How is it possible to resume code execution after an exception is thrown?
For example, take the following code:
namespace ConsoleApplication1
{
public class Test
{
public void s()
{
throw new NotSupportedException();
string #class = "" ;
Console.WriteLine(#class);
Console.ReadLine();
}
}
public class Program
{
public static void Main(string[] args)
{
try
{
new Test().s();
}
catch (ArgumentException x)
{
}
catch (Exception ex)
{
}
}
}
}
After catching the exception when stepping through, the program will stop running. How can I still carry on execution?
EDIT: What I specifically mean is the line Console.WriteLine(#class); does not seem to be hit, because when I run to it when in debug mode, the program exits from debug mode. I want to run to this line and stop at it.
Thanks
Well, you don't have any code after the catch blocks, so the program would stop running. Not sure what you're trying to do.
The following should be proof that the program doesn't simply "stop" after the catch blocks. It will execute code after the catch blocks if there is code to be executed:
static void Main(string[] args)
{
try
{
new Test().s();
}
catch (ArgumentException x)
{
Console.WriteLine("ArgumentException caught!");
}
catch (Exception ex)
{
Console.WriteLine("Exception caught!");
}
Console.WriteLine("I am some code that's running after the exception!");
}
The code will print the appropriate string depending on the exception that was caught. Then, it will print I am some code that's running after the exception! at the end.
UPDATE
In your edit you asked why Console.WriteLine(#class); does not seem to be hit. The reason is that you are explicitly throwing an exception in the very first line of your s() method; anything that follows is ignored. When an exception is encountered, execution stops and the exception is propagated up the call stack until the appropriate handler can handle it (this may be a catch block that corresponds to the try that wraps the statement in question within the same method, or it may be a catch block further up the call-stack. If no appropriate handler is found, the program will terminate with a stacktrace [at least in Java - not sure if the same happens in C#]).
If you want to hit the Console.WriteLine line, then you shouldn't be explicitly throwing an exception at the beginning of the method.
It sounds like you're wanting resumeable exceptions. C# doesn't do resumeable exceptions, and I'm doubtful that CLR supports them.
The purpose of throwing an exception is to abort a function and an entire operation (call stack) if/when something in the call environment (parameters, object state, global state) makes the function's operation impossible or invalid. Passing a zero param to a function that needs to divide a quantity by that param, for example. Division by zero won't produce a meaningful result, and if that's the sole purpose of the function, then the function can't return a meaningful result either. So, throw an exception. This will cause execution to jump to the nearest catch or finally block on the call stack. There is no returning to the function that threw the exception.
If you want to step into your code in the debugger to trace the Console.WriteLine() calls, you need to remove the throw new NotSupportedException() line from your code and recompile.
If you're worried that an exception will be thrown in the method but you want the method to continue, add an error handler inside the method.
class Test
{
public void s()
{
try
{
// Code that may throw an exception
throw new NotSupportedException();
}
catch(Exception ex)
{
// Handle the exception - log?, reset some values?
}
string #class = "" ;
Console.WriteLine(#class);
Console.ReadLine();
}
}
You could also return a bool or some other value to indicate the state.
Disclaimer: I am not suggesting that you actually do this.
You can mimic the old VB style On Error Resume Next with the following code.
public static class ControlFlow
{
public static Exception ResumeOnError(Action action)
{
try
{
action();
return null;
}
catch (Exception caught)
{
return caught;
}
}
}
And then it could be used like the following.
public static void Main()
{
ControlFlow.ResumeOnError(() => { throw new NotSupportedException(); });
ControlFlow.ResumeOnError(() => { Console.WriteLine(); });
ControlFlow.ResumeOnError(() => { Console.ReadLine(); });
}
Some simple code I put together to catch exceptions that are thrown inside a catch block:
try
{
//do code here
}
catch (Exception ex)
{
try { SomeMethod1(); }
catch { }
try { SomeMethod2(); }
catch { }
try { SomeMethod3(); }
catch { }
}
finally
{
//cleanup goes here
}
Execution is still carying on but there is no code after the exception is caught. If you want to repeatedly call s then consider wrapping the try/catch block in a while loop.
