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You have two options. You could return default(DateTime); or change the return type to Nullable<DateTime>
[edit] You could also rethrow the exception. I find this a tad ugly, but whatever works for you
modified on Thursday, December 3, 2009 12:05 PM
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Thank Your friend ...
for Your replay ..
0x3c0 wrote: Nullable<datetime>
what this line will return? sorry i do not know.
how to use this Nullable<datetime>?
syntax please?
thank U
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You replace private DateTime convertByteToDatetime(byte[] Data) with private Nullable<DateTime> convertByteToDatetime(byte[] Data). Then you can either return new Nullables, or just keep your code and rely on the implicit operators. If you do that, you don't need to do anything apart from checking HasValue in your calling code. If that's true, then retrieve value.
Example:
Nullable<DateTime> nullableDate = convertByteToDatetime(new byte[] {0, 0, 0});
if(nullableDate.HasValue)
else
However, please be aware that this will call your code to fail silently. I would personally raise an event detailing the exception before I return null so that I know precisely what went wrong without having to sprinkle try-catch blocks everywhere.
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Thank You frined .....
i shall try that .
 
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=>Joe<= wrote: if there is an error...
... throw an Exception.
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thanks for your replay...
PIEBALDconsult wrote: ... throw an Exception.
i do not want stop the process ... in other words i have skip to othere work ..
thank U
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That's up to the calling routine to decide.
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=>Joe<= wrote: Any idea.? suggestion?
AFAIK there are four popular ways, you've had most of them already:
1.
set aside a special value, could be Default<DateTime> or any other value you don't need as a real value. For DateTime, there is also DateTime.MaxValue
2.
nullable types, written either as Nullable<DateTime> or DateTime? (with question mark)
I don't like them much as they take more memory, and cause some trouble when parts of your environment don't know how to handle them (e.g. files, databases, ...).
3.
Use an Exception. That often is the most elegant approach, assuming the "no value" situation really is exceptional.
[ADDED, AS SUGGESTED BY PIEBALD]
4.
Return a boolean and have the value in an out parameter. Like with TryParse.
[/ADDED]
modified on Thursday, December 3, 2009 2:59 PM
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thank you friend for your replay...and the suggestion ...
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Now it's worth a ten.
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can you explique more please?? do you like to return DataTime null in the catch??
why??
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Luc should probably also have said:
4. Return a boolean and have the value in an out parameter. Like with TryParse.
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fixed
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Hi there
I am having problems developing a C# application that sorts the data in an
SQL database by a particular column and then outputs it to a csv file on my
hard drive.
I want to do something like this:
SqlCommand sqlComm = new SqlCommand("SELECT * FROM TableA ORDER BY ColumnB
INTO OUTFILE ‘C:\output_file.csv’", myConnection);
Is this the right way to go about it?
Thanks in advance
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totally_stumped wrote: SELECT * FROM TableA ORDER BY ColumnB
INTO OUTFILE ‘C:\output_file.csv’
Doesn't looks like it will execute on SQL server. AFAIK, you will need to use BCP to export data to csv file. Or, you can get the data from database in your code and then write that to CSV.
50-50-90 rule: Anytime I have a 50-50 chance of getting something right, there's a 90% probability I'll get it wrong...!!
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You should know that the sql command is executed on the computer where the MS SQL server is installed. This means that "C:\output_file.csv" is server's C drive. You may have no access to it! The second problem is the query itself. I think that this query is only available for MySQL user.
A better approach will be to get the data from the server into a DataSet (on client side) and write a simple method to convert the data to scv file.
I am quite sure that an article explaining how to do this exists in CP's articles.
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Maybe you should ask in the database forum.
But seriously; you'll need to write some code. Use a DataReader to read the data, then write each row to the file. It's not a big deal.
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I am kind of digging into this threading business and having some trouble. I created the threads (one for prefilling a bunch of information from a SQL database, and another one for inserting files into the database if the user chooses to do so). So the first on happens on load and the other one could happen at any time by either clicking the Attach button, or by dragging and dropping a file from the desktop to the listview.
Problem is when I spawn the thread and the user closes the form before the file is uploaded (the form is a child form and is not the parent main form), it gives an error about the control cannot be invoked when the handle is not created. OK, I understand that it has disposed of the forms controls and thats why it has done this... but how do I make sure these threads are stopped before disposing of all the components on the form?
