I have met this a lot recently reading other people's scripts. A short example is below:
Say we need input and store them in var A and B, the scheme is below:
int ok;
ok = false;
while(!ok){
//ask input for A
//ask input for B
ok = true;
}
I understand what it wants, but why is this scheme necessary? can I only have "ask input for A and B".
but why is this scheme necessary?
It is not necessary.
can I only have "ask input for A and B".
You sure can.
However, if user gives you input that is not useful (for example: you ask for the users age, and they type "horse"), then you might want to ask again. Allowing re-trying of input is generally a useful feature. The canonical control structure for repeating a piece of program is a loop.
Your example program however, sets ok unconditionally, so in that case there is really no use for the loop. The loop makes sense only if there is some form of validation that must be passed before the input is OK.
When there are no checks in the code you omitted, but you see this same construct all over the place, then it's a copy&paste artifact.
Someone had a piece of code that was reading input and validating it, then copied the code somewhere else, removed the validation bits, and left the rest as-is. Then they copy&pasted that code all over the place.
In my experience, this happens very often.
I'm trying to wrap my head around Try-Catch statements in vb.net. I am trying to handle a SQL exception error that occurs, but I'm not sure what to do with it? The particular method that contains the code sample below is also expecting a return value.
Try
Dim records As DataRecordCollection = Sql Insert SP call
Catch sqlEx As SqlException
Select Case sqlEx.Number
Case 547
*****What goes here?*****
Case Else
Throw
End Select
End Try
You can put whatever you want there. We can't give you the answer to this question, because it could be anything. It could also be nothing; you don't have to put anything at all in that place, though it's usually poor practice to just swallow an exception like that.
The point is what you put there depends entirely on your application. You might make a log entry, or clean up the error to show something nicer to the user, or put other code there to recover or try again, or even all of the above. Whatever you want. But we can't know what you want to do. That's up to you and your design specification.
Also, you can simplify this code using a conditional exception:
Try
Dim records As DataRecordCollection = Sql Insert SP call
Catch sqlEx As SqlException When sqlEx.Number = 547
// Put whatever you want here
End Try
Finally, in my experience the best option is usually to skip the Try/Catch block at this level entirely.
If you have a well-designed application, your database access is abstracted away into it's own class, assembly, namespace, or some combination thereof, that is separate from the UI or Business layer. My experience is that handling these exceptions in the database code is not as helpful as allowing the exception to bubble up to a higher level of abstraction. You'll be better positioned to deal with it there. That's kind of what Try/Catch is all about... that exceptions can be caught at the level that is most appropriate to that kind of exception.
This is especially true when you don't even know what you want to do. If you don't have a plan to handle an exception, then don't handle it. Ditch the Try/Catch block and let the exception bubble up to a higher level where maybe someone else has a better strategy for it.
The Microsoft SQL server stores the error messages in the database so you can query it in your server:
select * from sys.messages where message_id = 427 and language_id = 1033
When I run it on my Sql Server 2016 Express, I got the following result:
Could not load the definition for constraint ID %d in database ID %d. Run DBCC CHECKCATALOG to verify the integrity of the database.
I suggest you to debug the error message not only the number and you will get some additional information about your problem. So please log the error message too, write that here and with that we will be able to help you.
Thank you,
Morzel
I'm working on refactoring a decently large project, and I'm interested in searching out and reducing Code Clone for better standardization as well as ease of development.
I've got a code snippet that keeps coming up in "Exact Matches" (using Visual Studio 2012's
"Find Code Clones" feature).
Here it is:
End If
End Using
Catch ex As Exception
MsgBox(ex.Message)
End Try
End Using
End Using
Return Nothing
Basically, I have a whole host of similar functions (similar in structure but not in actual function) that all open with something like this:
Const sql As String = ...
Using cn As SqlConnection...
Using cmd As SqlCommand(sql,cn)
... Maybe add some SqlParameters
cn.Open()
... Something with cmd.Execute...
Now, I recognize that the first code block is IDENTICAL across many, many methods, but I can't figure out a way to pull that code out, write it once and simply call it each time I need that functionality. It seems to me that there is too much control flow occurring.
So, I'm stumped as to how to fix this (and I'm only assuming that I can "fix" this because Microsoft has identified it as "cloned code").
I'm thinking of something along the lines of making a small number of functions that do the same type of thing (like returning a count from a table, returning a top value, etc...) and only really differ by the SQL that is executed. That could be a bit tricky, though, since sometimes the parameters (type and number) differ.
Any thoughts?
I wouldn't concern yourself with those. Your common-sense first impression is correct. While, technically speaking, the syntax is repeated multiple times, it is not really a repetition of logic or an algorithm of any kind. That's not to say there's no way to reduce that repetition, it's just that by doing so, you will likely end up with a worse design in your code.
