I've read How to rethrow InnerException without losing stack trace in C#? and the answer here may be the same; on the other hand my situation is sufficiently different that someone might be able to suggest a better approach.
My simplified structure is
try {
something();
} catch (MyException e1) {
try {
somethingElse();
} catch (MyException e2) {
throw e1;
}
}
That is to say, calling somethingElse() is an attempt to recover from the original exception, and if the recovery attempt fails, I want to throw the original exception, not the one that arose from the recovery attempt.
I get a warning from the compiler about rethrowing exceptions losing the stack trace (and of course, I don't like leaving my code with warning conditions). But what should I do about it? My IDE (Rider) suggests changing the throw e1; to throw;, but that would presumably rethrow e2.
Are there any solutions short of the convoluted ideas proposed in the cited question for inner exceptions?
To be honest, I don't really care about the imperfect stack trace - I just want to get rid of the warnings.
There is no way to rethrow an exception from an outer catch block inside an inner catch block. The best way to achieve this pattern is to note whether or not the inner operation succeeded :
try {
something();
} catch (MyException e1) {
bool recovered=false;
try {
somethingElse();
recovered=true;
} catch {
}
if (!recovered) {
throw;
}
}
There is the ExceptionDispatchInfo class.
This lets you capture an exception and re-throw it without changing the stack-trace:
public static void Method()
{
try
{
Something();
}
catch (MyException e1)
{
try
{
SomethingElse();
}
catch (MyException)
{
//throw e1;
ExceptionDispatchInfo.Capture(e1).Throw();
throw;
}
}
}
A regular throw; after the .Throw() line is because the compiler won't know that .Throw() always throws an exception. throw; will never be called as a result, but at least the compiler won't complain if your method requires a return object or is an async function.
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I'm trying to figure out the "best" way execute a (close) statement in my sql data access layer methods.
I was wondering which of the two ways is considered more correct (thanks):
Option #1
public void dbOperation( ... )
{
try
{
_cmd.Open();
_cmd.Execute();
}
catch (Exception ex)
{
throw ex;
}
finally
{
_cmd.Close()
}
}
Option #2
public void dbOperation( ... )
{
try
{
_cmd.Open();
_cmd.Execute();
}
catch (Exception ex)
{
_cmd.Close()
throw ex;
}
_cmd.Close();
}
Neither is correct. You shouldn't have a catch clause just to re-throw the exception again, clearing out its stack trace, and doing nothing productive, which is what your first option does.
You should just be closing in the finally:
try
{
_cmd.Open();
_cmd.Execute();
}
finally
{
_cmd.Close()
}
Your second snippet has the same problem with it, since you're improperly re-throwing the exception.
The best option would be to use a using, which is just a syntactic sugar for a try/finally without a catch:
using(var command = ...)
{
command.Open();
command.Execute();
}
This has the added benefit of also ensuring that the scope of the command is exactly the same as when it's valid to use it. A try/finally block requires the command to be a valid identifier after it has been disposed of.
Option #1 is the only correct one of the two. In fact, you can get the equivalent of option #1 using less code:
try
{
_cmd.Open();
_cmd.Execute();
}
finally
{
_cmd.Close()
}
If an exception is thrown, there's no need to have a catch block to just re-throw it. If you want to do something in the catch block, such as logging the exception, be sure to do this:
try
{
_cmd.Open();
_cmd.Execute();
}
catch (Exception ex)
{
logException(ex);
throw; //Just say throw, not throw ex, to preserve the original stack trace
}
finally
{
_cmd.Close()
}
Option #1. If you use option2 you won't execute cmd.close unless it throws an exception
I have the following code:
try
{
retval = axNTLXRemote.IsUnitPresent(_servers[0].IPAddress, 1, _servers[0].RemotePort, _servers[0].CommFailDelay * 1000);
}
catch (COMException ce)
{
throw ce;
}
Which gives me the followig warning which I want to get rid of:
CA2200 : Microsoft.Usage : 'Connect()' rethrows a caught exception and specifies it explicitly as an argument. Use 'throw' without an argument instead, in order to preserve the stack location where the exception was initially raised.
