Posts filed under 'MSAccess'
Access programming is inherently single-threaded. That’s usually OK as most operations are sequential anyway and it keeps things simple at the programming level.
There are times though where the lack of ability to run code on another thread is sorely missing: anything that takes a long time to run will just freeze the application, making it unresponsive and appearing to be locked and about to crash to the user.
Checking for the existence of network paths
Checking for the existence of network paths (directories or files) is one of these issues that can freeze an application for 30 seconds or more if the folder is not accessible.
This is a type of problem that benefits greatly from running in a separate thread: it can take such a long time that the best way to check for these remote paths is to launch the verification for their existence outside of Access and somewhat get the result back and cache it for the current session so we don’t have to suffer these delays again every time we check for that path’s existence.
One easy way to do achieve that goal is to create plain DOS batch files that execute hidden from view, create a result file when they complete their task and delete themselves automatically when they are finished.
How to use it
Download the sample database below then just add the FileUtilities, HashTable and MD5 modules to your project and you can use the code as such:
The status variable will return either of the following values:
AsyncDirectoryStatus.OK if the path was found.AsyncDirectoryStatus.NotFound if the path was not found (either because it doesn’t exist or you don’t have the rights to access it).AsyncDirectoryStatus.Checking if the verification is in progress and we haven’t received a definite answer yet.
It’s up to you to decide how you want to handle that case. You could periodically check it, like I did in the example database, or you could disable the controls until you’re getting a confirmed result (by checking every time the user performs some action, like moving from record to record in a datasheet for instance).
You can call PathExistAsync as often as you want to check the status: it will not slow down your application (read the optional arguments section below though).
The result of the verification is cached, so querying the existence of the path is actually only done once; the result of subsequent queries for the same path is just instantly retrieved from memory.
Optional arguments
If you want to force the actual re-checking of a path without using the cached value, you can simply pass the ForceCheck optional parameter:
The first time you query for a path (or force it to be rechecked) there will be a short 150ms delay to give a chance to the function to return its result straight away (in case the path can be resolved quickly).
This may not be desirable if you’re checking a bunch of directories at a time. For instance, this is what I do when my application launches:
By querying the existence of all these paths as soon as my application launches, I am starting the verification process without introducing delays in the application itself: each verification will start in its own process, in parallel to the main application.
Later in the application, when I need to actually use these paths, their result is likely to be known.
How it works
The FileUtilities module contains the main code.
In it, the PathExistAsync function works in slightly different ways depending on whether it’s the first time it is being called for a particular path or not.
The first time
The first time the function is called for a given path, we create in the user’s temporary folder the small batch file whose name is simply a MD5 hash (see below) of the path with .bat appended to it.
This batch file simply checks for the existence of the path and will create a small file (whose name is the MD5 hash of the path) with either 0 or 1 in it depending on the result of the verification.
We initially cache the status of the verification for the Path into the AsyncDirectories hastable (see below) as Checking.
Example of batch file automatically created to verify a path:
The Batch file name is 463C7367D8329BD6209A65A70A7DA08C.bat where the long number is actually the MD5 hash of the path we’re checking \\123.56.78.9\going nowhere\.
Getting back the result
Whenever the PathExistAsync function is called, we check the currently cached result from the AsyncDirectories hastable.
If it is still Checking then we try to verify if we the result file has been created from the running batch. If not, we just return the same status, if yes, we read the result from the file, save it in the hashtable and delete the result file.
Useful libraries
The code makes use of 2 extremely useful libraries that I end up using quite often:
a HashTable implementation.
It makes it easy to create hashtable objects (otherwise known as Associative Arrays) to store and retrieve key/value pairs quickly.
Hashtables are often used to cache data and can be thought of arrays where the index is a string value instead of an number.
Here I use a hashtable to keep track of the paths we’ve checked and their result.
a MD5 hash implementation.
MD5 is a way to get a somewhat unique fixed-length value from a chunk of data.
It’s a mathematical function that guarantees that a small change in input (say a single bit in the input data) has a large effect on the output value (a totally different number will be generated) and that you can’t reverse the function (you can’t obtain the input just by looking at the output).
It is often used in security applications to transform sensitive data like passwords into unique values that can be (somewhat) safely stored because you can’t easily reverse a md5.
