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.
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;
}
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");
}
I have a function called connect like so:
public boolean connnect(){
{
..... connecting codde
if(connectionSuccessfull)
{
return true;
}
else
{
return false;
}
}
This is a very basic form of error handling, I want to upgrade this function to handle errors correctly. Such as not just tell me false there was an error, but be able to say, error, Authentication failed, or Time-out error etc.
This information then needs to be sent back up the line to the Caller so it can know what happened.
What is the correct way to go about doing this?
{EDIT}
In my care its quite probable that an exception will occur I would say 50% of the time.
I have come up with this, does it look partially correct?
namespace MobileWebServices.Exceptions
{
//Timeout
public abstract class TimeOutException : Exception
{
}
public class ConnectingTimeOutException : TimeoutException
{
}
public class DissconnectingTimeOutException : TimeoutException
{
}
//Authetntication
public abstract class AuthenticationException : Exception
{
}
public class BadAuthenticationException : AuthenticationException
{
}
}
Something along the lines of:
public void Connect()
{
try
{
//code here to look-up the connection details
if(!ValidateConnectionDetails(details))
throw new InvalidOperationException("The connection details are not valid.");
//code here to establish the connection
if(SomeTestThatShowsWereNotHappyWithTheConnection())
throw new Exception("The connection is bad, for some reason");
}
catch(SocketException se)
{
//We'd only have this block if a socket exception is possible. We might just allow it to pass on up.
throw; // User now gets the exception we got, exactly.
//We might re-throw the error, but from here so the stack-trace goes to here rather than the innards of this method:
throw se;
//Most usefully we might throw a new exception that contains this as an inner exception:
throw new Exception("Connecting failed", se);
//Or even better, we might throw a more well-defined exception, that relates to this operation more specifically, with or without the inner exception, depending on whether that is likely to be useful:
throw new ConnectionException("Some message, or maybe just a default is defined in the constructor");
//OR:
throw new ConnectionException("Some message, or maybe just a default is defined in the constructor", se);
}
catch(Exception ex)
{
//If we get to an exception ourselves that isn't of a particular type we're expecting, we probably shouldn't catch it at all. We might though want to note the exception before re-throwing it, or throw a more specific connection with this as an inner-exception:
Log(ex);
throw;
}
}
Because you're no longer returning a value to indicate success, you could also now return an object that represents the connection you created:
public ConnectionObject Connect()
{
// Much as above, but returning the object before the end of the `try`.
}
Returning values representing failure should only be done if that failure is both likely to happen, and something you expect the calling code to be able to reasonably react to right at the point of calling. This isn't that likely with code to connect since the calling code could be code that e.g. connects and then does an operation, and the code calling that in turn is where the exception (whether from here or the subsequent operation) should be caught - it's the code that ultimately cares about the failing.
In the latter case, then returning a value indicating the failure makes a lot more sense. Here though, I'd probably still consider an exception, because it can encapsulate more information, be used by coders in the normal way they use other .NET methods, and because the calling code is probably not written thinking "try to get the connection and then if it works..." it's written thinking "get the connection and then..." with the error case being exactly that; an error case. (For comparison, a method like int.TryParse() is to answer the question "does this string represent an integer, and if so what is it?" where the method int.Parse() answers the question "what is the integer in this string?" with there not being an integer being an error condition).
To think of it another way. Are you currently using a web-browser to browse the web, or are you using it to try to browse the web? Your internet connection could die on you, stopping you from continuing to read these answers, but you'd consider that a problem in what you were trying to do.
The normal approach is to throw an exception (perhaps of a user-defined type), and then to catch those exceptions at a higher level.
If for some reason you cannot use exceptions, you could instead write a wrapper class that encompassed an error message (which would be null if no error occurred) and the bool result (which would only be relevant if the error message is null).
However, I would recommend using exceptions. (The only issue might be whether or not you need to globalise the error message string in the exception, but the consensus is that you should not.)
Here is a sample on how things should be done :
First use your connect() method to return an object (like a Socket for example).
Return a null one if it fails connecting without throwing an Exception.
In your connect() method try/catch your connecting instructions, and rethrow those catched.
Then in the calling method, catch all the Exceptions that can be thrown, and check if the returned object is null or not.
Here is an example of code using Sockets :
public static Socket connect()
{
Socket s = null;
try
{
IPEndPoint iEP = new IPEndPoint("127.0.0.1", 8080);
s = new Socket(iEP.AddressFamily, SocketType.Stream, ProtocolType.Tcp);
s.Connect(iEP);
if(!s.Connected)
{
return null;
}
}
catch(Exception e)
{
throw e;// Rethrow the Exception to the caller
}
return s;
}
public static void Main(String[] args)
{
Socket mySocket = null;
try
{
mySocket = connect();
}
catch(SocketException e)
{
// TODO - Detailed error about a SocketException
Console.Error.WriteLine("SocketException: " + e.Message + "(" + e.ErrorCode + ")");
}
catch(SecurityException e)
{
// TODO - Detailed error about a SecurityException
Console.Error.WriteLine("SecurityException: " + e.Message);
}
catch(Exception e)
{
// TODO - Detailed error about those Exceptions :
// ArgumentNullException, ObjectDisposedException and InvalidOperationException
Console.Error.WriteLine(e.GetType() + ": " + e.Message);
}
if(mySocket == null)
{
// TODO - Error while initializing the Socket
Console.Error.WriteLine("Error while initializing the Socket");
}
// TODO - Use your Socket here
}
I think the best way is using the try catch exception surrounding your call with the exception you want :
catch(TimeoutException ex)
{
//Do something
}
catch(SqlException ex)
{
//do something
}
//....
catch(Exception ex)
{
//do something
}
Make sure of the order of your catch ( the global Exception in last)
I have two radio buttons for the user to select the type of movie they like. This is just an example program, as I want to understand throwing exceptions better. When the user clicks the display button it shows the type of movie they selected, between Action or Comedy. If no selection is made it throws an exception, this is the best way I could figure it out, is this going in the right direction?
string selection;
try
{
if (radAction.Checked)
{
selection = radAction.Text;
}
else
if (radComedy.Checked)
{
selection = radComedy.Text;
}
else
throw new ArgumentNullException("Please Choose Movie Type");
MessageBox.Show(selection);
}
catch(ArgumentNullException msg)
{
MessageBox.Show(msg.Message);
}
It's not at all a good practice to show Error message in your scenario. Using try...catch and throw...Exception always comes with a performance penalty. Try to avoid as much as possible. Refer this SO post for further reference.
