const results in error CS0648: '' is a type not supported by the language

[staff0rd]

REPRO

1. Open VS2015 (Windows 10, ASP.NET 5 Beta 7)
2. Click File > New > Project > Web > Console Application (Package) > Ok
3. Edit Program.cs to reflect code snippet below.
4. Add a breakkpoint just after Console.WriteLine(a);
5. Run the project
6. Right-click variables a and b click Add Watch

CODE

public void Main(string[] args)
{
    const int a = 3;
    int b = 4;
    Console.WriteLine(a);
}

EXPECTED
Watch window displays values for variables a and b

ACTUAL
Watch window Value column for variables a and b displays:

error CS0648: '' is a type not supported by the language

NOTES

1. The value of a is written correctly to Console
2. Removing const from the snippet reverts to EXPECTED bevahiour

[Sharpiro]

+1
Just discovered this is occuring in Xunit tests as well.
Cannot view any local variables in scope while debugging if "const" is involved

[Eilon]

@barrytang looks like maybe something with the debugger or Roslyn is changed? Do you know a contact there?

[dandago]

I ran into this a few weeks ago, and wrote a blog post with some detail yesterday: http://gigi.nullneuron.net/gigilabs/nasty-const-bug-in-asp-net-5/

[Eilon]

I contacted the Roslyn team to have them take a look at this.

[KevinH-MS]

Since constants aren't actually locals (they're just inline values in the emitted IL), we have to retrieve their name/value/type from the .pdb. It seems that the low-level API we use for doing this (ISymUnmanagedConstant.GetSignature) is always returning a zero-length array on CoreClr.

I should be able to make the watch for b work despite this, but we'll need a fix from the core debugger to show the value of a. I'll follow up with the folks who own the symreader and post back when I know more.

[KevinH-MS]

I have a PR out to ignore the const local in this case (enables you to at least inspect the other locals in that scope): dotnet/roslyn#5658

I've also opened an internal bug for the core debugger to figure out why reading the signature of a const isn't supported here.

[Eilon]

This is now being tracked on the Roslyn GitHub org and also in an internal MSFT bug database, so I'm closing this out. Thanks, everyone!