in this test will allow the reader to detect the encoding of the file which will fail and should throw a DecoderFallbackException. The second attempt will use ASCII encoding for reading the file and should return a string of length 4 with the characters new char[] { 216, 0, 0, 0 }. The compiler will then interpret the consecutive 0 elements as binary.

The Mono runtime though is throwing an unexpected exception on this code path with the Message property having the text "GetCPInfoExW". This is unexpected and causes us to fall back to a different error message: source file not found.