Related
A little confusion as to the behaviour of Moq with MsTest.
Edit: This is not a question of "How do I test?" or "How do I assert?", this is a scratch pad to see how MoQ works so don't focus on the exception type etc.
I think a better question may be => "Does Moq Throws<> behave similar to MsTest ExpectedExceptionAttribute?" That is, they're expecting an exception in the test or the SUT?
I'd like to know just how MoQ "Throws" works when used with MsTest. Is it better to not use the MsTest expected exception attribute? Is it better to perform a try..catch within the test? I have a few more questions surrounding this.
I am Mocking a database call and when an error occurs I would like to return zero (0).
The TestMethod is straight forward with the MsTest exception attribute, and the throws exception with Moq. It only works when I throw an exception within the SaveCart method and not when I return zero.
I would like to understand the underlying behaviour because it feels as though I shouldn't, nor want to throw an exception within the SaveCart method.
Here is the Test under question:
[TestMethod]
[ExpectedException(typeof(ApplicationException))]
public void CartRepoSaveCartExceptionShouldReturnZero()
{
_cartDatabaseMock.Setup(c => c.SaveCart(_cart))
.Throws<ApplicationException>();
var result = _cartRepository.SaveCart(_cart);
Assert.AreEqual(result, _cartSaveExceptionValue);
}
Here is the basic SaveCart which does NOT throw an exception causing the test to fail:
public long SaveCart(Cart cart )
{
long returnValue;
try
{
returnValue = _cartDatabase.SaveCart(cart);
}
catch (Exception)
{
return 0;
}
return returnValue;
}
Here is a basic SaveCart where the test works because it's throwing an exception:
public long SaveCart(Cart cart )
{
long returnValue;
try
{
returnValue = _cartDatabase.SaveCart(cart);
}
catch (Exception)
{
throw new ApplicationException();
}
return returnValue;
}
Feel free to suggest a better title for the question if it doesn't quite explain it clearly.
You should use ExpectedExceptionAttribute when the unit under test throws an exception.
In your first example the method didn't throw any exception therefore the test failed.
Since your method under test doesn't throw any exception you don't need to use this attribute at all...(just verify the return value in this scenario)
When you want to verify that exception was thrown and you want to verify that some additional operations occurred, use the following pattern:
[TestMethod]
[ExpectedException(typeof(<The specific exception>))]
public void FooTest()
{
//arrange
try
{
// act
}
catch(<the specific exception>)
{
// some asserts
throw;
}
}
The above snippet will failed if:
wrong exception raised
exception was not raised
one of your asserts failed.
BTW, since your catch in the method is no Exception instead of ApplicationException, I offer you to change the setup to:
_cartDatabaseMock.Setup(c => c.SaveCart(_cart)).Throws<Exception>();
You are right - the second test "SaveCart" works because it's throwing an exception and the the first test fail because you are turning 0. From your response to previous answers, I am sure you already know all of this. If you are asking for the behavior how it failed your first test... it goes like this:
SaveCart is called
It returns an exception (result of your moq setup)
Your try catch caught the exception (you did this on purpose to alter the result)
Your try catch returns 0 (result is now 0 as you intended to alter it)
Assert checks your result against _cartSaveExceptionValue
You get a fail test stating something similar to this "Message: Assert.AreEqual failed. Expected. Actual<0 (System.Int32)>."
If you want to double check this... you can try the following test
comment out the [ExpectedException(typeof())]
change the Assert.AreEqual(result, _cartSaveExceptionValue) to Assert.AreEqual(result, 0);
the test should pass because you are comparing "result" (aka 0) to 0
I hope this answer your question.
catch (Exception)
{
return 0;
}
you are not throwing the exception, rather swallowing the exception, so why would you expect exception? It has nothing to do with MOQ. Your test and code are not in sync.
This is a bad practice btw, to swallow exception.
catch (Exception)
{
throw new ApplicationException();
}
That's also a code smell. You are catching all kinds of exception and then throwing a different type.
I'm writing a function that takes user input, runs a procedure in our database, and compares the values. Along the way, I need to check that we've received proper input and then that the query has returned an acceptable value.
private void DoTheThing(int? userInput1, int? userInput2, int valuePassedIn)
{
if (userInput1 == null || userInput2 == null)
{
Exception ex = new Exception();
ex.Data.Add("Message", "You screwed up.");
throw ex;
}
var queryResult = 0; //execute a query using the non-null inputs
if (queryResult == null) //or otherwise doesn't return an acceptable value
{
Exception ex = new Exception();
ex.Data.Add("Message", "some other thing happened");
throw ex;
}
else
{
//We're good, so do the thing
}
}
A quick note about this: I'm aware of the argument against exceptions as flow control and that I'd be better off checking the user's input before I even get this far. I won't get into all the details, but please accept that I'm kind of stuck writing the function this way.