Here is the method I used to pass the information about the file. So I call this method on the main thread and this methods sets the global variables (so the new thread can access them and have the information to insert into the database), show the progressbar, and then spawn the thread. As you can see the thread is not a declared globally so the dispose method cannot access it directly correct?
private void SaveFile(string fileName, string safeFileName, bool fileMR)
{
this.fileName = fileName;
this.safeFileName = safeFileName;
this.fileMR = fileMR;
if (fileMR)
picProgressFilesMR.Visible = true;
else
picProgressFiles.Visible = true;
Thread fileThread = new Thread(new ThreadStart(saveFiles));
fileThread.IsBackground = true;
fileThread.Start();
}
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Hi,
there are different aspects involved:
1. FWIW: when a thread needs to update the GUI, you have to use Control.InvokeRequired/Control.Invoke [^]
2. A BackgroundWorker may or may not make life a little easier.
3. saving data to files probably should always run to completion, so I would not use a background thread here.
4. when a Form should not be closed yet because something it started (e.g. saving files) hasn't finished yet,
you could check for that in the FormClosing event, and inform/ask the user what to do, possibly setting Cancel true.
5. Aborting a thread is always a bad idea, you don't know in what state its objects are going to be left behind. A cooperative cancel is OK, however it isn't always feasible.
Jacob Dixon wrote: the thread is not a declared globally so the dispose method cannot access it directly correct?
wrong. An object is collectible as soon as it goes out of scope. A disposable object will have its finalizer (and hence Dispose) called by the GC when the GC determines it is collectible.
Thread objects are a bit special as MSDN says: It is not necessary to retain a reference to a Thread object once you have started the thread. The thread continues to execute until the thread procedure is complete.
Which I would rephrase as: a thread keeps itself alive and uncollectible until it reaches the end of its code.
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Oh... In the method that I created the thread for it does use invoke.. Now while we are on that subject I've noticed there are multiple ways of invoking a control. The way I chose to do it required less lines of code in my opinion and without the need of created another method outside of this one:
Invoke((Action)(() => { labelFiles.Text = string.Format("Saving File... Size: {0}MB", Math.Round((fileSize / 1024) / 1024, 2)); }));
The way I have been doing it was like this:
private delegate void FilesLabelDelegate(string text);
private void FilesLabel(string text)
{
labelFiles.Tex = "blahblahblah";
}
Invoke(new FilesLabelDelegate(FilesLabel), new object[] { "blahblahblah" }));
I'm assuming that between the two isn't really a performance difference that would matter as much?
Anyways,
So before the thread starts I could set something saying that a file is currently being saved, and when the thread comes to an end I need to set that something back to false (like a variable). And on the FormClosing if that variable is true then display that we can't exit until the file is saved and call the e.Cancel = true;
That is a great idea thanks
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1.
There are a couple of ways to correctly access the GUI from another thread; I'm strongly in favor of the one I call the "canonical form", as it works on all versions of C# and VB.NET
2.
yes your FormClosing needs to check your app's state and, when necessary, communicate with the user. How exactly is a design choice: "busy" and wait (  ); "busy" + cancel button; "busy" and ignore closing; simply ignore closing ( which makes it look like a bug).
my preference is to save files synchronously, without a thread; the one exception is when you can and often will start saving long before the user is likely to close the form. Anyway I as a user want to make sure things got saved before I drastically change the GUI state.
Rationale: saving what I entered/changed isn't taking as long as me entering/changing it, and I'm willing to spend that time to make sure and avoid the possibility I have to redo it.
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Uhmm.. Ok I think I am going to go with a couple of you guys ideas put together. I think I might use a background worker that supports cancelling. If the bw is busy then I will ask the user if they want to continuing waiting until it is finished or stop the current process and close.
The reason I want to use it on a seperate thread is because it is a inventory screen. Often we must go in and enter updated information (which can be a lot) and save files. I don't really want saving a file to freeze the GUI when they could start saving the file and continue working on something else.
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Things to keep in mind:
- cancelling a BGW (or any threaded action) requires cooperation; if there is only one big action (one SQL statement executing), then cancelling means waiting for completion.
- the "continue working on something else" may encounter a DB table that is being updated and possibly inconsistent.
Also I'm not convinced saving should take that long; if anything it tells me you are probably loading way too much data in your Form to be handy (yielding long Form load times as well then).
A paging system (such as the CP forums themselves) might be more appropriate.
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the other thng you may want to do.. And it might be conceptually easier is to
thread[0] : run Main -> all your saving loading etc..
thread[1] : run Form
thread[2] : other(background worker from Form)
to the user this will look like the application "Quit" however it will continue running in the background until it completes it's task.
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It sounds like your thread is accessing the child form - maybe the progress control? - after the form has closed and been disposed.
Are you sure you want to cancel the thread when the form closes? Wouldn't it be better to allow the thread to complete? If you do want to cancel, you will have to add cancel logic to your thread method. Don't use Thread.Abort() ! If you want to let the thread complete, you will have to make it check if the form still exists before trying to access it.
BTW, Thread.IsBackground just means the thread will be killed when the main form is closed instead of keeping the process running until it has completed.
Nick
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Be excellent to each other
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