C# Compiler Hard crash when debugging and inspecting C# 7.2 ref readonly properties

Runtime: .NET 4.x
Backend: Mono
API Compatibility Level: .NET 4.x
Relevant Packages: Incremental Compiler

We've been using the new C# 7.2 featureset alongside the Incremental Compiler package and it has been pretty flawless so far!

However we have just discovered that when we inspect/watch

ref readonly

properties returned from an array when debugging we get a hard crash. This is a pretty critical issue for us right now, as most of our codebase uses

ref readonly

.

The current work around we have is that we assign the

ref readonly

to a local variable and so force it to copy and deref and debug that, however it is barely more convenient than Debug.Log.

Crash log:

Code (CSharp):

1. * Assertion: should not be reached at ..\mono\metadata\marshal.c:4147
2. (0,0): watchdog triggered an exit

Example code:

Code (CSharp):

1. public struct MyStruct {
2.     public Vector2 Position;
3. }
4.  
5. public class ExampleClass<T> where T: struct {
6.     T[] MyArray = new T[256];
7.  
8.     public ref readonly T this[int index] => ref MyArray[index];
9. }
10.  
11. var example = new ExampleClass<MyStruct>();
12. var newPos = example[123].Position * 3; // This crashes when inspecting Position
13. var localPos = example[123].Position; // Inspecting localPos after executing this line won't crash

 

Latest of both.
VS: 15.7.5
VSTU: 3.7.0.1
Unity: 2018.2.1f1

 

Worth noting that it is Unity that crashes - VS does not crash.

 

@joncham did you ever repro this? We get it daily.

 

Hi, we have exactly the same bug when inspecting a 'ref return' property.

This code is in an external DLL.

Code (CSharp):

1.  
2. public struct Write<T>
3. {
4.     public ref T Value => ref _array[_index];
5.  
6.     readonly T[] _array;
7.     readonly int _index;
8.  
9.     public Write(T[] array, int index)
10.     {
11.         _array = array;
12.         _index = index;
13.     }
14. }
15.  

This code is in a Unity script.

Code (CSharp):

1.  
2. var array = new int[] { 1, 2, 3 };
3. var write = new Write<int>(array, 0);
4. // break here and inspect 'write.Value'
5.  

Tried with Unity 2018.2.2f1
and Visual Studio 15.8.1

This is critical bug for us as it prevents us from debugging properly.

 

Tried with Unity 2018.2.6f1
and Visual Studio 15.8.2

and bug still occurs.

 

Case 1077782

 

Hello, this bug is happening again it's apparently being ignored. Pleas read this thread.