I'm trying to invoke an instance using constructor.Invoke and also passing some parameters in it, but I'm getting
System.Reflection.TargetInvocationException: 'Exception has been thrown by the target of an invocation.'
Any inputs would be helpful
Thanks
TargetInvocationException basically means that the member could be invoked successfully and the exception occurred in the invoked member itself. To differentiate it from other issues (eg. wrong instance, invalid number or type of parameters, etc.) the exception thrown by the invoked member is wrapped into a TargetInvocationException. Just look at its InnerException to see what happened.
If you want to 'unwrap' such exceptions as if you called the constructor without reflection you can do something like this:
try
{
return myConstructorInfo.Invoke(parameters);
}
catch (TargetInvocationException e)
{
// we could just throw e.InnerException but that would corrupt the
// original stack trace, showing this line as the source of the exception
ExceptionDispatchInfo.Capture(e.InnerException).Throw();
throw; // unreachable, just to satisfy the compiler if there is no return later
}
Can you hide "Exception thrown" messages in output for certain methods (certain code areas)?
I use HttpWebRequest for server communication. I periodically check if the server is available (a few times every second). When a server is not reachable HttpWebRequest throws an exception. I catch it and set GUI elements enabled to false. The problem is when the server is unreachable, output window gets cluttered up with "Exception thrown" messages.
I know you can right-click output window and uncheck "Exception Messages". But I am not only one working on the project and there might be someone who wants to see some other exception messages (in their part of the project).
Example of what I need:
// Keep showing "Exception thrown" message in this method.
static void Foo()
{
try
{
throw new NotImplementedException();
}
catch (NotImplementedException ex)
{
// Process exception
}
}
// Suppress "Exception thrown" message when it is thown in this method.
static void FooSuppress()
{
try
{
throw new ArgumentException();
}
catch (ArgumentException ex)
{
// Process exception
}
}
static void Main(string[] args)
{
Foo();
FooSuppress();
}
Current output:
Exception thrown: 'System.NotImplementedException' in ExceptionTest.dll
Exception thrown: 'System.ArgumentException' in ExceptionTest.dll
Desired output:
Exception thrown: 'System.NotImplementedException' in ExceptionTest.dll
Edit:
Enabling Just my code in Tools/Options/Debugging might help.
We used Npgsql to access PostgreSQL database and some calls had timeout. Everytime call timeouted "Exception thrown" was written to output window (and there were a lot). Just my code prevents that.
To disable the Exception messages:
(1)Like your previous reply, you could disable it in the Output windows.
(2)You could also disable it under TOOLS->Options->Debugging->Output Window.
(3)Or you could just throw the Exception using the Exception Settings under Debug menu->Windows->Exception Settings.
I don't find other workaround to disable it unless you really resolve/handle the Exceptions in your code. I test it using the VS2015 version.
No other good suggestion, but I help you submit a feature here: https://visualstudio.uservoice.com/forums/121579-visual-studio-ide/suggestions/16752127-visual-studio-suppress-certain-exception-thrown-me
You could vote it.
If you are willing to wait a little bit or use a pre-release version, the next version of Visual Studio (VS 15) will have a feature "Add Conditions to Exception Settings"
Add Conditions to Exception Settings When you configure the
debugger to break on thrown exceptions, you can add conditions so that
the debugger will only break when exceptions are thrown in specified
modules.
This will allow you to set filters on when exceptions should break.
I know that in Java, if an exception is caught by a catch clause and its catch block throws an exception, control would pass throw the associated finally block (if any) before the thread is terminated. This does not appear to be the case in C#, however.
It is possible to almost mirror this behavior in C# is by putting a try-finally statement inside the try block of the try-catch statement with the catch block that throws the exception, but that would be a problem if, for example, the finally block is supposed to contain code that disposes a Stream Writer that is supposed to log the exception.
Is there a clean way to achieve java-like try-catch-finally exception handling behavior in C#?
