NTSD and SOS: StopOnException

2009-01-15 original post code sample →

!StopOnException comes in handy when you want to break when a specific exception occurs.

The program below throws an OutOfMemoryException on every iteration of the loop except one, where it throws a single ArgumentException. That gives you something to practise on: plenty of noise, and one exception you actually care about.

using System;
class Program
{
    static void Main(string[] args)
    {
        Program p = new Program();
        p.ExceptionSample();
    }
    private void ExceptionSample()
    {
        int i=0;
        while (i < 100)
        {
            if (i == 60)
            {
                try
                {
                    throw new ArgumentException();
                }
                catch (Exception)
                {
                }
            }
            else
            {
                try
                {
                    throw new OutOfMemoryException();
                }
                catch (Exception)
                {
                }
            }
            i++;
        }
    }
}

Build it with debug information and start it under the debugger:

Console

csc /debug Program.cs
ntsd program.exe

Add the current directory to the symbol path, break when the runtime is loaded, and let the program go:

Console

.sympath+ .
sxe ld mscorwks
g

Now that mscorwks is loaded, load SOS, tell it to stop the first time an ArgumentException is created, and continue:

Console

.loadby sos mscorwks
!StopOnException -Create System.ArgumentException 1
g

The debugger breaks on the iteration that throws ArgumentException. !ClrStack -a shows the managed stack along with the parameters and locals of each frame:

Console

!ClrStack -a
OS Thread Id: 0x103c (0)
Child-SP         RetAddr          Call Site
000000000019ee50 000007ff00180182 Program.ExceptionSample()
    PARAMETERS:
        this = 0x0000000002603358
    LOCALS:
        0x000000000019ee78 = 0x000000000000003c
        0x000000000019ee7c = 0x0000000000000000
000000000019eec0 000007fefa3f2672 Program.Main(System.String[])
    PARAMETERS:
        args = 0x0000000002603338
    LOCALS:
        0x000000000019eee0 = 0x0000000002603358

The first local of ExceptionSample is 0x3c, which is 60 — exactly the iteration that throws the ArgumentException.

!pe prints the details of the exception itself:

Console

!pe
Exception object: 00000000026087d0
Exception type: System.ArgumentException
Message: Value does not fall within the expected range.
InnerException: <none>
StackTrace (generated):
<none>
StackTraceString: <none>
HResult: 80070057

This will stop when the exception occurs. You can print the details of the exception using !pe and look into the locals using !ClrStack -a. This comes in handy when you want to debug a problem and break at the point the exception is thrown, rather than sifting through every exception the process happens to raise.