Visual Studio 2017 will crash with this code.

[CreateAndInject]

15.7.2:

Steps to Reproduce:

1. write this code:

      foreach(_ int "")
            {
            }

2. move mouse to _ ,wait about 2 seconds

Expected Behavior: nohting

Actual Behavior:Visual Studio 2017 will crash

[CyrusNajmabadi]

I cannot repro this:

I'm on 15.8.0 preview 1.1

[CreateAndInject]

I wanted to say:

            foreach(_ in "")
            {

            }

but both "in" or "int" will crash here

[CreateAndInject]

I ask some people, every one will crash!

[alrz]

I repro'd twice, VS 15.8p1.1, funny thing, this does not repro on roslyn F5

[CreateAndInject]

My version is 15.7.2

[svick]

I can also reproduce this with VS 15.8.0 Preview 1.1. The stack trace from Event Viewer is:

Application: devenv.exe
Framework Version: v4.0.30319
Description: The application requested process termination through System.Environment.FailFast(string message).
Message: System.NullReferenceException: Object reference not set to an instance of an object.
   at Microsoft.CodeAnalysis.CSharp.SymbolDisplayVisitor.VisitDiscard(IDiscardSymbol symbol)
   at Microsoft.CodeAnalysis.CSharp.Symbols.DiscardSymbol.Accept(SymbolVisitor visitor)
   at Microsoft.CodeAnalysis.CSharp.SymbolDisplay.ToDisplayParts(ISymbol symbol, SemanticModel semanticModelOpt, Int32 positionOpt, SymbolDisplayFormat format, Boolean minimal)
   at Microsoft.CodeAnalysis.CSharp.SymbolDisplay.ToMinimalDisplayParts(ISymbol symbol, SemanticModel semanticModel, Int32 position, SymbolDisplayFormat format)
   at Microsoft.CodeAnalysis.CSharp.Symbol.Microsoft.CodeAnalysis.ISymbol.ToMinimalDisplayParts(SemanticModel semanticModel, Int32 position, SymbolDisplayFormat format)
   at Microsoft.CodeAnalysis.Editor.CSharp.LanguageServices.CSharpSymbolDisplayService.ToMinimalDisplayParts(SemanticModel semanticModel, Int32 position, ISymbol symbol, SymbolDisplayFormat format)
   at Microsoft.CodeAnalysis.LanguageServices.AbstractSymbolDisplayService.AbstractSymbolDescriptionBuilder.ToMinimalDisplayParts(ISymbol symbol, SymbolDisplayFormat format)
   at Microsoft.CodeAnalysis.LanguageServices.AbstractSymbolDisplayService.AbstractSymbolDescriptionBuilder.AddDescriptionForArbitrarySymbol(ISymbol symbol)
   at Microsoft.CodeAnalysis.LanguageServices.AbstractSymbolDisplayService.AbstractSymbolDescriptionBuilder.<AddDescriptionPartAsync>d__31.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at Microsoft.CodeAnalysis.LanguageServices.AbstractSymbolDisplayService.AbstractSymbolDescriptionBuilder.<AddPartsAsync>d__24.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at Microsoft.CodeAnalysis.LanguageServices.AbstractSymbolDisplayService.AbstractSymbolDescriptionBuilder.<BuildDescriptionSectionsAsync>d__30.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at Microsoft.CodeAnalysis.LanguageServices.AbstractSymbolDisplayService.<ToDescriptionGroupsAsync>d__11.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at Microsoft.CodeAnalysis.Editor.Implementation.IntelliSense.QuickInfo.AbstractSemanticQuickInfoProvider.<CreateContentAsync>d__2.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.ValidateEnd(Task task)
   at Microsoft.CodeAnalysis.Editor.Implementation.IntelliSense.QuickInfo.AbstractSemanticQuickInfoProvider.<BuildContentAsync>d__0.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at Microsoft.CodeAnalysis.Editor.Implementation.IntelliSense.QuickInfo.AbstractQuickInfoProvider.<GetQuickInfoItemAsync>d__2.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at Microsoft.CodeAnalysis.Editor.Implementation.IntelliSense.QuickInfo.AbstractQuickInfoProvider.<GetItemAsync>d__0.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at Microsoft.CodeAnalysis.Editor.Implementation.IntelliSense.QuickInfo.Controller.<ComputeModelInBackgroundAsync>d__9.MoveNext()
Stack:
   at System.Environment.FailFast(System.String, System.Exception)
   at Microsoft.CodeAnalysis.FailFast.OnFatalException(System.Exception)
   at Microsoft.CodeAnalysis.ErrorReporting.FatalError.Report(System.Exception, System.Action`1<System.Exception>)
   at Microsoft.CodeAnalysis.ErrorReporting.FatalError.ReportUnlessCanceled(System.Exception)
   at Microsoft.CodeAnalysis.Editor.Implementation.IntelliSense.QuickInfo.Controller+<ComputeModelInBackgroundAsync>d__9.MoveNext()
   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(System.Threading.Tasks.Task)
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(System.Threading.Tasks.Task)
   at Microsoft.CodeAnalysis.Editor.Implementation.IntelliSense.QuickInfo.Controller+<ComputeModelInBackgroundAsync>d__9.MoveNext()
   at System.Runtime.CompilerServices.AsyncTaskMethodBuilder`1[[System.__Canon, mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089]].Start[[Microsoft.CodeAnalysis.Editor.Implementation.IntelliSense.QuickInfo.Controller+<ComputeModelInBackgroundAsync>d__9, Microsoft.CodeAnalysis.EditorFeatures, Version=2.8.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35]](<ComputeModelInBackgroundAsync>d__9 ByRef)
   at Microsoft.CodeAnalysis.Editor.Implementation.IntelliSense.QuickInfo.Controller.ComputeModelInBackgroundAsync(Int32, Microsoft.VisualStudio.Text.ITextSnapshot, System.Collections.Generic.IList`1<Microsoft.CodeAnalysis.Editor.IQuickInfoProvider>, Boolean, System.Threading.CancellationToken)
   at Microsoft.CodeAnalysis.Editor.Implementation.IntelliSense.QuickInfo.Controller+<>c__DisplayClass7_0.<StartSession>b__0(Microsoft.CodeAnalysis.Editor.Implementation.IntelliSense.QuickInfo.Model, System.Threading.CancellationToken)
   at Microsoft.CodeAnalysis.Editor.Implementation.IntelliSense.ModelComputation`1+<>c__DisplayClass17_0[[System.__Canon, mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089]].<ChainTaskAndNotifyControllerWhenFinished>b__0(System.Threading.Tasks.Task`1<System.__Canon>)
   at Roslyn.Utilities.TaskExtensions+<>c__DisplayClass16_0`2[[System.__Canon, mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089],[System.__Canon, mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089]].<SafeContinueWithFromAsync>b__0(System.Threading.Tasks.Task)
   at System.Threading.Tasks.ContinuationResultTaskFromTask`1[[System.__Canon, mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089]].InnerInvoke()
   at System.Threading.Tasks.Task.Execute()
   at System.Threading.Tasks.Task.ExecutionContextCallback(System.Object)
   at System.Threading.ExecutionContext.RunInternal(System.Threading.ExecutionContext, System.Threading.ContextCallback, System.Object, Boolean)
   at System.Threading.ExecutionContext.Run(System.Threading.ExecutionContext, System.Threading.ContextCallback, System.Object, Boolean)
   at System.Threading.Tasks.Task.ExecuteWithThreadLocal(System.Threading.Tasks.Task ByRef)
   at System.Threading.Tasks.Task.ExecuteEntry(Boolean)
   at System.Threading.Tasks.Task.System.Threading.IThreadPoolWorkItem.ExecuteWorkItem()
   at System.Threading.ThreadPoolWorkQueue.Dispatch()
   at System.Threading._ThreadPoolWaitCallback.PerformWaitCallback()

