If Single doesn't find the element you're expecting to exist then it throws an InvalidOperationException. Only trouble is that other things result in this exception too. For example an EF Code First model being out of date.
I've tried to narrow it down by checking the exception message. However this could change and I'd be none the wiser. Is there a better way of catching this problem?
try
{
return DbSet.Single(filter);
}
catch (InvalidOperationException exc)
{
if (exc.Message == "Sequence contains no elements")
{
throw new UserNotFoundException();
}
throw;
}
Use .FirstOrDefault() instead. Then check to see if the result is null. If it is, the user wasn't found.
Related
Is there any practical difference between using:
throw Exception('message');
vs
throw 'message';
When I want to throw an error in flutter?
throw 'message' throws a String.
Callers trying to catch it either would need to do:
try {
throw 'message';
} on String catch (s) {
...
}
or would need to use a blanket, untyped catch clause (which isn't recommended). Doing this would be very unusual and would not be behavior callers would expect. There is an only_throw_errors lint to warn about this.
throw Exception('message'); throws a constructed Exception object. That's far more typical.
It might be obvious, but I still fail to understand the difference between throw and rethrow and when do either of those should be used?
According to Effective Dart:
If you decide to rethrow an exception, prefer using the rethrow statement instead of throwing the same exception object using throw. rethrow preserves the original stack trace of the exception. throw on the other hand resets the stack trace to the last thrown position.
The biggest difference is the preservation of the original stack trace.
They provided 2 examples to show the intended usage:
Bad:
try {
somethingRisky();
} catch (e) {
if (!canHandle(e)) throw e;
handle(e);
}
Good:
try {
somethingRisky();
} catch (e) {
if (!canHandle(e)) rethrow;
handle(e);
}
If my single errors because of a networkexception return Single.just(false)
If my single errors because of another reason return Single.error
If my single succeeds return the original Single value.
this should be as easy as
getStudent(studentId)
.onErrorResumeNext { if (it is NetworkException) return #onErrorResumeNext Single.just(true)
return Single.error(it) }
Type inference failed. Expected type mismatch SingleSource found Single
Your Single needs to return the same type as your source (I'm assuming getStudent() isn't returning a Boolean). If you want to represent a "success" and "error" states, Kotlin has a Result class just for this.
E.g.
getStudent()
.map { student ->
// Your logic here may look different
Result.success(student)
}
.onErrorResumeNext { error ->
if (error is NetworkException){
Single.just(Result.failure(error))
} else {
Single.error(error)
}
}
This will catch network errors and wrap the exception in a Result, all other exceptions will be propagated downstream. You can then choose how to handle the error in your subscribe method.
Depending on your use case however, you may want to also look into using Maybe or the retry() operator.
I'm trying to catch my custom Error, but for some reason my catch statements where I name the error that I know is being thrown, it skips those, goes to the default catch, and then gives me a EXC_BAD_INSTRUCTION (code=EXC_I386_INVOP, subcode=0x0) when I try to do print("Unexpected error \(error)")
Here's some abbreviated code:
This is the error that I have declared in my file that houses the class that I'm calling the method on (the class is called CC8DB):
public enum CC8RSVPError: Error {
case noEventOnDate
case invalidRSVPValue
}
I have a method declared as:
public func rsvpForEvent(_ inEventDate:Date?, forUserID inUserID:String, withValue inRSVPValue:String) throws -> CC8RSVPStatus
In another class were I'm calling this method, I have this:
do {
let rsvpResponse = try self.cc8DB.rsvpForEvent(inRSVPDate, forUserID: String(inMessage.author.id.rawValue), withValue: inRSVPValue);
...(other code to do when this doesn't fail)...
} catch CC8RSVPError.invalidRSVPValue {
...(Report specific error to user)...
} catch CC8RSVPError.noEventOnDate {
...(Report specific error to user)...