I have read the following The difference between try/catch/throw and try/catch(e)/throw e and I understand that the 'throw ce; will reset the stack trace and make it appear as if the exception was thrown from that function.
I want to simply change it to a 'throw' instead of a 'throw ce' which will get rid of the warning.
What is the difference in the following catches:
catch (COMException ce)
{
throw;
}
and
catch (COMException)
{
throw;
}
Do I only need to have 'COMException ce' if I wish to somehow use the ce variable?
Also, when I perform a 'throw' or 'throw ce', is it the calling function that will handle or catch it?? I'm a little unclear about this.
The only difference is that with catch (COMException ce), you are assigning the exception to a variable, thereby letting you access it within the catch block. Other than that, it is in every way identical.
I'm not sure what the question is here. If you want to access the exception object, you must give it a variable name in the catch clause.
No matter how or where an exception is thrown, the exception will bubble up through the call stack to the closest catch block that matches.
Here's an example.
void Method1()
{
try
{
Method2();
}
catch // this will catch *any* exception
{
}
}
void Method2()
{
try
{
Method3();
}
catch (COMException ex) // this will catch only COMExceptions and exceptions that derive from COMException
{
}
}
void Method3()
{
// if this code were here, it would be caught in Method2
throw new COMException();
// if this code were here, it would be caught in Method1
throw new ApplicationException();
}
I'm sure someone will jump in with an uber-technical answer, but in my experience the answer to your first two questions is that there is no difference, and as you stated you'd only include ce if you intended to use it to write the stack trace to a log or display the message to the user or similar.
The throw will send the exception up the chain. That may be the calling method or, if your method has several nested try/catch blocks, it will send the exception to the next try/catch block that the current try/catch block is nested within.
Here are a couple good resources to check out if you want to read further on the subject:
Exception Handling
Design Guidelines for Exceptions
There is no difference in both cases, but only when exception variable should be used for stack/message etc.
So:
catch(ComException);
and
catch(ComException ex);
statements will produce similar MSIL, except local variable for ComException object:
.locals init ([0] class [mscorlib]System.Exception ex)
Let us say we have an external server which we use (e.g.-telephony station, etc.). Also we have the next code:
try
{
externalService.CreateCall(callParams);
}
catch (Exception ex)
{
_log.Error("Unexpected exception when trying execute an external code.", ex);
_callService.UpdateCallState(call, CallState.Disconnected, CallOutcome.Failed);
throw;
}
Theoretically UpdateCallState could throw but we would hide this exception using that code and would treat only exceptions generated by CreateCall in a right way.
The question is, what is the right pattern for these situations so that we treat all the exceptions correctly?
You can always nest another try..catch inside the first catch and deal with it appropriately.
try
{
externalService.CreateCall(callParams);
}
catch (Exception ex)
{
_log.Error("Unexpected exception when trying execute an external code.", ex);
try
{
_callService.UpdateCallState(call, CallState.Disconnected, CallOutcome.Failed);
}
catch(Exception updateEx)
{
// do something here, don't just swallow the exception
}
throw; // this still rethrows the original exception
}
Break it up. Something like
if !TryCreateExternalCall(callParams)
{
_log.Error("Unexpected exception when trying execute an external code.", ex);
_callService.UpdateCallState(call, CallState.Disconnected, CallOutcome.Failed);
}
else
{
throw new ExternalServiceException(???);
}
TryCreateExternalCall should of course log the exception and stacktrace, before it swallows and returns false.
It is not a good practice to throw exception in Catch block.
The try, Catch suggest that
try
{
//make some changes. If something goes wrong go to Catch.
}
Catch(exception)
{
//I will clean the mess. Rollback the changes.