That having been said, here's my question:
Given that the only differences between the 2 exceptions here is the message and the time at which they are thrown, how can I clean this code up to be both DRY and avoid running unnecessary code after determining that there will be a problem?
I thought about using a goto and placing the error code there, but that really only moves the problem around. If I move the exception code to the bottom and check for a message variable (or something similar), then I'm just running code that doesn't need to be run in the first place.
I suggest not throwing Exception (which means something went wrong, no comments are available), but ArgumentNullException and InvalidOperationException classes. Another amendment is avoding arrow-head
antipattern:
private void DoTheThing(int? userInput1, int? userInput2, int valuePassedIn)
{
// What actually went wrong? An argument "userInput1" is null
if (null == userInput1)
throw new ArgumentNullException("userInput1");
else if (null == userInput2)
throw new ArgumentNullException("userInput2"); // ...or userInput2 is null
var queryResult = executeSomeQuery(userInput1, userInput2, valuePassedIn);
// What went wrong? We don't expect that null can be returned;
// so the operation "executeSomeQuery" failed:
// we've provided validated (not null) values and got unexpected return.
// Let it have been known.
if (null == queryResult)
throw new InvalidOperationException(
String.Format("Query ({0}, {1}, {2}) returned null when bla-bla-bla expected",
userInput1, userInput2, valuePassedIn));
// We're good, so do the thing
// Note that's there's no "arrow-head antipattern":
// we're not within any "if" or "else" scope
}
Edit: Since every *Exception is inherited from Exception you can put some info into Data:
Exception ex = new ArgumentNullException("userInput1");
ex.Data.Add("Some key", "Some value");
throw ex;
but often Message is a far better place to explain what had heppened.
You might be better off creating a BadInputException class and a NullQueryResultException class. These do two different things and throwing a specific exception is better than throwing a generic Exception(...). In fact I think FXCop or Visual Studio's Code Analysis will give you a warning about throwing generic Exceptions.
It's not really all that much new code to write.
public class BadInputException : Exception
{
public BadInputException()
{
this.Data.Add("Message", "You screwed up.")
}
}
Then instead of this:
Exception ex = new Exception();
ex.Data.Add("Message", "You screwed up.");
throw ex;
Do this:
throw new BadInputException();
Edit: moved the "You screwed up" message from the Message property to the Data collection to match what the OP wants.
I would create a method:
private void CheckResult(bool cond, string msg, string info) {
if (!cond)
return;
Exception ex = new Exception();
ex.Data.Add(msg, info);
throw ex;
}
and call
CheckResult(userInput1 == null || userInput2 == null, "Message", "You screwed up.");
and
CheckResult(queryResult == null, "Message", "You screwed up.");
I think the QuestionRefactoring Guard Clauses is helpful for you .
There are something about this in Replace Nested Conditional with Guard Clauses.
Hope it's useful.
The code below will complain
try
{
session.Save(obj);
return true;
}
catch (Exception e)
{
throw e;
return false; // this will be flagged as unreachable code
}
whereas this will not:
try
{
session.Save(obj);
return true;
}
catch (Exception e)
{
return false;
throw e;
}
I dont get it...I thought my csc101 told me that return statements should always be the last statement in a function and that it exits the function and return control to the calling code. Why does this defy my professor's logic, and why does only one of these generate a warning?
return will exit the method; throw will also exit the method, assuming it is not inside the try. It can only exit once!
So regardless of the order - the first of the throw / return effectively end the method.
As more general feedback, though: if the intent is to return false upon failure, all you need is:
try
{
session.Save(obj);
return true;
}
catch
{
return false;
}
Personally, I would say that this is bad code - it hides the actual problem from the caller, making it very hard to debug. It tells us nothing of why it failed. I would say that the better approach is simply to let the exception bubble. In that case, there is no point returning true, because we would never return false - and there is no point catching an exception just to re-throw it. So the entire method becomes:
session.Save(obj);
(nothing else required whatsoever)
If your question is "why does only one of these generate a warning": a fair question, but the compiler isn't required to spot either of them for you. Perhaps it should spot it. I suspect that gmcs would spot this and warn about it - the compiler in mono is far more willing to point out stupidity.