But if you are really stick with try...catch then create your own User-defined exception.
public class MovieSelectionNotFoundException : Exception
{
public MovieSelectionNotFoundException()
{
}
public MovieSelectionNotFoundException(string message)
: base(message)
{
}
public MovieSelectionNotFoundException(string message, Exception inner)
: base(message, inner)
{
}
}
And you can use this in your code as follows:
string selection = string.Empty;
try
{
if (radAction.Checked)
{
selection = radAction.Text;
}
else if (radComedy.Checked)
{
selection = radComedy.Text;
}
else
throw new MovieSelectionNotFoundException("Please Choose Movie Type");
MessageBox.Show(selection);
}
catch (MovieSelectionNotFoundException msg)
{
MessageBox.Show(msg.Message);
}
Exceptions are for unexpected and exceptional situations. If you know the case, you should control that case and avoid using exceptions.
my requirements specifically call for try, catch
Having a try catch block is a good thing for the worst case scenarios, but you can still satisfy your requirements without throwing an exception. But do you really need to throw exception, when you can just control the case with a simple condition.
If you really need to throw an exception, you can implement a custom exception or use the ones already implemented under System.Exception.
I have some cases where I don't care what exception is thrown (as long as some exception is thrown). Unfortunately,
Assert.Throws<Exception>(someDelegate);
doesn't pass unless exactly an instance of Exception (so not an instance of a derived class) is thrown. I know I can obtain the behavior I want with
Exception exception = Record.Exception(someDelegate);
Assert.NotNull(exception);
but it doesn't read right. Am I missing something in xUnit that has the behavior I want? Here are two tests that indicate what I mean:
[Fact]
public void Throws_exception_and_passes() {
Exception exception = Record.Exception(
() => { throw new InvalidOperationException(); }
);
Assert.NotNull(exception);
}
[Fact]
public void Throws_exception_and_fails() {
Assert.Throws<Exception>(
() => { throw new InvalidOperationException(); }
);
}
Per the documentation here:
http://xunit.codeplex.com/wikipage?title=HowToUse&referringTitle=Home
You have to specify the type of exception you want to be thrown. In general, this is good practice. You should be able to predict what scenarios a test would throw what type of exception. You should be able to design both you method and your test in a way that will allow you to predict this.
There are ways around this, like doing a try catch yourself, but you should look into changing your design a bit.
It didn't exist at the time of this question, but now one can use Assert.ThrowsAny<Exception> to test for any exception derived from Exception (and hence any exception at all), along with variants such as Assert.ThrowsAny<ArgumentException> which would test for any exception derived from ArgumentException and so on.
As you've identified if Assert.Throws<T> doesn't fit the bill, the only OOTB thing in xUnit you're left with is using Record.Exception.
As you've identified, the main way of doing a 'Assert throws anything` is to do
Assert.NotNull( Record.Exception( lambda ))
Look at it - not pretty. This is likely by design; there are very few things in xUnit.net that are by accident (as opposed to carefully considered opinionated design).
Record.Exception returns a result for a reason (and if you were using F#, you'd have to |> ignore to chuck away the value). You should always be able to Assert something about the nature of the Exception that's happening so that an actual problem in your code doesn't get ignored by chance as you change your code over time, which is the reason for all this testing stuff in the first place. Perhaps that might take the form of
var exception = Record.Exception( sut.Something );
Assert.True( typeof(SomeException).IsAssignableFrom( exception ) );
Looking at that, it's safer that an Assert.NotNull(), but still doesn't feel right. It's time to, as discussed in GOOS, listen to your tests (and in the case of an opinionated test framework, your test framework).
The biggest problem in your question is however that in a real example from a real test, there is always a way to make your interface clearer or express your expectation in another way, so the real answer is Mu.
xUnit won't stand in your way if you want to do your own Custom Assertion, something like:
public static bool Throws<T>(this Action action, bool discardExceptions = false)
where T : Exception
{
try
{
action.Invoke();
}
catch (T)
{
return true;
}
catch (Exception)
{
if (discardExceptions)
{
return false;
}
throw;
}
return false;
}
Or:
public static bool Throws(this Action action)
{
try
{
action.Invoke();
}
catch (Exception)
{
return true;
}
return false;
}
I was just looking in the xUnit.net source and here is the culprit:
private static Exception Throws(Type exceptionType, Exception exception)
{
Guard.ArgumentNotNull("exceptionType", exceptionType);
if (exception == null)
throw new ThrowsException(exceptionType);
if (!exceptionType.Equals(exception.GetType()))
throw new ThrowsException(exceptionType, exception);
return exception;
}
What would solve your problem is if this change were applied:
if(!exceptionType.Equals(exception.GetType()))
to:
if(!exception.GetType().IsAssignableTo(exceptionType))
You could possibly offer to submit a patch?
public static void SuppressException<TSut>(this TSut value, Action<TSut> action) where TSut : class
{
try
{
action.Invoke(value);
}
catch (Exception)
{
//do nothing
}
}