For instance, you could create a single method that does all the setup and tear-down and then just calls a method in the middle that actually uses the connection to do the work, such as:
Public Function PerformDbTask(task As IMyDbTask)
Using cn As SqlConnection...
Using cmd As SqlCommand = cn.CreateCommand()
Try
cn.Open()
task.Perform(cmd)
Catch ex As Exception
MsgBox(ex.Message)
End Try
End Using
End Using
Return Nothing
End Function
However, what have you really gained? Probably not much at all, yet you've lost a lot of flexibility. So, unless that kind of design is actually necessary for what you are trying to do, I wouldn't waste time trying to solve a problem that doesn't exist.
You could create a class that implements a builder pattern, that winds up looking like
Dim list =
SqlBuilder.Query("SELECT ...")
.WithConnection("...connection string...")
.WithParameter(userName)
.WithParameter(lastTrackId)
.RetrieveTopRows(10)
Your problem is you are using a clone detector that matches token sequences (thus it matches the end-of-block sequences you exhibited), as opposed to clone detectors that match code structures. While the token sequence is technically a clone, it isn't an interesting clone. Token clone detectors produce many many "false positive" clones like this, which simply waste your time.
It is easier to build token sequence detectors, which is why they built that into MS Studio.
If you want better detectors, you have to step outside of Studio.
You should look into clone detectors that match on abstract syntax trees. They don't produce this kind of false positive, and they can find clones with complex parameters (as opposed to single-token parameters).
You should also understand that just because something has been identified as a clone, that it is not always easy or possible to refactor it away. The language you have may not have sufficiently strong abstraction mechanisms to handle that case.
so, I am parsing Hayes modem AT commands. Not read from a file, but passed as char * (I am using C).
1) what happens if I get something that I totally don't recognize? How do I handle that?
2) what if I have something like
my_token: "cmd param=" ("value_1" | "value_2");
and receive an invalid value for "param"?
I see some advice to let the back-end program (in C) handle it, but that goes against the grain for me. Catch teh problem as early as you can, is my motto.
Is there any way to catch "else" conditions in lexer/parser rules?
Thanks in advance ...
That's the thing: the whole point of your parser and lexer is to blow up if you get bad input, then you catch the blow up and present a pretty error message to the user.
I think you're looking for Custom Syntax Error Recovery to embed in your grammar.
EDIT
I've no experience with ANTLR and C (or C alone for that matter), so follow this advice with caution! :)
Looking at the page: http://www.antlr.org/api/C/using.html, perhaps the part at the bottom, Implementing Customized Methods is what you're after.
HTH
I have process that needs to create a bunch of records in the database and roll everything back if anything goes wrong. What I want to do is this:
Public Structure Result
Public Success as Boolean
Public Message as String
End Structure
Private _Repository as IEntityRepository
Public Function SaveOrganization( _
ByVal organization As rv_o_Organization) As Result
Dim result = Result.Empty
_Repository.Connection.Open()
_Repository.Transaction = _Repository.Connection.BeginTransaction()
''//Performs validation then saves it to the database
''// using the current transaction
result = SaveMasterOrganization(organization.MasterOrganization)
If (Not result.Success) Then
GoTo somethingBadHappenedButNotAnException
End If
''//Performs validation then saves it to the database
''//using the current transaction
result = SaveOrganziation(dbOrg, organization)
If (Not result.Success) Then GoTo somethingBadHappenedButNotAnException
somethingBadHappenedButNotAnException:
_Repository.Transaction.Commit()
_Repository.Connection.Close()
Return result
End Sub
Is this an ok use of the GoTo statement, or just really bad design? Is there a more elegant solution? Hopefully this sample is able to get the point across
If you have to ask, don't do it.
For your specific code, you could do it like this:
Public Function SaveOrganization(ByVal organization As rv_o_Organization) As Result
Dim result As Result = Result.Empty
_Repository.Connection.Open()
_Repository.Transaction = _Repository.Connection.BeginTransaction()
'Performs validation then saves it to the database
'using the current transaction
result = SaveMasterOrganization(organization.MasterOrganization)
'Performs validation then saves it to the database
'using the current transaction
If result.Success Then result = SaveOrganziation(dbOrg, organization)
_Repository.Transaction.Commit()
_Repository.Connection.Close()
Return result
End Sub
Goto has such a terrible reputation that it will cause other developers to instantly think poorly of your code. Even if you can demonstrate that using a goto was the best design choice - you'll have to explain it again and again to anyone who sees your code.
For the sake of your own reputation, just don't do it.
Really bad design. Yes.
There might be some extreme edge cases where it is applicable, but almost unequivocally, no, do not use it.
