Is there a way to override the message of an exception?
I don't want to make a custom exception but to override the message of an existing exception.
For example:
Every time when a ArgumentOutOfRangeException is thrown, I'd like it to contain my message instead of the default one.
Is it possible?
For exceptions you're throwing, you can just pass the message in to the constructor:
throw new ArgumentOutOfRangeException("name", "My custom message");
Note that here, name is the name of the parameter that caused the problem. In C# 6, you should use the nameof operator to make this refactoring-safe:
public void Foo(int x)
{
if (x > 10)
{
throw new ArgumentOutOfRangeException(nameof(x), "That's too big");
}
}
You can't modify the message of an exception thrown by other code, but you can catch the exception and rethrow another one:
try
{
...
}
catch (FooException e)
{
// Keep the original exception
throw new BarException("Some message", e);
}
I would try to avoid doing this too much though. If you're considering showing exception messages to users, I would generally shy away from that - they're really aimed at developers. As an example, the ArgumentOutOfRangeException you suggested should generally indicate a bug in your code rather than some external condition (like a network failure or whatever) - the user isn't going to be able to do anything about that bug; it's something you should fix. A network failure or something similar is at least more reasonable for the user to take action about, but frankly it's often not going to be clear what the chain of events is.
The Exception.Message property is declared read-only, so no, you cannot change the Message of a pre-existing Exception object. (The same applies to derived exception types.)
But you can set the message text of an exception you're throw-ing yourself: usually, you pass the message text to the exception constructor:
throw new ArgumentException("Frobbled arguments are not accepted", paramName: …);
// ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
When you define your own exception types, you should follow this protocol; see the section at the end of this answer.
You want to change an existing exception object's Message — Alternatives:
Catch the original exception and derive a new exception from it that is identical except for the message:
…
catch (ArgumentException ex)
{
throw new ArgumentException("New message text", paramName: ex.ParamName);
}
This is in general not a terribly good idea, because you might lose state data contained in ex (such as its stack trace and InnerException). But it might work just fine in this scenario.
Wrap the exception in a new exception so that it becomes the InnerException:
…
catch (ArgumentException ex)
{
throw new ArgumentException("New message text", innerException: ex);
}
This is better, because all the original exception's state is preserved. However, in the particular case of ArgumentException it is not obvious at all that the actual exception information is contained in the InnerException.
How to make sure your own exception classes allow setting the message et al.:
When you define a new exception type, it's important for the reasons mentioned above to define constructors that accept a message text and/or an inner exception. So a custom exception class would have at least these constructors:
class MyCustomException : Exception
{
public MyCustomException() : base() { }
public MyCustomException(string message) : base(message) { }
public MyCustomException(string message, Exception innerException) : base(message, innerException) { }
public MyCustomException(string message) : base(message) { }
}
You can use try .. . catch
try
{
//Code here regarding array indexing
}
catch (ArgumentOutOfRangeException ex)
{
throw new ArgumentOutOfRangeException("exception", "New Custom Message");
//Or show new message
MessageBox.Show("Your custom Message");
}
Related
This might be a broad question, but recently I ahve wondered about the following: In our C# backend we have many places that wrap some code in a try/catch block, specifically calls to external WcF services. Some of these calls are crucial for the application so in the catch block we log the error and rethrow, like:
catch(Exception ex)
{
_logger.Error("Some good error message");
throw ex;
}
On the other hand there are services we allow to fail, but we still want to log the error, so they look like:
catch(Exception ex)
{
_logger.Error("Some good error message");
}
Now reading the code of team members I can not be sure if they forgot to throw or if this is the intended behaviour.
Q: Is there a way, resp. what is the default way, to explicitly NOT rethrow (without including a comment in the code).
I have considered something like this:
catch(Exception ex)
{
_logger.Error("Some good error message");
NotThrowingHereOnPurpose();
}
// ...
// and further below a private method
// ...
private void NotThrowingHereOnPurpose(){}
One approach that may be useful here is to change the way of invoking the code that you explicitly allow to fail in such a way that it does not look like a try/catch block at all.
