How to prevent iOS crash reporters from crashing MonoTouch apps?

Put this in AppDelegate.cs:

[DllImport ("libc")]
private static extern int sigaction (Signal sig, IntPtr act, IntPtr oact);

enum Signal {
    SIGBUS = 10,
    SIGSEGV = 11
}

static void EnableCrashReporting ()
{
    IntPtr sigbus = Marshal.AllocHGlobal (512);
    IntPtr sigsegv = Marshal.AllocHGlobal (512);

    // Store Mono SIGSEGV and SIGBUS handlers
    sigaction (Signal.SIGBUS, IntPtr.Zero, sigbus);
    sigaction (Signal.SIGSEGV, IntPtr.Zero, sigsegv);

    // Enable crash reporting libraries
    EnableCrashReportingUnsafe ();

    // Restore Mono SIGSEGV and SIGBUS handlers            
    sigaction (Signal.SIGBUS, sigbus, IntPtr.Zero);
    sigaction (Signal.SIGSEGV, sigsegv, IntPtr.Zero);

    Marshal.FreeHGlobal (sigbus);
    Marshal.FreeHGlobal (sigsegv);
}

static void EnableCrashReportingUnsafe ()
{
    // Run your crash reporting library initialization code here--
    // this example uses HockeyApp but it should work well
    // with TestFlight or other libraries.

    // Verify in documentation that your library of choice
    // installs its sigaction hooks before leaving this method.

    var manager = BITHockeyManager.SharedHockeyManager;
    manager.Configure (HockeyAppId, null);
    manager.StartManager ();
}

Call EnableCrashReporting () in the beginning of FinishedLaunching method.
Wrap this call in #if !DEBUG directive if you want.

How does it work?

I followed Rolf's suggestion:

One possible solution is to allow mono to handle all SIGSEGV signals (technically speaking the crash reporting lib should either not handle the SIGSEGV signal, or it should chain to mono's handler and not do any processing by itself). If mono determines that the SIGSEGV signal is not from managed code (i.e. something very bad happened), it will raise a SIGABORT signal (which the crash reporting lib should already handle and treat as a crash). As you can understand this is something that has to be done in the crash reporting library.

And Landon Fuller's Objective C implementation:

#import <signal.h>

@implementation AppDelegate

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions {
    /* Save Mono's signal handler actions */
    struct sigaction sigbus_action, sigsegv_action;
    sigaction(SIGBUS, NULL, &sigbus_action);
    sigaction(SIGSEGV, NULL, &sigsegv_action);

    // Enable the crash reporter here. Ie, [[PLCrashReporter sharedReporter] enableCrashReporterAndReturnError:],
    // or whatever is the correct initialization mechanism for the crash reporting service you're using

    /* Restore Mono's signal handlers */
    sigaction(SIGBUS, &sigbus_action, NULL);
    sigaction(SIGSEGV, &sigsegv_action, NULL);

    return YES;
}

I used Banshee source code as a reference point for how to call sigaction from MonoTouch.

Hope it helps!

Starting with Xamarin.iOS 10.4 there's now a supported way of doing this:

static void EnableCrashReporting ()
{
    try {
    } finally {
        Mono.Runtime.RemoveSignalHandlers ();
        try {
            EnableCrashReportingUnsafe ();
        } finally {
            Mono.Runtime.InstallSignalHandlers ();
        }
    }
}

static void EnableCrashReportingUnsafe ()
{
    // Run your crash reporting library initialization code here--
    // this example uses HockeyApp but it should work well
    // with TestFlight or other libraries.

    // Verify in documentation that your library of choice
    // installs its sigaction hooks before leaving this method.

    // Have in mind that at this point Mono will not handle
    // any NullReferenceExceptions, if there are any 
    // NullReferenceExceptions on any thread (not just the current one),
    // then the app will crash.

    var manager = BITHockeyManager.SharedHockeyManager;
    manager.Configure (HockeyAppId, null);
    manager.StartManager ();
}