Edit: as expected, [g]mcs outputs:
Program.cs(15,13): warning CS0162: Unreachable code detected
Program.cs(28,13): warning CS0162: Unreachable code detected
for the code below - so it does indeed report both uses as warnings:
class Program
{
static void Main() { }
static void DoSomething() { }
bool ReturnFirst()
{
try
{
DoSomething();
return true;
}
catch
{
return false;
throw; // line 15
}
}
bool ThrowFirst()
{
try
{
DoSomething();
return true;
}
catch
{
throw;
return false; // line 28
}
}
}
You are wrong: both your examples raise the Dead code compiler error because both throw and return mark the exit point of a method and no further code is allowed beyond that point.
However, whether the compiler allows it or not, the code below either the throw or the return is still dead and will never get a chance to execute.
(NOTE: this question was initially tagged as Java and my first sentence pertains to Java compiler semantics)
Because any code after the return statement within a code block will be unreachable.
This answer is based on C# and may or may not be applicable to Java.
In this case, you do not actually need the return statement. throw will be the last step of the function.
In this example, both return and throw will end the current function. Regardless of which way around you put them, then first will always prevent the second from being reachable.
NOTE: The exception to when a throw statement would end the function is if it was to be wrapped in a try block. In this case, the throw function would end execution of the remaining try block code, and move to the most relevant catch block - or finally block if a catch is not applicable.
Your code should look like this:
try
{
session.Save(obj);
return true;
}
catch(Exception e)
{
throw e;
}
However, there is not much point in having the try/catch anyway if all you are doing is re-throwing the exception.
To specifically answer your only question:
Why does this defiles my professor's logic?
Well either your professor is wrong, or you have misunderstood them
The "return false;" in the catch block is unreachable because of the "throw e;" just before it. When the code executes in the catch block the first line is a throw which means you immediately throw the exception to the calling method and therefore any following code does not get executed.
try
{
session.Save(obj);
return true;
}
catch(Exception e)
{
throw e; //Throws exception to calling method
return false; //this will be flagged as unreachable code
}
I hope this helps.
I have some code that is structured like the following:
if (someStatement)
{
//...
if (SomeOtherStatement)
{
//..., possibly more cases like this
}
else
{
//goto warning;
//would otherwise repeat
//the MessageBox.Show here
}
}
else
{
//goto warning;
}
//...
warning:
MessageBox.Show("some warning");
As i abhor copying code, is this one of the few useful applications of goto or is there a better structure i can use?
What about this?
if (someStatement)
{
//...
if (SomeOtherStatement)
{
//..., possibly more cases like this
return; // in the inner-most case
}
}
MessageBox.Show("some warning");
Yes, but only if by goto you mean "create a function and call it multiple times".
the best approach depends on what the rest of your code looks like. if the code shown is the last code in that method:
if(someStatement) {
// ...
if(SomeOtherStatement)
{
// a couple of lines of code
return; // !
}
}
MessageBox.Show("someWarning");
if not, you probably have to retort to something like this:
if(someStatement) {
// ...
if(SomeOtherStatement)
{
// a couple of lines of code
}
else
{
showWarning("not someOtherStatement");
}
}
else
{
showWarning("not someStatement");
}
I personally consider centralized error handling as "almost the only case" where goto is acceptable. However, for a better structure, I would put the label fully outside of the else, at the very end of your function.
Are you displaying the same error message on both lines? Then you should write a function that calls just that line and call that function on both occasions.
if (someStatement)
{
//...
if (SomeOtherStatement)
{
//..., possibly more cases like this
}
else
{
showError();
}
}
else
{
showError();
}
BTW: this is a bad example, it can be solved with a single else-branch catching the error. Here is a better one:
if (!doSomething())
{
showError();
}
doSomethingElse();
if (!anotherCall())
{
showError();
}
If you are going to get into a deep nasty error testing statement like you have, then you might want to consider wrapping it with a single try/catch, and throw the appropriate application specific exceptions from within the if statement, and catch those specific exceptions in the catch block, letting all the others go to be caught further up the chain.
Exceptions are the proper way to handle errors in .NET, throw them, but use them wisely:
try
{
if (someStatement)
{
//...
if (SomeOtherStatement)
{
//..., possibly more cases like this
}
else
{
throw new MyException();
//would otherwise repeat
//the MessageBox.Show here
}
}
}
catch(MyException e)
{
MessageBox.Show(e.Message);
// log the exception, if necessary
}
finally
{
// tidy up
}
With the exception you get a strongly typed answer to what went wrong which you can easily implement into logging systems.
Since (starting with FrameWork 1.1) logical-and "&&" : "only evaluates its second operand if necessary."