For example, you could write a helper method that does error reporting, and call it with actions expressed as lambdas:
void InvokeFailSafe(Action action, Action<Exception> onFailure = null) {
try {
action();
} catch (Exception e) {
if (onFailure != null) {
onFailure(e);
}
}
}
Now instead of try/catch you would write this:
InvokeFailSafe(
() => {
... The code that may fail
}
, exception => _logger.Error("Some good error message: {0}", exception)
);
or like this, if you don't want anything logged:
InvokeFailSafe(
() => {
... The code that may fail
}
);
If you code things this way, there would be no doubts about a missing throw statement.
It's an opposite solution to dasblinkenlight's answer. Instead of notifying others that the exception mustn't be rethrown it would say that it must be.
If you only want to log it then use the Error method as usual. Otherwise, you can write an extension method for your logger to log and throw exceptions.
The method would take the catched exception and rethrow it using the ExceptionDispatchInfo class. The ExceptionDispatchInfo is used to rethrow the exception with the original stack trace information and Watson information. It behaves like throw; (without the specified exception).
public static void ErrorAndThrow(this ILogger logger, string message, Exception exception)
{
var exceptionInfo = ExceptionDispatchInfo.Capture(exception);
logger.Error(message);
exceptionInfo.Throw();
}
And use it this way:
try
{
}
catch (Exception ex)
{
// ex would be rethrown here
_logger.ErrorAndThrow("Some good error message", ex);
}
Q: Is there a way, resp. what is the default way, to explicitly NOT
rethrow (without including a comment in the code).
Ideal way would be not to catch a generic exception. Now, to throw or not that entirely depends on your case. You need to understand that Exception handling is used when you know what to do in case an exception occurs. So, only specific exceptions should be handled. Catching exceptions without knowing what you are catching will change the behavior of your application.
Now reading the code of team members I can not be sure if they forgot
to throw or if this is the intended behaviour.
This is something the author of the code can explain to you. But here is a learning to take from this. Your code should be self explanatory. In specific cases where you are unable to express yourself with the code, add a meaningful comment.
You can check this link for better understanding.
I actually found another way that kind of includes what other have suggested here, but uses a built in feature: exception filters. I was free to modify the example given in here to illustrate this:
public void MethodThatFailsSometimes()
{
try {
PerformFailingOperation();
}
catch (Exception e) when (e.LogAndBeCaught())
{
}
}
and then one could have two extension methods on Exception, say LogAndBeCaught and LogAndEscape like so:
public static bool LogAndBeCaught(this Exception e)
{
_logger.Error(#"Following exception was thrown: {e}");
return true;
}
public static bool LogAndEscape(this Exception e)
{
_logger.Error(#"Following exception was thrown: {e}");
return false;
}
I have a function "ReturnString":
public static string ReturnString(string sa, string sb)
{
try
{
...
...
return "xyz";
}
catch (Exception ex)
{
throw new clsException(ex.Message);
}
}
it is call by more than 600 times from other more then 40 classes and win farms Mean's it has more than 600 references in more then 40 classes and win farms.
When Exception thrown by it, I want to know what is the it's last calling ref. when exception happen?
Please help me to solve this without changing function arguments.
You should initialize an instance of StackTrace class -
https://msdn.microsoft.com/en-us/library/system.diagnostics.stacktrace(v=vs.110).aspx
Then, get the first StackFrame -
https://msdn.microsoft.com/en-us/library/system.diagnostics.stackframe(v=vs.110).aspx
Finally, get the MethodBase of this frame; Its "Name" property is what you need -
https://msdn.microsoft.com/en-us/library/system.reflection.methodbase(v=vs.110).aspx
Try this:
public static string ReturnString(string sa, string sb)
{
try
{
//...
//...
return "xyz";
}
catch (Exception ex)
{
StackTrace oStackTrace = new StackTrace();
string sMethodName = oStackTrace.GetFrame(1).GetMethod().Name;
//It's not a good practice to keep only the error message (you may need other exception details later)
throw new clsException(string.Format("{0}: {1}", sMethodName, ex.Message));
}
}
Your problem is here:
throw new clsException(ex.Message);
As others have mentioned, ex already contains the info you want inside the StackTrace property (check this link for more info).