}
Catch the exception, only if you can handle the exception. Else bubble it up let the caller decide on what to do with the exception.
You should catch the most specific exception first, followed by the most general exceptions.
try
{
externalService.CreateCall(callParams);
}
catch (CreateCallExceptionType ccEx)
{
_callService.UpdateCallState(call, CallState.Disconnected, CallOutcome.Failed);
}
catch (Exception ex)
{
//do something
}
And then you could handle the UpdateCallState exception within the method.
following is a code snippet:
class xxx
{
public xxx(){}
try
{
throw new Exception(InvalidoperationException);
}
catch(Exception x)
{
}
catch(InvalidoperationException x)
{
}
}
can anyone tell which exception will raise here and what is the reason behind it.
Wow, lots of problems here. Where to start?
That code won't compile. The try-catch block that you've defined is outside of any method, which is not allowed. You need to move it inside of a method.
Never throw a method that you intend to catch yourself later in the method. That's commonly known as using exceptions for "flow control", which is roundly discouraged. There is a performance cost associated with doing so, and it also makes it very confusing to monitor the exceptions that are being thrown when using a debugger when you have code that's throwing and catching it's own exceptions. Use boolean variables (known as flags) for flow control, if necessary.
Always catch the most derived exception class first. That means you should catch InvalidOperationException first, before trying to catch Exception. You need to reverse the order of your catch blocks in the code that you have.
You should practically never catch System.Exception. The only exceptions that you should catch are those that you explicitly understand and are going to be able to handle. There's virtually no way that you're going to know what went wrong or how to handle it when the only information you have is that a generic exception was thrown.
Along those same lines, you also should never throw this exception from your own code. Choose a more descriptive exception class that inherits from the base System.Exception class, or create your own by inheriting from the same.
I see that other answers are showing you sample code of what your code should look like, were it to be rewritten. I'm not going to do that because if I rewrote your code to be correct, I'd end up with this:
class Xxx
{
public Xxx()
{
}
}
Not particularly helpful.
If the code is like this
class xxx
{
public xxx(){
try
{
throw new Exception(InvalidoperationException);
}
catch(InvalidoperationException x)
{
}
catch(Exception x)
{
}
}
}
It should compile and raise your exception and catch. Otherwise your code will not compile at all.
No exception will be thrown as this code will not even compile.
Regardless - several points:
When using exception handling, put the more specific exception before the less specific ones (so the catch of InvalidOperationException should be before the one for Exception).
Catching Exception is normally no very useful.
If you catch an exception, do something with it.
You probably meant:
throw new InvalidOperationException();
However, the way you structured your exceptions, the catch(Exception x) block would have run.
You should write:
class xxx
{
public void Test()
{
try
{
throw new InvalidoperationException();
}
catch(InvalidoperationException exception)
{
// Do smth with exception;
}
catch(Exception exception)
{
throw; // Rethrows your exception;
}
}
}
InvalidOperationException inherits from Exception.
catch tries to processes the most specific branch, so catch (InvalidOperationException x) will be executed here.
Nope. It wouldn't compile. So, it there's no question about as to which exception will be generated.
Your code should be something like this :
class xxx
{
public void Test()
{
try
{
throw new InvalidoperationException();
}
catch(InvalidoperationException exception)
{
// Something about InvalidOperationException;
}
catch(Exception exception)
{
// Something about the Exception
}
}
}
Point to be noted :
Write more specific class of Exception first, hence we write InvalidOperationException prior to Exception class.
Ignoring the compile issue.... the first matching exception block (catch(Exception x)) will get the exception. You then ignore the exception and don't re-throw, so exception will be seen by the outside world. That doesn't make it good practice, though... in particular, catching an arbitrary Exception and ignoring it is risky - it could have been anything... it isn't necessarily the exception you thought it was.
Well, the code won't compile, but I'll just ignore that...