Here's an update with the requested sample code:
StreamWriter writer = new StreamWriter("C:\\log.txt");
try
{
throw new Exception();
}
catch (Exception e)
{
writer.WriteLine(e.Message);
throw e;
}
finally
{
if (writer != null)
{
writer.Dispose();
}
}
Add that code to a console application, run it, let the re-thrown exception go unhandled, attempt to delete C:\log.txt. You won't be able to, because control never passed through the finally block. Also, if you add a breakpoint to some line inside the finally block you will see that it doesn't get hit. (I'm using VS2005).
As far as I know, the only way to force control to pass through the finally block is if the re-thrown exception is handled by the catch block of an enclosing try block (if you took the code above and placed it inside the try block of another try-catch statement).
If the exception is not caught and is allowed to terminate the application, as in the sample code I provided, control won't pass through the finally block.
In Java it would. In C#, at least based on what I have seen, it would not.
No, this is incorrect. C# will always execute the finally block, even after an exception is thrown/re-thrown from the catch block. See When is finally run if you throw an exception from the catch block?.
In the .NET Framework, when an exception occurs, the system will determine what if anything is going to catch that exception before any finally blocks execute. Depending upon various application settings, an attempt to throw an exception which will not be caught may kill the application instantly, without giving any finally blocks (or anything else) a chance to run.
If one wraps the Main method, as well as each thread, in
try
{
...
}
catch
{
throw;
}
then any exception which is thrown within the try block will get caught. Even though it will be immediately re-thrown, any nested finally blocks will execute before the catch. There are some cases where this is desirable behavior; there are other cases where one may wish to e.g. perform some special logging if an exception isn't going to be caught (in some cases, the information one wishes to log may be destroyed if the finally blocks get a chance to run first). Within C#, there isn't any way to vary one's actions based upon whether an exception is going to be caught, but in VB.NET there are some ways via which that can be done; a VB.NET assembly which makes calls to C# code can give that code a way of knowing whether any exceptions thrown by an inner method will propagate out to the vb.net wrapper without being caught.
This does not appear to be the case in C#, however.
Is this what you are looking for.
As already stated, the finally block will always run once the catch block has been executed.
Edit: I just tried the sample code provided by the OP in a console application and lo and behold, it did not hit the finally block and had an error of "An unhandled exception of type 'System.Exception' occurred in ConsoleApplication1.exe". This was truly puzzling (besides the part of re-throwing the same exception in an endless loop) so I did a little investimagation and this is what I found:
If a exception occurs the CLR traverses up the call stack looking for
a matching catch expression. If the CLR doen't finds a matching one,
or the Exception gets re thrown each time, the Exception bubbles out
of the Main() method. In that case Windows handles the Exception.
Event Handling of Console Applications is the easiest to understand,
because there is no special Handling by the CLR. The Exception is
leaving the Applications Thread if not caught. The CLR opens a window
asking for debug or exit the application. If the user chooses to
debug, the debugger starts. If the user chooses to close, the
Application exits and the Exception is serialized and written to the
console.
Moral of the story, do not re-throw the same exception from a catch block in a console application!.
I have a delegate which is attached to an event in the Excel interop component. The goal is to update a winforms control with updated info from Excel. Since I'm changing Control properties I need to use Invoke:
public delegate void DataGridViewUpdate(object[,] data);
...
excel.InteractiveEdit( delegate(object[,] data) {
Invoke(new Common.DataGridViewUpdate(back_from_excel), new object[] { data });
});
...
private void back_from_excel(object[,] data) {
// datagridview updating code
// an exception is thrown here !
}
(This code is in the Form class that I'm updating so it's Invoking on this)
Basically my problem is that when an exception occurs in the back_from_excel(object[,] data) method, the debugger doesn't catch it. I know the delegate is running in the correct UI thread because I have no problems manipulating form controls.