} catch {
...(Report general error to user)...
print("Error doing RSVP: \(error)");
}
And finally, in the CC8DB.rsvpForEvent() method, I'm triggering an error that does this:
throw CC8RSVPError.invalidRSVPValue;
The germane part of this method is:
public func rsvpForEvent(_ inEventDate:Date?, forUserID inUserID:String, withValue inRSVPValue:String) throws -> CC8RSVPStatus
{
var retStatus = CC8RSVPStatus(eventDate: nil, previousRSVP: "", newRSVP: "");
var upperRSVPValue:String = inRSVPValue.uppercased();
if (["YES", "MAYBE", "NO"].contains(upperRSVPValue)) {
//...(Code to do things when the info is correct)...
} else {
throw CC8RSVPError.invalidRSVPValue;
}
return retStatus;
}
For my test case where I'm seeing this, the inRSVPValue is "bla", to test what happens when a user doesn't enter a valid status value.
What I'm seeing is that rather than going into the catch that's specific for the CC8RSVPError.invalidRSVPValue case, it's going down to the general catch. In addition, I'm getting the EXC_BAD_INSTRUCTION on the line where I try and print the error value.
I've stepped through it to verify that I am indeed hitting the throw line that I think I am, and I can see in the debugger that the value of error is CC8DB.CC8RSVPError.invalidRSVPValue, but even if I try to do po error from the lldb command, I get the same exception error.
Has anyone seen this or know what I could have done to make do-try-catch not work right?
You could assign a constant named error inside your catch statement and inside the catch block read the constant and figure out what to do with it.
do something like:
} catch let error {
switch error {
case CC8RSVPError.noEventOnDate:
// code
case CC8RSVPError.invalidRSVPValue:
// code
}
}
Ok, I figured it out. I realized that somewhere along the way, some build setting got set so that I was statically linking into the binary (this is a command-line tool, a bot for Discord to be specific).
I saw some warnings about some of the Swift runtime libs being found in both the binary and the XCode developer runtime area, and realized that it might be that the error object was being used both in my CC8DB module in the binary and in the built-modules folder (or something to that effect).
I need to statically link for when I actually deploy the bot to where it's going to run, so I must have turned something on that won't turn off (I deleted the extra flags that I thought turned that on, but that wasn't fixing it).
Basically, I recreated my .xcodeproj file with swift package generate-xcodeproj to clear out whatever I broke, and now it works as expected.
Thanks to everyone who looked at this and offered help (especially #gmogames for his time and help). I'm sure it helped lead me down the path of figuring this out.
Is there a hook in NUnit to execute code only when assertion fails without catching the exception itself. Basically, it should accept action delegate to be executed when assertion fails and then re-throw exception. Why do I need this?
I need to compare two objects and dump the result on the screen, for easier debugging, when assertion fails.
Something like this works but is a bad hack, The problem is that it eagerly evaluates ProcessCompareError so I have unnecessary overhead, plus it does it no matter if there is an error or not. So, is there overload that will accept the delegate that would be executed when assertion fails?
Assert.That(benefitLimitComparer.Compare(copyBenefitLimit, origBenefitLimit), Is.EqualTo(0),limitError, ProcessCompareError(origBenefitLimit, copyBenefitLimit));
}
}
}
private string ProcessCompareError(BenefitLimit origBenefitLimit, BenefitLimit copyBenefitLimit)
{
Console.WriteLine("Original: ");
ObjectDumper.Write(origBenefitLimit);
Console.WriteLine("Copy");
ObjectDumper.Write(copyBenefitLimit);
return "";
}
I'm not sure how it might be done through a delegate. One alternative is to store the result of the Compare. If the result is false, write out the contents of the objects and then call Assert.Fail()
There is a possibilty to wrap an assert as an Action in a try-catch. In the catch you can handle the additional compare:
public static void ExecuteAssert(Action assert)
{
if (assert == null) return;
try
{
assert();
}
catch (Exception ex)
{
// perform the compare
}
}
As remark: I use a similar method to continue test execution and avoid the entire test to stop, if some non-fatal checks fail. Actually I iterate through a number of actions:
private static void VerifyAll(params Action[] asserts)