If I'll just look at the line:
throw new Exception(InvalidoperationException);
1st of all, according to MSDN there is no such constructor. So I will assume you meant the constructor: Exception(String msg, Exception innerException). Meaning:
throw new Exception("blabla", InvalidoperationException);
The exception that is being thrown is of type Exception and not InvalidOperationException. So ONLY catch(Exception x) can catch it.
If you would've thrown InvalidoperationException than the way you wrote the order of the catches, the Exception class would get caught first.
The order of the catches does matter.
The best advice I can give you is simply try it yourself and see what happens.
What is the difference between
try { }
catch
{ throw; }
and
try { }
catch(Exception e)
{ throw e;}
?
And when should I use one or the other?
The constructions
try { ... }
catch () { ... } /* You can even omit the () here */
try { ... }
catch (Exception e) { ... }
are similar in that both will catch every exception thrown inside the try block (and, unless you are simply using this to log the exceptions, should be avoided). Now look at these:
try { ... }
catch ()
{
/* ... */
throw;
}
try { ... }
catch (Exception e)
{
/* ... */
throw;
}
try { ... }
catch (Exception e)
{
/* ... */
throw e;
}
The first and second try-catch blocks are EXACTLY the same thing, they simply rethrow the current exception, and that exception will keep its "source" and the stack trace.
The third try-catch block is different. When it throws the exception, it will change the source and the stack trace, so that it will appear that the exception has been thrown from this method, from that very line throw e on the method containing that try-catch block.
Which one should you use? It really depends on each case.
Let's say you have a Person class with a .Save() method that will persist it into a database. Let's say that your application executes the Person.Save() method somewhere. If your DB refuses to save the Person, then .Save() will throw an exception. Should you use throw or throw e in this case? Well, it depends.
What I prefer is doing:
try {
/* ... */
person.Save();
}
catch(DBException e) {
throw new InvalidPersonException(
"The person has an invalid state and could not be saved!",
e);
}
This should put the DBException as the "Inner Exception" of the newer exception being throw. So when you inspect this InvalidPersonException, the stack trace will contain info back to the Save method (that might be sufficient for you to solve the problem), but you still have access to the original exception if you need it.
As a final remark, when you are expecting an exception, you should really catch that one specific exception, and not a general Exception, ie, if you are expecting an InvalidPersonException you should prefer:
try { ... }
catch (InvalidPersonException e) { ... }
to
try { ... }
catch (Exception e) { ... }
The first preserves the stack trace while the second resets it. This means that if you use the second approach the stack trace of the exception will always start from this method and you will lose the original exception trace which could be disastrous for someone reading exception logs as he will never find out the original cause of the exception.
The second approach might be useful when you want to add additional information to the stack trace but it is used like this:
try
{
// do something
}
catch (Exception ex)
{
throw new Exception("Additional information...", ex);
}
There's a blog post discussing the differences.
You should use
try { }
catch(Exception e)
{ throw }
if you want to do something with the exception before re-throwing it (logging for example). The lonely throw preserves stack trace.
The difference between a parameterless catch and a catch(Exception e) is that you get a reference to the exception. From framework version 2 unmanaged exceptions are wrapped in a managed exception, so the parameterless exception is no longer useful for anything.
The difference between throw; and throw e; is that the first one is used to rethrow exceptions and the second one is used to throw a newly created exception. If you use the second one to rethrow an exception, it will treat it like a new exception and replace all stack information from where it was originally thrown.
So, you shold not use either of the alternatives in the question. You should not use the parameterless catch, and you should use throw; to rethrow an exception.
Also, in most cases you should use a more specific exception class than the base class for all exceptions. You should only catch the exceptions that you anticipate.
try {
...
} catch (IOException e) {
...
throw;
}
If you want to add any information when rethrowing the exception, you create a new exception with the original exception as an inner exception to preservere all information:
try {
...
} catch (IOException e) {
...
throw new ApplicationException("Some informative error message", e);
}