Roslyn: C# Tuple is zeroed when accessed via collection

Created on 24 Jul 2019 · 3Comments · Source: dotnet/roslyn

Version Used: Visual Studio 15.9.04012.0. File -> New -> Project -> Console (DotNet Core)

Steps to Reproduce:

Paste the following code:

using System;
using System.Collections.Generic;

namespace CompilerBugZero
{
   class Program
   {
       static void Main(string[] args)
       {
           var hdl = FindHandle();
           if (IntPtr.Zero == hdl)
           {
               throw new InvalidOperationException();
           }
       }

       static IntPtr FindHandle()
       {
           var candidates = new List<(IntPtr hwnd, string title, string className)>();
           candidates.Add((new IntPtr(123), "title", "className"));

           if (candidates.Count == 1)
           {
               // Breakpoint here
               return candidates[0].hwnd;
           }

           throw new InvalidOperationException();
       }
   }
}

Set a breakpoint the line below // Breakpoint here
See how everything is Zero when it shouldn't:

Note how the list still has the correct value but accessing [0] returns a zeroed tuple apparently.

Expected Behavior:
Debugger shows the correct values. Code works at runtime.

Actual Behavior:
Debugger doesn't show correct value. In fact in our real application I feel like we had the wrong behavior (ie IntPtr.Zero) at runtime. In the real application this happened in a PInvoke scenario. However, I couldn't reproduce this here. This might be a wrong positive, but worth considering when investigating this.

Workaround:

Change the code to

            if (candidates.Count == 1)
            {
                // Breakpoint here
-               return candidates[0].hwnd;
+               var t = candidates[0];
+               return t.hwnd;
            }

Area-External Interactive-Debugging Resolution-Duplicate

Source