The program stops running because there is no following code to be executed in the Main() method! You can add the following line to your code to keep the program running until there is a console input:
Console.ReadLine();
For that code, you can't. If you break the tasks up to smaller chunks, you can resume at the next chunk. But normally it's easier to have a different mechanism than exceptions to report non-fatal errors, such as a callback function which returns whether or not to continue.
You can use the "step-over" feature in debugging to achieve this on a per-run basis.
Instead of thowing the NotSupportedException, you could track that an exception was encountered, use a default value, and throw the exception at the end of the method call:
namespace ConsoleApplication1
{
public class Test
{
public void s()
{
bool exceptionEncountered = false;
if(someConditionNotSupported){//stub condition
exceptionEncountered=true
#class="DefaultValue";
}
Console.WriteLine(#class);
Console.ReadLine();
if(exceptionEncountered){
throw new NotSupportedException();
}
}
}
public class Program
{
public static void Main(string[] args)
{
try
{
new Test().s();
}
catch (ArgumentException x)
{
}
catch (Exception ex)
{
}
}
}
}
public static void Main()
{
for (int j = 0; j <= 100000; j++)
{
try
{
// TODO: Application logic...
}
catch
{
System.Threading.Thread.Sleep(1000);
}
}
}
Related
I couldn't find any Microsoft official resource that shows how code below the finally block execute, the only information in regards to it is the book CLR via C#, the author says:
Code below the finally block executes if no exception is thrown within the try block or if a catch block catches the exception and doesn't throw or re-throw an exception.
Let's say we have the following code:
class Program {
static void Main(string[] args) {
SomeMethod1();
Console.ReadLine();
}
static void SomeMethod1() {
try {
SomeMethod2();
}
finally {
Console.WriteLine("SomeMethod1 finally");
}
Console.WriteLine("SomeMethod1 last");
}
static void SomeMethod2() {
try {
SomeMethod3();
}
catch (DivideByZeroException e) {
Console.WriteLine("SomeMethod2 caught");
}
finally {
Console.WriteLine("SomeMethod2 finally");
}
Console.WriteLine("SomeMethod2 last");
}
static void SomeMethod3() {
try {
SomeMethod4();
}
finally {
Console.WriteLine("SomeMethod3 finally");
}
Console.WriteLine("SomeMethod3 last");
}
static void SomeMethod4() {
try {
Int32 i = 0;
var c = 3 / i;
}
finally {
Console.WriteLine("SomeMethod4 finally");
}
Console.WriteLine("SomeMethod4 last");
}
}
and the output is:
SomeMethod4 finally
SomeMethod3 finally
SomeMethod2 caught
SomeMethod2 finally
SomeMethod2 last
SomeMethod1 finally
SomeMethod1 last
You can see that "SomeMethod4 last" and "SomeMethod3 last" doesn't get printed.
"SomeMethod4 last" doesn't get printed is easy to understand as SomeMethod4 throw an exception and there is no catch block to catch the exception, so it doesn't meet the requirement that specified by the author, fair enough.
But why "SomeMethod3 last" doesn't get printed? there is no exception thrown at SomeMethod3, just like SomeMethod1, so why "SomeMethod1 last" get printed while "SomeMethod3 last" doesn't? Is any Microsoft official resource that explain the mechanics of it?
When the exception is thrown in SomeMethod4, it is flowing up to the calling method which is SomeMethod3. Think of it as if SomeMethod4 was actually inline inside SomeMethod3 like this.
static void SomeMethod3()
{
try
{
try
{
Int32 i = 0;
var c = 3 / i;
}
finally
{
Console.WriteLine("SomeMethod4 finally");
}
Console.WriteLine("SomeMethod4 last");
}
finally
{
Console.WriteLine("SomeMethod3 finally");
}
Console.WriteLine("SomeMethod3 last");
}
The reason SomeMethod2 Last is being run is because it is the part that actually catches the exception.
If you add a catch to SomeMethod3, you will see that SomeMethod3 Last gets printed as well.
Exceptions break the flow of the application, and basically don't stop going back up the call stack until it reaches a catch statement. Technically the exception is being thrown inside SomeMethod1, but its being caught in SomeMethod2 which means the exception never reaches SomeMethod1.
