Updating the managed debugging API for .NET v4

In any successful investigation, the right tools play a big part in collecting evidence about the state of the “crime scene” as it was before the detectives arrived. Unfortunately for the Crash Scene Investigator, we don’t have the budget to fly out to the customer’s site, chalk the outline, and eat their doughnuts. We have to rely on the end-user to collect the evidence for us, which means giving them the fingerprint dust and the evidence baggies and leaving them to it. With that in mind, the Red Gate support team have been writing tools that can collect vital clues with a minimum of fuss.

Years ago we would have asked for a memory dump, where we used to get the customer to run CDB.exe and produce dumps that we could analyze in-house, but those dumps were pretty unwieldy (500MB files) and the debugger often didn’t dump exactly where we wanted, or made five or more dumps. What we wanted was just the minimum state information from the program at the time of failure, so we produced a managed debugger that captured every first and second-chance exception and logged the stack and a minimal amount of variables from the memory of the application, which could all be exported as XML. This caused less inconvenience to the end-user because it is much easier to send a 65KB XML file in an email than a 500MB file containing all of the application’s memory. We don’t need to have the entire victim shipped out to us when we just want to know what was under the fingernails.

The thing that made creating a managed debugging tool possible was the MDbg Engine example written by Microsoft as part of the Debugging Tools for Windows distribution. Since the ICorDebug interface is a bit difficult to understand, they had kindly created some wrappers that provided an event-driven debugging model that was perfect for our needs, but .NET 4 applications under debugging started complaining that “The debugger’s protocol is incompatible with the debuggee”. The introduction of .NET Framework v4 had changed the managed debugging API significantly, however, without an update for the MDbg Engine code!

After a few hours of research, I had finally worked out that most of the version 4 ICorDebug interface still works much the same way in “legacy” v2 mode and there was a relatively easy fix for the problem in that you can still get a reference to legacy ICorDebug by changing the way the interface is created.

In .NET v2, the interface was acquired using the CreateDebuggingInterfaceFromVersion method in mscoree.dll. In v4, you must first create IClrMetaHost, enumerate the runtimes, get an ICLRRuntimeInfo interface to the .NET 4 runtime from that, and use the GetInterface method in mscoree.dll to return a “legacy” ICorDebug interface. The rest of the MDbg Engine will continue working the old way. Here is how I had changed the MDbg Engine code to support .NET v4:

private void InitFromVersion(string debuggerVersion)
{
if( debuggerVersion.StartsWith(“v1”) )
{
throw new ArgumentException( “Can’t debug a version 1 CLR process (“” + debuggerVersion +
“”). Run application in a version 2 CLR, or use a version 1 debugger instead.” );
}

ICorDebug rawDebuggingAPI=null;
if (debuggerVersion.StartsWith(“v4”))
{
Guid CLSID_MetaHost = new Guid(“9280188D-0E8E-4867-B30C-7FA83884E8DE”);
Guid IID_MetaHost = new Guid(“D332DB9E-B9B3-4125-8207-A14884F53216”);
ICLRMetaHost metahost = (ICLRMetaHost)NativeMethods.ClrCreateInterface(CLSID_MetaHost, IID_MetaHost);
IEnumUnknown runtimes = metahost.EnumerateInstalledRuntimes();
ICLRRuntimeInfo runtime = GetRuntime(runtimes, debuggerVersion);
//Defined in metahost.h
Guid CLSID_CLRDebuggingLegacy = new Guid(0xDF8395B5, 0xA4BA, 0x450b, 0xA7, 0x7C, 0xA9, 0xA4, 0x77, 0x62, 0xC5, 0x20);
Guid IID_ICorDebug = new Guid(“3D6F5F61-7538-11D3-8D5B-00104B35E7EF”);
Object res;
runtime.GetInterface(ref CLSID_CLRDebuggingLegacy, ref IID_ICorDebug, out res);
rawDebuggingAPI = (ICorDebug)res;
}
else
rawDebuggingAPI = NativeMethods.CreateDebuggingInterfaceFromVersion((int)CorDebuggerVersion.Whidbey,debuggerVersion);
if (rawDebuggingAPI != null)
InitFromICorDebug(rawDebuggingAPI);
else
throw new ArgumentException(“Support for debugging version ” + debuggerVersion + ” is not yet implemented”);
}

The changes above will ensure that the debugger can support .NET Framework v2 and v4 applications with the same codebase, but we do compile two different applications: one targeting v2 and the other v4.

As a footnote I need to add that some missing native methods and wrappers, along with the EnumerateRuntimes method code, came from the Mindbg project on Codeplex.

Another change is that when using the MDbgEngine.CreateProcess to launch a process in the debugger, do not supply a null as the final argument. This does not work any more because GetCORVersion always returns “v2.0.50727” as the function has been deprecated in .NET v4. What’s worse is that on a system with only .NET 4, the user will be prompted to download and install .NET v2! Not nice! This works much better:

proc = m_Debugger.CreateProcess(ProcessName, ProcessArgs, DebugModeFlag.Default,
String.Format(“v{0}.{1}.{2}”,
System.Environment.Version.Major,System.Environment.Version.Minor,
System.Environment.Version.Build));

Microsoft “unofficially” plan on updating the MDbg samples soon, but if you have an MDbg-based application, you can get it working right now by changing one method a bit and adding a few new interfaces (ICLRMetaHost, IEnumUnknown, and ICLRRuntimeInfo).

The new, non-legacy implementation of MDbg Engine will add new, interesting features like dump-file support and by association I assume garbage-collection/managed object stats, so it will be well worth looking into if you want to extend the functionality of a managed debugger going forward.