What's wrong with :
if(someStatement && someOtherStatement)
{
// more whatever
}
else
{
// raise an exception, call a method, show messagebox, whatever
}
Depending on the complexity of logical evaluation going on if the first clause evaluates to true : and you, possibly, implement lots of other tests, any number of which could result in errors : all of which you want handled in the identical way : I'd consider it mandatory to either raise a specific exception, call a method to handle the error, or put the whole thing in a try-catch block (which can be quite ugly if there's a lot of code).
A low-down-dirty-sneaky method I would use if it was a reasonable presumption that the possibility of error was quite remote 99% of the time the code was called : I'd set a boolean flag to reflect an error state : evaluate it on completion of the complex code block, and do what needed to be done : of course if your code branches off somewhere as a result of these internal evaluations : that would not be a good thing.
I think goto is a bad idea. If you have to jump, throw an exception, as proposed by joshcomley; if you just want to output the warning message, but continue without a jump, call a method.
Situation:
My application need to process the first step in the business rules (the initial try-catch statement). If an certain error occurs when the process calls the helper method during the step, I need to switch to a second process in the catch statement. The back up process uses the same helper method. If an same error occurs during the second process, I need to stop the entire process and throw the exception.
Implementation:
I was going to insert another try-catch statement into the catch statement of the first try-catch statement.
//run initial process
try
{
//initial information used in helper method
string s1 = "value 1";
//call helper method
HelperMethod(s1);
}
catch(Exception e1)
{
//backup information if first process generates an exception in the helper method
string s2 = "value 2";
//try catch statement for second process.
try
{
HelperMethod(s2);
}
catch(Exception e2)
{
throw e2;
}
}
What would be the correct design pattern to avoid code smells in this implementation?
I caused some confusion and left out that when the first process fails and switches to the second process, it will send different information to the helper method. I have updated the scenario to reflect the entire process.
If the HelperMethod needs a second try, there is nothing directly wrong with this, but your code in the catch tries to do way too much, and it destroys the stacktrace from e2.
You only need:
try
{
//call helper method
HelperMethod();
}
catch(Exception e1)
{
// maybe log e1, it is getting lost here
HelperMethod();
}
I wouldn't say it is bad, although I'd almost certainly refactor the second block of code into a second method, so keep it comprehensible. And probably catch something more specific than Exception. A second try is sometimes necessary, especially for things like Dispose() implementations that might themselves throw (WCF, I'm looking at you).
The general idea putting a try-catch inside the catch of a parent try-catch doesn't seem like a code-smell to me. I can think of other legitimate reasons for doing this - for instance, when cleaning up an operation that failed where you do not want to ever throw another error (such as if the clean-up operation also fails). Your implementation, however, raises two questions for me: 1) Wim's comment, and 2) do you really want to entirely disregard why the operation originally failed (the e1 Exception)? Whether the second process succeeds or fails, your code does nothing with the original exception.
Generally speaking, this isn't a problem, and it isn't a code smell that I know of.
With that said, you may want to look at handling the error within your first helper method instead of just throwing it (and, thus, handling the call to the second helper method in there). That's only if it makes sense, but it is a possible change.
Yes, a more general pattern is have the basic method include an overload that accepts an int attempt parameter, and then conditionally call itself recursively.
private void MyMethod (parameterList)
{ MyMethod(ParameterList, 0)l }
private void MyMethod(ParameterList, int attempt)
{
try { HelperMethod(); }
catch(SomeSpecificException)
{
if (attempt < MAXATTEMPTS)
MyMethod(ParameterList, ++attempt);
else throw;
}
}
It shouldn't be that bad. Just document clearly why you're doing it, and most DEFINITELY try catching a more specific Exception type.
If you need some retry mechanism, which it looks like, you may want to explore different techniques, looping with delays etc.
It would be a little clearer if you called a different function in the catch so that a reader doesn't think you're just retrying the same function, as is, over again. If there's state happening that's not being shown in your example, you should document it carefully, at a minimum.
You also shouldn't throw e2; like that: you should simply throw; if you're going to work with the exception you caught at all. If not, you shouldn't try/catch.
Where you do not reference e1, you should simply catch (Exception) or better still catch (YourSpecificException)
If you're doing this to try and recover from some sort of transient error, then you need to be careful about how you implement this.
For example, in an environment where you're using SQL Server Mirroring, it's possible that the server you're connected to may stop being the master mid-connection.
In that scenario, it may be valid for your application to try and reconnect, and re-execute any statements on the new master - rather than sending an error back to the caller immediately.
You need to be careful to ensure that the methods you're calling don't have their own automatic retry mechanism, and that your callers are aware there is an automatic retry built into your method. Failing to ensure this can result in scenarios where you cause a flood of retry attempts, overloading shared resources (such as Database servers).