[svick]

Here is a small repro for the exception:

string code = @"
class C
{
    void M()
    {
        foreach(_ in "")
        {
        }
    }
}";

var tree = SyntaxFactory.ParseSyntaxTree(code);

var compilation = CSharpCompilation.Create(null).AddSyntaxTrees(tree);

var variable = tree.GetRoot().DescendantNodes().OfType<ForEachVariableStatementSyntax>().Single().Variable;

var model = compilation.GetSemanticModel(tree);
var symbol = model.GetSymbolInfo(variable).Symbol;

symbol.ToMinimalDisplayParts(model, variable.SpanStart);

The code above crashes with:

Unhandled Exception: System.NullReferenceException: Object reference not set to an instance of an object.
   at Microsoft.CodeAnalysis.CSharp.SymbolDisplayVisitor.VisitDiscard(IDiscardSymbol symbol) in /_/src/Compilers/CSharp/Portable/SymbolDisplay/SymbolDisplayVisitor.cs:line 221
   at Microsoft.CodeAnalysis.CSharp.SymbolDisplay.ToDisplayParts(ISymbol symbol, SemanticModel semanticModelOpt, Int32 positionOpt, SymbolDisplayFormat format, Boolean minimal) in /_/src/Compilers/CSharp/Portable/SymbolDisplay/SymbolDisplay.cs:line 116
   at Microsoft.CodeAnalysis.CSharp.Symbol.Microsoft.CodeAnalysis.ISymbol.ToMinimalDisplayParts(SemanticModel semanticModel, Int32 position, SymbolDisplayFormat format) in /_/src/Compilers/CSharp/Portable/Symbols/Symbol.cs:line 1221
   at Program.Main() in C:\code\tmp\coreapp\Program.cs:line 39

[gafter]

I am unable to repro this in master or the latest VS preview 2.0 candidate. Here is a test I am adding:

        [Fact, WorkItem(27104, "https://github.com/dotnet/roslyn/issues/27104")]
        public void BadDiscardInForeachLoop_01()
        {
            var source = @"
class C
{
    void M()
    {
        foreach(_ in """")
        {
        }
    }
}";
            var compilation = CreateCompilation(source);
            compilation.VerifyDiagnostics(
                // (6,17): error CS8186: A foreach loop must declare its iteration variables.
                //         foreach(_ in "")
                Diagnostic(ErrorCode.ERR_MustDeclareForeachIteration, "_").WithLocation(6, 17)
                );
            var tree = compilation.SyntaxTrees[0];
            var variable = tree.GetRoot().DescendantNodes().OfType<ForEachVariableStatementSyntax>().Single().Variable;
            var model = compilation.GetSemanticModel(tree);
            var symbol = model.GetSymbolInfo(variable).Symbol;
            Verify(
                symbol.ToMinimalDisplayParts(model, variable.SpanStart),
                "var _",
                SymbolDisplayPartKind.ErrorTypeName, // var
                SymbolDisplayPartKind.Space,
                SymbolDisplayPartKind.Punctuation); // _
        }