从AppDomain.AssemblyLoad事件中抛出异常

Question

Can someone explain to me why I cannot seem to throw an exception from inside the AppDomain.Assembly load event? For example:

class Program
{
    static Program()
    {
        AppDomain.CurrentDomain.UnhandledException += (s, a) =>
        {
            Console.WriteLine("Caught exception!");
        };

        AppDomain.CurrentDomain.AssemblyLoad += (s, a) =>
        {
            Console.WriteLine(string.Format("Assembly {0} loaded", a.LoadedAssembly.FullName));

            throw new Exception();

            Console.WriteLine("Should never get here...");
        };
    }

    static void Main(string[] args)
    {
        Console.WriteLine(new ClassLibrary1.Class1().TestString());
        Console.WriteLine();
        Console.WriteLine("Done...");
        Console.ReadLine();
    }
}

When I execute this, the output is as follows:

Assembly ClassLibrary1, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null loaded
TestString
Done...

Can anyone explain this behavior to me? Thank you.

EDIT To clarify a couple things:

• The assembly load event runs fine, when I expect it to run. But my exception never gets thrown

• This is a distilled example taken from a larger application. I want to inspect the assembly after it is loaded and if I don't like something about it, I want to fail fast... But my exception doesn't 'happen'

Answer 1

This happens because of the way the JIT compiler works. It needs to generate the code for the Main() method before it can start to run. Since you are referencing the ClassLibrary1.Class1() type, it needs to load that assembly to retrieve type info. That requires it to load the assembly before your code starts running. Change it like this to get the exception:

using System.Runtime.CompilerServices;
...
    static void Main(string[] args) {
        Test();
    }
    [MethodImpl(MethodImplOptions.NoInlining)]
    static void Test() {
        Console.WriteLine(new ClassLibrary1.Class1().TestString());
        Console.WriteLine();
        Console.WriteLine("Done...");
        Console.ReadLine();
    }

Now the static constructor can run first and register the AssemblyLoad event handler before the ClassLibrary1 assembly gets loaded.

Answer 2

The exception is being thrown. But it seems .Net sometimes ignores exceptions happening in startup (Main()). I'm not sure of the reason, but I usually go to Debug->Exceptions and check the box "Throw" for Common Language Runtime Exceptions" to be able to break at the exception.

Answer 3

Why do you think the exception is not being thrown? If it weren't being thrown, one would expected to see your "Should never get here..." output. However, since it's not there, the exception presumably is being thrown.

Your code not catching the exception is a whole other story. It's quite possible that the code that raises the AppDomain.AssemblyLoad event is catching exceptions.

Answer 4

I believe the assembly load event is happening on a separate thread,using asynccallback. You are not getting the exception because you need to use Application.ThreadException+=new System.Threading.ThreadExceptionEventHandler(Application_ThreadException);

I think, I'm not an expert on this at all