Why is .NET exception not caught by try/catch block?

Question

I\'m working on a project using the ANTLR parser library for C#. I\'ve built a grammar to parse some text and it works well. However, when the parser comes across an illeg

Answer 1

I downloaded your code and everything work as expected.

Visual Studio debugger correctly intercepts all exceptions. Catch blocks work as expected.

I'm running Windows 2003 server SP2, VS2008 Team Suite (9.0.30729.1 SP)

I tried to compile you project for .NET 2.0, 3.0 & 3.5

@Steve Steiner, debugger options you mentioned have nothing to do with this behavior.

I tried to play with these options with no visible effects - catch blocks managed to intercept all exceptions.

Answer 2

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.

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:

         [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] 

If you right click in the callstack window and run turn on show external code you see this:

        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

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.

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.

Things to play with to see how this looks for those who didn't repro the problem:

• Go to Tools/Options/Debugging and turn off "Enable Just My code (Managed only)". or option.
• Go to Debugger/Exceptions and turn off "User-unhandled" for Common-Language Runtime Exceptions.

Answer 3

Is it possible that the exception is being thrown in another thread? Obviously your calling code is single threaded, but maybe the library you are consuming is doing some multithreaded operations under the covers.

Answer 4

I downloaded the sample VS2008 project, and am a bit stumped here too. I was able to get past the exceptions however, although probably not in a way that will work will great for you. But here's what I found:

This mailing list post had a discussion of what looks to be the same issue you are experiencing.

From there, I added a couple dummy classes in the main program.cs file:

class MyNoViableAltException : Exception
{
    public MyNoViableAltException()
    {
    }
    public MyNoViableAltException(string grammarDecisionDescription, int decisionNumber, int stateNumber, Antlr.Runtime.IIntStream input)
    {
    }
}
class MyEarlyExitException : Exception
{
    public MyEarlyExitException()
    {
    }

    public MyEarlyExitException(int decisionNumber, Antlr.Runtime.IIntStream input)
    {
    }
}

and then added the using lines into TimeDefParser.cs and TimeDefLexer.cs:

using NoViableAltException = MyNoViableAltException;
using EarlyExitException = NoViableAltException; 

With that the exceptions would bubble into the fake exception classes and could be handled there, but there was still an exception being thrown in the mTokens method in TimeDefLexer.cs. Wrapping that in a try catch in that class caught the exception:

            try
            {
                alt4 = dfa4.Predict(input);
            }
            catch
            {
            }

I really don't get why wrapping it in the internal method rather than where it is being called from handle the error if threading isn't in play, but anyways hopefully that will point someone smarter than me here in the right direction.

Answer 5

Are you using .Net 1.0 or 1.1? If so then catch(Exception ex) won't catch exceptions from unmanaged code. You'll need to use catch {} instead. See this article for further details:

http://www.netfxharmonics.com/2005/10/net-20-trycatch-and-trycatchexception/

Answer 6

"Also, you can put some code in to catch all unhandled exceptions. Read the link for more info, but the basics are these two lines."

This is false. This used to catch all unhandled exceptions in .NET 1.0/1.1 but it was a bug and it wasn't supposed to and it was fixed in .NET 2.0.

AppDomain.CurrentDomain.UnhandledException 

Is only intended to be used as a last chance logging saloon so you can log the exception before the program exits. It wont catch the exception as of 2.0 onwards (although in .NET 2.0 at least there is a config value you can modify to make it act like 1.1 but it isn't recommended practice to use this.).

Its worth noting that there are few exceptions that you cannot catch, such as StackOverflowException and OutOfMemoryException. Otherwise as other people have suggested it might be an exception in a background thread somewhere. Also I'm pretty sure you can't catch some/all unmanaged/native exceptions either.