But when you throw a new exception, you are only throwing the message, and ignoring all the info you want to get.
Just throw without a new exception, or include ex as the inner exception of your clsException.
I want to know what is the it's last calling ref. when exception
happen?
Then check the exception StackTrace, that will let you know the entire call stack and the latest one responsible for exception. Also the innerException property if any.
Check the documentation on Exception class. It has a property StackTrace which you should check.
In your case, the exception object should have it ex.StackTrace
You may also want to get the TargetSite property value from your exception object saying ex.TargetSite
I have created few custom exception class
public class CreateNewUserWebException : Exception
{
public CreateNewUserWebException(string email): base(
string.Format("[{0}] - User could not be added.", email))
{
}
}
public class CreateNewUserEntityFrameworkException : System.Data.DataException
{
public CreateNewUserEntityFrameworkException(string email)
: base(
string.Format("[{0}] - User could not be added.", email))
{
}
}
and here is my controller code
try
{
var user = _createUserModule.CreateUser(model);
CookieManager.SetAuthenticationCookie(user, model.Email, rememberMe: false);
return RedirectToAction("Index", "Bugs");
}
catch (CreateNewUserEntityFrameworkException exception)
{
this.ModelState.AddModelError("", "Some error occured while registering you on our sytem. Please try again later.");
Elmah.ErrorSignal.FromCurrentContext().Raise(exception);
}
catch (CreateNewUserWebException exception)
{
this.ModelState.AddModelError("", "Some error occured while registering you on our sytem. Please try again later.");
Elmah.ErrorSignal.FromCurrentContext().Raise(exception);
}
catch(Exception exception)
{
this.ModelState.AddModelError("", "Some error occured while registering you on our sytem. Please try again later.");
Elmah.ErrorSignal.FromCurrentContext().Raise(exception);
}
I have purposely fully induced an primary key violation exception which is
but exception is not catched by my custom exception class. It is not caught by the last exception catch block.
I cannot understand why so. Can some one help me out on this please.
The part you've highlighted in the debugger is the inner exception. That isn't used by the CLR to determine which catch block to enter. The outer exception is just a DbUpdateException - which you haven't specified a particular catch block for.
Even the inner exception is just a DataException - it's not an instance of your custom exception.
You haven't shown any code which actually throws your exception - are you sure it's being used at all? What code have you written to tell EF to throw your exception rather than the exception it would otherwise throw?
(Given your comments, I'm not sure you quite understand exception handling. Creating a custom exception doesn't somehow let you catch an instance of that without it being thrown - something still has to throw an instance of that exception before it's any use.)
I have a couple questions about exceptions.
1) when you hit a catch block, swallowing means what exactly? I thought it was always rethrow or the existing exceptions is passed up to the next catch block.
2) If you add Exception.Data values to an excepction, I notice I have to do another throw; to grab that data futher up in another catch block later. Why?
Swallowing an exception means catching it and not doing anything useful with it. A common thing you might see is this:
try
{
DoSomeOperationThatMightThrow();
}
catch (Exception ex) // don't do this!
{
// exception swallowed
}
You usually don't want to catch a base Exception at all, it's better to catch and handle specific Exception types, and ideally you should only catch exception types that you can do something useful with at the level of code you're in. This can be tricky in complex applications, because you might be handling different errors at different levels in the code. The highest level of code might just catch serious/fatal exceptions, and lower levels might catch exceptions that can be dealt with with some error handling logic.
If you do catch an exception and need to rethrow it, do this:
try
{
DoSomething();
}
catch (SomeException ex)
{
HandleError(...);
// rethrow the exception you caught
throw;
// Or wrap the exception in another type that can be handled higher up.
// Set ex as the InnerException on the new one you're throwing, so it
// can be viewed at a higher level.
//throw new HigherLevelException(ex);
// Don't do this, it will reset the StackTrace on ex,
// which makes it harder to track down the root issue
//throw ex;
}
Swallowing an exception normally means having a handling block for the exception, but not doing anything in the block. For example:
try { 3/0; } catch DivideByZeroException { //ignore } //Note: I know this really wont' compile because the compiler is smart enough to not let you divide by a const of 0.