While looking on C# try catch tutorial, I got following question. My sample code as follows,
Inside mainMethod() , I need to call three separate methods. Inside testMethodOne(), I need to handle exception as. If testMethodOne() throws exception, without executing testMethodTwo(dt), mainMethod() throwing exception. I need to call testMethodTwo(dt); and testMethodThreee(dt); if testMethodOne() throws exception, how can I do it.
public void MainMethod(data dt){
try{
testMethodOne(dt);
testMethodTwo(dt);
testMethodThreee(dt);
}catch(Exception ex){
throw ex;
}
}
public void testMethodOne(dt){
try
{
// Block of code to try
}
catch (Exception e)
{
// Block of code to handle errors
}
}
I understood your question as follows (but I might be wrong, your questions is not very clear):
Even if one of your testMethods throws an exception, you still want to continue in the normal program flow with the other methods. If at least one of the method failed, mainMethod could then report this as AggregateException.
public void MainMethod(data dt)
{
var exceptions = new List<Exception>();
try
{
testMethodOne(dt);
}
catch (Exception ex)
{
exceptions.Add(ex);
}
try
{
testMethodTwo(dt);
}
catch (Exception ex)
{
exceptions.Add(ex);
}
try
{
testMethodThreee(dt);
}
catch (Exception ex)
{
exceptions.Add(ex);
}
if (exceptions.Count > 0)
{
throw new AggregateException(exceptions);
}
}
It seems as if you want exceptions to alter the flow of your main method without breaking everything. One easy method is to make each 'testmethod' return a boolean.
public bool testMethodOne(dt){
try
{
// Block of code to try
return true;
}
catch (Exception e)
{
// Block of code to handle errors
return false;
}
}
Then in your main code you can go
if(!testMethodOne(dt))
if(!testMethodTwo(dt))
if(!testMethodThree(dt))
//log that all methods failed
The above snippet would try each method until it finds one that succeeds. If that's not the behaviour you are looking for can you reword your question to make it clearer? If you want the opposite to happen just get rid of the ! and it will go until one fails. Alternatively you could put a throw in your catch statement in each of the testMethods, and that would stop execution once one is reached as well.
My simple example is:
void FixedUnalterableMethod()
{
try
{
throw new Exception("Exception 1"); //line 12.
}
finally
{
throw new Exception("Exception 2"); //line 16.
}
}
void Method1()
{
try
{
FixedUnalterableMethod(); //line 24.
}
catch (Exception ex)
{
var messageWithStackTrace = ex.ToString(); //line 28.
Console.WriteLine(messageWithStackTrace);
}
}
The console output is:
System.Exception: Exception 2
at Program.FixedUnalterableMethod() in ...\Program.cs:line 16
at Program.Main(String[] args) in ...\Program.cs:line 24
The question is, how to be informed that the Exception 1 has occured?
Is there a way how to include Exception 1 in my StackTrace (in line 28.) ?
Of coure I can't modify the FixedUnalterableMethod() method!
Yes, this is possible, though pretty nasty!
It is a little known fact that CLR exceptions do not cause the execution of finally blocks until the exception has actually been caught. This is disguised somewhat because if an exception is not caught (and makes it out of Main) then the default behaviour of the CLR hosting code is to run finally blocks for you, giving the illusion that they always run.
However, there is a way to examine an exception before catching it, to decide if you want to catch it. Try this:
static bool StoreFirstException(Exception x, Action<Exception> store)
{
if (x.Message == "Exception 1")
{
store(x);
}
return true;
}
static void Method1()
{
Exception firstException = null;
try
{
FixedUnalterableMethod(); //line 24.
}
catch (Exception ex) when (StoreFirstException(ex, x => firstException = x))
{
Console.WriteLine(firstException);
Console.WriteLine(ex);
}
}
The catch... when feature lets you write a boolean expression to examine the exception. Here I check the message (the only distinguishing fact you've given me) and if it's the first exception I pass it to the store action.
The caller uses this callback to stash the first exception.
Then it votes to catch, which only then causes the finally block to execute, which throws the second exception. The same when clause examines it but this time doesn't offer it to store. So then I have both exceptions in the catch block and I log them both. My console shows the two exceptions with the correct source line numbers.