Specifically what happens is that when back_from_excel hits an unhandled exception, it stops executing at that point. The rest of the application continues running and is responsive. The debugger doesn't pause. The output pane shows:
A first chance exception of type 'System.NullReferenceException' occurred in My Application.exe
A first chance exception of type 'System.NullReferenceException' occurred in System.Windows.Forms.dll
It doesn't give me any hints about which line caused the problem, just that it's somewhere in the .exe.
My question is: am I doing the Invoke thing right? It seems kind of strange to have delegate - Invoke - delegate chained together like that, but I do need to pass a delegate that Invokes a delegate. Is my problem in Visual Studio rather than in C#? If so how do I get the debugger re-attached to that UI thread?
A "first chance exception" indicates that an exception was thrown, but that it was caught at some point. Since you're passing a delegate to the InteractiveEdit method, that method can easily swallow any exceptions produced by the delegate.
By default, Visual Studio will only catch exceptions that don't get caught. If you want to catch all exceptions, regardless of whether they are caught, you need to configure Visual Studio to break on all exceptions. See this link for details on how to do this.
Take this code:
using System;
namespace OddThrow
{
class Program
{
static void Main(string[] args)
{
try
{
throw new Exception("Exception!");
}
finally
{
System.Threading.Thread.Sleep(2500);
Console.Error.WriteLine("I'm dying!");
System.Threading.Thread.Sleep(2500);
}
}
}
}
Which gives me this output:
Unhandled Exception: System.Exception: Exception!
at OddThrow.Program.Main(String[] args) in C:\Documents and Settings\username
\My Documents\Visual Studio 2008\Projects\OddThrow\OddThrow\Program.cs:line 14
I'm dying!
My question is: why does the unhandled exception text occur before the finally's? In my mind, the finally should be excuted as the stack unwinds, before we even know that this exception is unhandled. Note the calls to Sleep() - these occur after the unhandled exception is printed, as if it was doing this following:
Unhandled exception text/message
Finally blocks.
Terminate application
According to the C# standard, §8.9.5, this behaviour is wrong:
In the current function member, each try statement that encloses the throw point is examined. For each statement S, starting with the innermost try statement and ending with the outermost try statement, the following steps are evaluated:
If the try block of S encloses the throw point and if S has one or more catch clauses, the catch clauses are examined in order of appearance to locate a suitable handler for the exception. The first catch clause that specifies the exception type or a base type of the exception type is considered a match. A general catch clause (§8.10) is considered a match for any exception type. If a matching catch clause is located, the exception propagation is completed by transferring control to the block of that catch clause.
Otherwise, if the try block or a catch block of S encloses the throw point and if S has a finally block, control is transferred to the finally block. If the finally block throws another exception, processing of the current exception is terminated. Otherwise, when control reaches the end point of the finally block, processing of the current exception is continued.
If an exception handler was not located in the current function member invocation, the function member invocation is terminated. The steps above are then repeated for the caller of the function member with a throw point corresponding to the statement from which the function member was invoked.
If the exception processing terminates all function member invocations in the current thread, indicating that the thread has no handler for the exception, then the thread is itself terminated. The impact of such termination is implementation-defined.
Where am I going wrong? (I've got some custom console error messages, and this is in-the-way. Minor, just annoying, and making me question the language...)
The standard's statements about the order of execution are correct, and not inconsistent with what you are observing. The "Unhandled exception" message is allowed to appear at any point in the process, because it is just a message from the CLR, not actually an exception handler itself. The rules about order of execution only apply to code being executed inside the CLR, not to what the CLR itself does.
What you've actually done is expose an implementation detail, which is that unhandled exceptions are recognised by looking at a stack of which try{} blocks we are inside, rather than by actually exploring all the way to the root. Exceptions may or may not be handled by looking at this stack, but unhandled exceptions are recognised this way.
As you may be aware, if you put a top-level try{}catch{} in your main function, then you will see the behaviour you expect: each function's finally will be executed before checking the next frame up for a matching catch{}.
I could be way off base in the way I'm reading things...but you have a try finally block without a catch.
