Accessing an unitialized array throws a NullReferenceException but at the wrong line

[vsfeedback]

This issue has been moved from a ticket on Developer Community.

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Consider the following code to reproduce:

static string[] SomeArray = null;

static void Main(string[] args)
{
    SomeArray[1] = string. Empty; // Line #15
    DoSomething(); // Line #16
}
static void DoSomething() { }

Obviously, SomeArray is never initialized and trying to access it (at line #15) would throw a NullReferenceException. However, the debugger shows the exception at the next line (line #16) and the stack trace is as follows:

> ConsoleApp1.exe! ConsoleApp1.Program.Main(string[] args) Line 16 C#

I was able to reproduce this on both VS2017 and VS2019.

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Original Comments

Visual Studio Feedback System on 3/31/2020, 00:55 AM:

We have directed your feedback to the appropriate engineering team for further evaluation. The team will review the feedback and notify you about the next steps.

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Original Solutions

(no solutions)

[YairHalberstadt]

Is this a debug or release build?

[jcouv]

I can repro this (16.6p2, Debug configuration for the project). The highlight for the breakpoint currently shows DoSomething() which is incorrect.
It could be a sequence point or a debugger issue.

[jcouv]

Tagging @cshung
FYI @gafter @chuckries

I verified that the sequence points are produced as I expect (see repro below).
This feels much like this previously reported issue: #33995 where the thrown exception is shown one instruction too late.

        [Fact, WorkItem(42994, "https://github.com/dotnet/roslyn/issues/42994")]
        public void Repro()
        {
            string source = @"
class C
{
    static string[] SomeArray = null;

    static void M()
    {
        SomeArray[1] = string. Empty;
        DoSomething();
    }
    static void DoSomething() { }
}
";
            var compilation = CompileAndVerify(source);

            compilation.VerifyIL("C.M",
@"
{
  // Code size       18 (0x12)
  .maxstack  3
  // sequence point: SomeArray[1] = string. Empty;
  IL_0000:  ldsfld     ""string[] C.SomeArray""
  IL_0005:  ldc.i4.1
  IL_0006:  ldsfld     ""string string.Empty""
  IL_000b:  stelem.ref
  // sequence point: DoSomething();
  IL_000c:  call       ""void C.DoSomething()""
  // sequence point: }
  IL_0011:  ret
}
", sequencePoints: "C.M", source: source);
        }

[cshung]

@dotnet/dotnet-diag

[tommcdon]

@mikem8361 seems similar to dotnet/runtime#34226

[mikem8361]

Yes, it is. This issue could be linked to that one and closed.

[jcouv]

Thanks. Closing as duplicate of dotnet/runtime#34226