You should also ensure you're catching exceptions specific to the transient error you're trying to retry. So, in the example I gave, SqlException, and then examining to see if the error was that the SQL connection failed because the host was no longer the master.
If you need to retry more than once, consider placing an 'automatic backoff' retry delay - the first failure is retried immediately, the second after a delay of (say) 1 second, then doubled up to a maximum of (say) 90 seconds. This should help prevent overloading resources.
I would also suggest restructuring your method so that you don't have an inner-try/catch.
For example:
bool helper_success = false;
bool automatic_retry = false;
//run initial process
try
{
//call helper method
HelperMethod();
helper_success = true;
}
catch(Exception e)
{
// check if e is a transient exception. If so, set automatic_retry = true
}
if (automatic_retry)
{ //try catch statement for second process.
try
{
HelperMethod();
}
catch(Exception e)
{
throw;
}
}
Here's another pattern:
// set up state for first attempt
if(!HelperMethod(false)) {
// set up state for second attempt
HelperMethod(true);
// no need to try catch since you're just throwing anyway
}
Here, HelperMethod is
bool HelperMethod(bool throwOnFailure)
and the return value indicates whether or not success occurred (i.e., false indicates failure and true indicates success). You could also do:
// could wrap in try/catch
HelperMethod(2, stateChanger);
where HelperMethod is
void HelperMethod(int numberOfTries, StateChanger[] stateChanger)
where numberOfTries indicates the number of times to try before throwing an exception and StateChanger[] is an array of delegates that will change the state for you between calls (i.e., stateChanger[0] is called before the first attempt, stateChanger[1] is called before the second attempt, etc.)
This last option indicates that you might have a smelly setup though. It looks like the class that is encapsulating this process is responsible for both keeping track of state (which employee to look up) as well as looking up the employee (HelperMethod). By SRP, these should be separate.
Of course, you need to a catch a more specific exception than you currently are (don't catch the base class Exception!) and you should just throw instead of throw e if you need to rethrow the exception after logging, cleanup, etc.
You could emulate C#'s TryParse method signatures:
class Program
{
static void Main(string[] args)
{
Exception ex;
Console.WriteLine("trying 'ex'");
if (TryHelper("ex", out ex))
{
Console.WriteLine("'ex' worked");
}
else
{
Console.WriteLine("'ex' failed: " + ex.Message);
Console.WriteLine("trying 'test'");
if (TryHelper("test", out ex))
{
Console.WriteLine("'test' worked");
}
else
{
Console.WriteLine("'test' failed: " + ex.Message);
throw ex;
}
}
}
private static bool TryHelper(string s, out Exception result)
{
try
{
HelperMethod(s);
result = null;
return true;
}
catch (Exception ex)
{
// log here to preserve stack trace
result = ex;
return false;
}
}
private static void HelperMethod(string s)
{
if (s.Equals("ex"))
{
throw new Exception("s can be anything except 'ex'");
}
}
}
Another way is to flatten the try/catch blocks, useful if you're using some exception-happy API:
public void Foo()
{
try
{
HelperMethod("value 1");
return; // finished
}
catch (Exception e)
{
// possibly log exception
}
try
{
HelperMethod("value 2");
return; // finished
}
catch (Exception e)
{
// possibly log exception
}
// ... more here if needed
}
An option for retry (that most people will probably flame) would be to use a goto. C# doesn't have filtered exceptions but this could be used in a similar manner.
const int MAX_RETRY = 3;
public static void DoWork()
{
//Do Something
}
public static void DoWorkWithRetry()
{
var #try = 0;
retry:
try
{
DoWork();
}
catch (Exception)
{
#try++;
if (#try < MAX_RETRY)
goto retry;
throw;
}
}
In this case you know this "exception" probably will happen so I would prefer a simple approach an leave exceptions for the unknown events.
//run initial process
try
{
//initial information used in helper method
string s1 = "value 1";
//call helper method
if(!HelperMethod(s1))
{
//backup information if first process generates an exception in the helper method
string s2 = "value 2";
if(!HelperMethod(s2))
{
return ErrorOfSomeKind;
}
}
return Ok;
}
catch(ApplicationException ex)
{
throw;
}
I know that I've done the above nested try catch recently to handle decoding data where two third party libraries throw exceptions on failure to decode (Try json decode, then try base64 decode), but my preference is to have functions return a value which can be checked.
I generally only use the throwing of exceptions to exit early and notify something up the chain about the error if it's fatal to the process.
If a function is unable to provide a meaningful response, that is not typically a fatal problem (Unlike bad input data).
It seems like the main risk in nested try catch is that you also end up catching all the other (maybe important) exceptions that might occur.