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copied!<p>I believe I understand the problem. The exception is being caught, the issue is confusion over the debugger's behavior and differences in the debugger settings among each person trying to repro it.</p> <p>In the 3rd case from your repro I believe you are getting the following message: "NoViableAltException was unhandled by user code" and a callstack that looks like this:</p> <pre> [External Code] > TestAntlr-3.1.exe!TimeDefLexer.mTokens() Line 852 + 0xe bytes C# [External Code] TestAntlr-3.1.exe!TimeDefParser.prog() Line 141 + 0x14 bytes C# TestAntlr-3.1.exe!TestAntlr_3._1.Program.ParseTest(string Text = "foobar;") Line 49 + 0x9 bytes C# TestAntlr-3.1.exe!TestAntlr_3._1.Program.Main(string[] args = {string[0x00000000]}) Line 30 + 0xb bytes C# [External Code] </pre> <p>If you right click in the callstack window and run turn on show external code you see this:</p> <pre> Antlr3.Runtime.dll!Antlr.Runtime.DFA.NoViableAlt(int s = 0x00000000, Antlr.Runtime.IIntStream input = {Antlr.Runtime.ANTLRStringStream}) + 0x80 bytes Antlr3.Runtime.dll!Antlr.Runtime.DFA.Predict(Antlr.Runtime.IIntStream input = {Antlr.Runtime.ANTLRStringStream}) + 0x21e bytes > TestAntlr-3.1.exe!TimeDefLexer.mTokens() Line 852 + 0xe bytes C# Antlr3.Runtime.dll!Antlr.Runtime.Lexer.NextToken() + 0xc4 bytes Antlr3.Runtime.dll!Antlr.Runtime.CommonTokenStream.FillBuffer() + 0x147 bytes Antlr3.Runtime.dll!Antlr.Runtime.CommonTokenStream.LT(int k = 0x00000001) + 0x2d bytes TestAntlr-3.1.exe!TimeDefParser.prog() Line 141 + 0x14 bytes C# TestAntlr-3.1.exe!TestAntlr_3._1.Program.ParseTest(string Text = "foobar;") Line 49 + 0x9 bytes C# TestAntlr-3.1.exe!TestAntlr_3._1.Program.Main(string[] args = {string[0x00000000]}) Line 30 + 0xb bytes C# [Native to Managed Transition] [Managed to Native Transition] mscorlib.dll!System.AppDomain.ExecuteAssembly(string assemblyFile, System.Security.Policy.Evidence assemblySecurity, string[] args) + 0x39 bytes Microsoft.VisualStudio.HostingProcess.Utilities.dll!Microsoft.VisualStudio.HostingProcess.HostProc.RunUsersAssembly() + 0x2b bytes mscorlib.dll!System.Threading.ThreadHelper.ThreadStart_Context(object state) + 0x3b bytes mscorlib.dll!System.Threading.ExecutionContext.Run(System.Threading.ExecutionContext executionContext, System.Threading.ContextCallback callback, object state) + 0x81 bytes mscorlib.dll!System.Threading.ThreadHelper.ThreadStart() + 0x40 bytes </pre> <p>The debugger's message is telling you that an exception originating outside your code (from NoViableAlt) is going through code you own in TestAntlr-3.1.exe!TimeDefLexer.mTokens() without being handled. </p> <p>The wording is confusing, but it does not mean the exception is uncaught. The debugger is letting you know that code you own mTokens()" needs to be robust against this exception being thrown through it.</p> <p>Things to play with to see how this looks for those who didn't repro the problem:</p> <ul> <li>Go to Tools/Options/Debugging and turn off "Enable Just My code (Managed only)". or option.</li> <li>Go to Debugger/Exceptions and turn off "User-unhandled" for Common-Language Runtime Exceptions.</li> </ul>

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