You have to rethrow because the first handler for an exception is the only one that will execute.
If you want the exception to bubble up you either don't handle it or you rethrow it. By the way, it's important to note that in .NET by just saying "throw" you'll preserve the stack trace. If you "throw Exception" you'll lose your stack trace.
Ok, you can handle the exception up to call stack you can do some thing like this:
public class A
{
public void methodA()
{
try
{
}
catch(Exception e)
{
throw new Exception("Some description", e);
}
}
}
public class B
{
public void methodB()
{
try
{
A a = new A();
a.methodA();
}
catch(Exception e)
{
//...here you get exceptions
}
}
}
I traditionally deploy a set of web pages which allow for manual validation of core application functionality. One example is LoggerTest.aspx which generates and logs a test exception. I've always chosen to raise a DivideByZeroException using an approach similar to the following code snippet:
try
{
int zero = 0;
int result = 100 / zero;
}
catch (DivideByZeroException ex)
{
LogHelper.Error("TEST EXCEPTION", ex);
}
The code works just fine but I feel like there must be a more elegant solution. Is there a best way to raise an exception in C#?
try
{
throw new DivideByZeroException();
}
catch (DivideByZeroException ex)
{
LogHelper.Error("TEST EXCEPTION", ex);
}
Short answer:
throw new Exception("Test Exception");
You will need
using System;
Build a custom exception for testing purposes ? Then you could add whatever custom properties you want the exception to carry with it on it's way through the exception handling / logging process...
[Serializable]
public class TestException: ApplicationException
{
public TestException(string Message,
Exception innerException): base(Message,innerException) {}
public TestException(string Message) : base(Message) {}
public TestException() {}
#region Serializeable Code
public TestException(SerializationInfo info,
StreamingContext context): base(info, context) { }
#endregion Serializeable Code
}
in your class
try
{
throw new TestException();
}
catch( TestException eX)
{
LogHelper.Error("TEST EXCEPTION", eX);
}
try
{
string a="asd";
int s = Convert.ToInt32(a);
}
catch (Exception ex)
{
Response.Write(ex.Message);
}
It will return exception "Input string was not in a correct format. "
throw exceptionhere;
Isn't it?
Example I found was
if (args.Length == 0)
{
throw new ArgumentException("A start-up parameter is required.");
}
So, let me put in a pitch for continuing to do it the way you were. You don't want to test what happens when a DivideByZeroException is thrown; you want to test what happens when a divide by zero actually occurs.
If you don't see the difference, consider: Are you really sure when you want to check for NullRefernceException and when for ArgumentNullException ?
Thanks for the feedback. I've marked GalacticCowboy's answer as correct as it is obviously the correct answer based on the way the question is phrased.
For those thinking "there's got to be more to this question", you're right. In essence I was looking for a best way to raise/cause/simulate an exception. As James Curran stated, it's the occurrence of the exception rather than the throwing of an exception which I'm after. Forcing a DivideByZeroException is my default strategy though I thought there might be another way or maybe even a better exception to force.
More than likely there's no difference between throwing and "raising" an exception. The majority of answers seem to be of this opinion at least.
Thanks again for the feedback and sorry if the question was vague.
throw new DivideByZeroException("some message"); ?
Or am I missing something?
If you're just testing LogHelper's Error method, why even throw the exception? You just need a one-liner:
LogHelper.Error("TEST EXCEPTION", new Exception("This is a test exception"));
public class CustomException: Exception
{
public CustomException(string message)
: base(message) { }
}
//
if(something == anything)
{
throw new CustomException(" custom text message");
}
you can try this
For testing purposes you probably want to create a specific class (maybe TestFailedException?) and throw it rather than hijacking another exception type.
Does
System.Diagnostics.Debug.Assert(condition);
give you an alternative?
Perhaps then use
catch (AssertionException) { }
to log a test failure.
See also C# - What does the Assert() method do? Is it still useful? and http://en.csharp-online.net/Assert.