Here's version that doesn't look at the message; it just assumes the first exception it see must be the interesting one. Also it's neater to use a nested function:
static void Method1()
{
Exception firstException = null;
bool StoreFirstException(Exception x)
{
if (firstException == null) firstException = x;
return true;
}
try
{
FixedUnalterableMethod(); //line 24.
}
catch (Exception ex) when (StoreFirstException(ex))
{
Console.WriteLine(firstException);
Console.WriteLine(ex);
}
}
If the "exception type" is literally the same you probably have little choice but the examine the Message property, which can be problematic to say the least.
Looking at that code again you will only ever see the 1 exception anyway, the one on line 16.
Thanks to #Daniel Earwicker the working solution is:
void FixedUnalterableMethod()
{
try
{
throw new Exception("Exception 1"); //line 12.
}
finally
{
throw new Exception("Exception 2"); //line 16.
}
}
void Method1()
{
bool CatchException(Exception ex)
{
//Log...
Console.WriteLine(ex.ToString());
return true;
}
try
{
FixedUnalterableMethod(); //line 24.
}
catch (Exception ex) when (CatchException(ex))
{
//do something with the lastest exception
}
}
I have a general retry on exception handler which I would like it to repeat a function for a definite number of time and here's the code for it
public static void Retry(this MethodInfo methodInfo, object[] parametrsList, short after = 0, short? retry = 1)
{
if (retry < 0)
return;
try
{
short waitingPeriodMs = after*1000;
Thread.Sleep(waitingPeriodMs);
Type classObjType = methodInfo.ReflectedType;
object classObj = Activator.CreateInstance(classObjType);
methodInfo.Invoke(classObj, parametrsList);
}
catch (TargetInvocationException ex)
{
Debug.WriteLine("Exception Caught");
methodInfo.Retry(parametrsList, after, --retry);
}
catch (Exception ex)
{
Debug.WriteLine("Exception Caught");
methodInfo.Retry(parametrsList, after, --retry);
}
}
The problem is whenever I an unhandled exception is throw in the method that I'm invoking neither of the two catch statements catches it
I've made sure that the Use Only My Code checkbox is not checked
I've used those two Debug.Writeline statements to make sure that it's not a debugger related issue and checked the Output window and made sure those two statements were not executed
P.S. I'm aware the using a general retry on exception code is risky and could lead to an infinite number of retries the I'm using it because of a project related reason
Update:
A unit test example that reproduces the question
[TestClass]
public class ExceptionTest
{
[TestMethod]
public void TestExceptionRetry()
{
Action act = () => { throw new Exception(); };
act.Method.Retry(new object[0]);
}
}
This issue drives me insane, as I can't see what's causing it. The behavior is unexpected and I just can't see how it can occur. When I execute my plugin using the code below, I get the error message "bada boom". Just as expected.
public void Execute(IPluginExecutionContext context)
{
throw new Exception("bada boom");
try
{
throw new Exception("bada bing");
...
} catch (Exception) { }
...
}
However, when I comment out the first throw, I don't get to see "bada bing". Instead, I get "object not set to a reference"! What the duck?! (Typo intended.)
public void Execute(IPluginExecutionContext context)
{
//throw new Exception("bada boom");
try
{
throw new Exception("bada bing");
...
} catch (Exception) { }
...
}
Here's graphics for the issue.
In your first example the exception is handled at the caller level and up till a catch block is found. In the second example the exception is handled in the mandatory catch or finally block associated with the try of the Execute method.
So in the code inside that catch or finally block you have a null reference exception
void Main()
{
try
{
Execute();
}
catch(Exception x)
{
Console.WriteLine("In main: " + x.Message);
}
}
public void Execute()
{
// Goes to the catch block in main
//throw new Exception("bada boom");
try
{
// Goes to the catch block associated with this try
throw new Exception("bada bing");
}
catch(Exception x)
{
// Uncomment this to see the null reference exception in main
// Console.WriteLine("In Execute: " + x.InnerException.Message);
Console.WriteLine("In Execute:" + x.Message);
}
}
Well, of course, when I say mandatory I want to say that you cant write
try
{
....
}
without a catch or a finally clause following the try. And you can write a catch or finally block without code, but you can't omit one of the two keywords and its block