In this specific case, you should be using the Using statement to handle this in a better manner. Generally, you would create a class which would implement IDisposable (or use one that already does) and then handle cleanup in the Dispose method. In this case, you would close the connection to the database (apparently, it is reopened again from your design).
Also, I would suggest using the TransactionScope class here as well, you can use that to scope your transaction and then commit it, as well as auto-abort in the face of exceptions.
The only time you should use a goto, is when there is no other alternative.
The only way to find out if there are no other alternatives is to try them all.
In your particular example, you should use try...finally instead, like this (sorry, I only know C#)
void DoStuff()
{
Connection connection = new Connection();
try
{
connection.Open()
if( SomethingBadHappened )
return;
}
finally
{
connection.Close();
}
}
I would say extremely sparingly. Anytime I have had to think about using a GOTO statement, I try and refactor the code. The only exception I can think of was in vb with the statement On Error Goto.
There is nothing inherently wrong with goto, but this isn't a very ideal use of it. I think you are putting too fine a point on the definition of an exception.
Just throw a custom exception and put your rollback code in there. I would assume you would also want to rollback anyway if a REAL exception occured, so you get double duty out of it that way too.
Gotos are simply an implementation detail. A try/catch is much like a goto (an inter-stack goto at that!) A while loop (or any construct) can be written with gotos if you want. Break and early return statements are the most thinly disguised gotos of them all--they are blatant(and some people dislike them because of the similarity)
So technically there is nothing really WRONG with them, but they do make for more difficult code. When you use the looping structures, you are bound to the area of your braces. There is no wondering where you are actually going to, searching or criss-crossing.
On top of that, they have a REALLY BAD rep. If you do decide to use one, even in the best of possible cases, you'll have to defend your decision against everyone who ever reads your code--and many of those people you'll be defending against won't have the capability to make the judgment call themselves, so you're encouraging bad code all around.
One solution for your case might be to use the fact that an early return is the same as a goto (ps. Worst psuedocode ever):
dbMethod() {
start transaction
if(doWriteWorks())
end Transaction success
else
rollback transaction
}
doWriteWorks() {
validate crap
try Write crap
if Fail
return false
validate other crap
try Write other crap
if Fail
return false
return true
}
I think this pattern would work in VB, but I haven't used it since VB 3 (around the time MS bought it) so if transactions are somehow bound to the executing method context or something, then I dunno. I know MS tends to bind the database very closely to the structure of the code or else I wouldn't even consider the possibility of this not working...
I use goto's all the time in particular places, for example right above a Try Catch, in case a you prompt the user, "Retry?, Cancel", if retry then Goto StartMyTask: and increment the Current Try of Maximum retries depending on the scenario.
They're also handy in for each loops.
For each Items in MyList
If VaidationCheck1(Item) = false then goto SkipLine
If ValidationCheck(Item) = false then goto skipline
'Do some logic here, that can be avoided by skipping it to get better performance.
'I use then like short circuit operands, why evaluate more than you actually have to?
SkipLine:
Next
I wouldn't substitute them for functions and make really huge blocks of long code, just in little places where they can really help, mostly to skip over things.
Everytime I've seen a goto used, simple refactoring could have handle it. I'd reccomend never using it unless you "know" that you have to use it
I'm tempted to say never, but I suppose there is always one case where it may be the best solution. However, I've programmed for the last 20 years or so without using a Goto statement and can't foresee needing one any time soon.
Why not wrap each function call in a try catch block and when that is done, if one of your exceptions are thrown, you can catch it and close the connection. This way, you avoid the GOTO statement altogether.
In short, the GOTO statement is NOT a good thing, except in unusual situations, and even then it is usually a matter of refactoring to avoid it. Don't forget, it is a leftover from early languages, in this case BASIC.
I'd say use very sparingly as it's generally associated with introducing spagetti code. Try using methods instead of Labels.
A good case I think for using GOTO is to create a flow through select which is available in C# but not VB.
The go to statement has a tendency to make the program flow difficult to understand. I cannot remember using it during the last ten years, except in visual basic 6 in combination with "on error".
Your use of the go to is acceptable as far as I am concerned, because the program flow is very clear. I don't think using try ... catch would improve things much, because you would need to throw the exceptions on the locations where the go tos are now.
The formatting, however, is not very appealing:-)
I would change the name of the go to label to something else, because this location is also reached when everything is successful. clean_up: would be nice.
Eh, there was a decent use for it in VBscript/ASP for handling errors. We used it to return error handling back to ASP once we were done using on error resume next.
in .net? Heavens, no!
Everyone says avoid it but why.
The GOTO syntax is a jump statement in assembly - very efficient.
Main reason to avoid it is code readability. You need to find the GOTO label in the code which is hard to eyeball.
Some people think that it might cause memory leaks but I've seen that experts say that this is not true in .NET.