Juding by the description you posted, since the Exception is never caught, it bubbles up to the caller, works it's way up through the stack, is eventually unhandled, the call terminates, and then the finally block is called.
The output is actually from the default CLR exception handler. Exception Handlers occur before the finally block. After the finally block the CLR terminates because of the unhandled exception (it can't terminate before, as c# guarantees [1] that the finally clause is called).
So I'd say it's just standard behaviour, exception handling occurs before finally.
[1] guranteed during normal operation at least in absence of internal runtime errors or power outage
To add more into the mix, consider this:
using System;
namespace OddThrow
{
class Program
{
static void Main()
{
AppDomain.CurrentDomain.UnhandledException +=
delegate(object sender, UnhandledExceptionEventArgs e)
{
Console.Out.WriteLine("In AppDomain.UnhandledException");
};
try
{
throw new Exception("Exception!");
}
catch
{
Console.Error.WriteLine("In catch");
throw;
}
finally
{
Console.Error.WriteLine("In finally");
}
}
}
}
Which on my system (Norwegian) shows this:
[C:\..] ConsoleApplication5.exe
In catch
In AppDomain.UnhandledException
Ubehandlet unntak: System.Exception: Exception!
ved OddThrow.Program.Main() i ..\Program.cs:linje 24
In finally
Although not completely expected, the program does behave as it should. A finally block is not expected to be run first, it is only expected to be run always.
I adjusted your sample:
public static void Main()
{
try
{
Console.WriteLine("Before throwing");
throw new Exception("Exception!");
}
finally
{
Console.WriteLine("In finally");
Console.ReadLine();
}
}
In this case you will get the nasty unhandled exception dialog, but afterwards the console will output and wait for input, thus executing the finally, just not before windows itself catches the unhandled exception.
A try/finally without a catch will use the default handler which does exactly what you see. I use it all the time, e.g., in cases where handling the exception would be covering an error but there's still some cleanup you want to do.
Also remember that output to standard error and standard out are buffered.
The try-catch-finally blocks are working exactly as you expected if they are caught at some point. When I wrote a test program for this, and I use various nesting levels, the only case that it behaved in a way that matched what you described was when the exception was completely unhandled by code, and it bubbled out to the operating system.
Each time I ran it, the OS was what created the error message.
So the issue is not with C#, it is with the fact that an error that is unhandled by user code is no longer under the control of the application and therefore the runtime (I believe) cannot force an execution pattern on it.
If you had created a windows form application, and wrote all your messages to a textbox (then immediately flushing them) instead of writing directly to the console, you would not have seen that error message at all, because it was inserted into the error console by the calling application and not by your own code.
EDIT
I'll try to highlight the key part of that. Unhandled exceptions are out of your control, and you cannot determine when their exception handler will be executed. If you catch the exception at some point in your application, then the finally blocks will be executed before the lower-in-the-stack catch block.
To put a couple of answers together, what happens is that as soon as you have a Unhandled Exception a UnhandledExceptionEvent is raised on the AppDomain, then the code continues to execute (i.e. the finally). This is the MSDN Article on the event
Next try:
I believe this case isn't mentioned in the c# standard and I agree it seems to almost contradict it.
I believe the internal reason why this is happening is somewhat like this:
The CLR registers its default exception handler as SEH handler into FS:[0]
when you have more catches in your code, those handlers are added to the SEH chain. Alternatively, only the CLR handler is called during SEH handling and handles the CLR exception chain internally, I don't know which.
In your code when the exception is thrown, only the default handler is in the SEH chain. This handler is called before any stack unrolling begins.
The default exception handler knows that there are no exception handler registered on the stack. Therefore it calls all registered UnhandledException handler first, then prints its error message and marks the AppDomain for unloading.
Only after that stack unrolling even begins and finally blocks are called according the c# standard.
As i see it, the way the CLR handles unhandled exception isn't considered in the c# standard, only the order in which finallys are called during stack unrolling. This order is preserved. After that the "The impact of such termination is implementation-defined." clause takes effect.