Well, MD5 are not that secure any longer but here we just use their ability to transform our path into a unique number that we can easily use as a filename and a key for our hash to retrieve the current status of the path being checked.
Sample database
Download the PathExistAsync01.zip (67KB) containing the Access 2007 ACCDB database.
Download the PathExistAsync02.zip (61KB) containing the MDB database (untested as I only have Access 2007).
License
Please refer to the source code in the database for the exact licensing terms.
Note that the license only refers to code by me. When code from other sources is used, you will have to conform to their own licensing terms.
This work is licensed under a Creative Commons Attribution 3.0 Unported License.
References
June 20th, 2008
This project provides a custom and enhanced message box replacement for the default MsgBoxfound in Access. A Test database for Access 2007 is available at the bottom of this post.
What’s wrong with the default MsgBox
The default message box in Access is sometimes useful to warn, inform or ask confirmation from the user.
It has, however, a few drawbacks:
- It is bland; the standard message box does not follow the currently selected Office 2007 scheme.
- The amount of text it can display is limited. If you try to display too much the text will be truncated.
- You can’t copy or save the content of the message being displayed.
- Because message boxes are viewed as intrusive, people tend not to read them and end-up closing them before they realize it may have contained useful information.
- It only displays plain text. You cannot format the message to draw attention to the key points.
Sometimes you need to display an important message or require users to take an action.
Message boxes are not to be abused but they serve a useful purpose.
An enhanced message box
Rather than using the bland standard message box you can now have something a bit more customized.
Plain Text version of the enhanced custom message box under the Office Blue Colour Scheme:
RichText version of the enhanced custom message box under the Office Black Colour Scheme:
Here are the features of the enhanced message box:
- It is entirely compatible with the standard one: just change
MsgBox to Box
using find and replace should be enough.
- It allows the user to simply click on a button to copy the content of the message to
the clipboard or to save it to a text file to a configurable default location.
- It looks and feel like it belongs to the main application, following its colour scheme.
- It attempts to prevent users from blindly closing the modal box without them making
an effort to read the message: buttons will first be inactive for a configurable amount
of time. It’s not a perfect solution, but it is quite effective.
- There is a
RichBox version that can display rich HTML content, not just plain text, so important parts of the message can be formatted in a useful way.
- It is able to display large amount of data. While it’s not something you usually want, it may be useful for the message box to display more text in some situations (log or tracing information, legal documentation, etc).
- Rather than sprinkling your code with “
& vbCrLf & _” uglies, you can embed newlines in the text itself by using C-style “\n” escape sequences that will automatically be transformed into the appropriate newlines. Makes for clearer code and less typing.
- Because you get the source, you can easily customise the message box with new icons and colours to better match your overall application’s personality.
- It is non-blocking: if your application forces users to log-off after a certain amount of inactivity, the enhanced message box will just close rather than prevent Access from closing like the standard
MsgBox does. Of course, it’s up to you to decide how to handle that gracefully, if at all.
- It properly displays the expected button captions based on the language of the operating system, so it behaves very much like the default
MsgBox (for instance, it will properly display “Cancel” on English systems and “Annuler” on French ones).
- It also properly plays the system sounds associated with the type of message. You can also enable or disable the sound effect as needed.
How to use it
Because the enhanced message box is just a replacement for the standard one, you just use it like you would use the MsgBox.
There are a few additional settings that can be used to change the behaviour of the enhanced message boxes.
One is that you can adjust the delay before the buttons become activated.
Another one is that you can enable or disable whether beeps should be played or not.
The last settings is the folder where we should save the content of the message when the user clicks the Save button on the message box.
These few settings make the enhanced message box more customizable.
Large text
The standard MsgBox cannot display much text. On the other hand, there is no real limitation to the amount of text the Box and RichBox can display.
When the amount of information is too much to fit the maximum allowed size for the message box the text will overflow and can be scrolled up/down as necessary.
Limitations of the RichBox
The RichBox version relies on the normal TextBox control’s ability under Access 2007 to display RichText wich is nothing more than lightweight HTML.
Because font size may be varying a lot in the message, it becomes very difficult to accurately predict the size of the box needed to display the whole message.
Short of implementing a complete HTML engine, we have to rely on some assumptions to display HTML.
The risk is that sometimes the content may not properly fit the TextBox control in some circumstances.
If you use the RichBox, thoroughly try displaying your messages and tweak the HTML as necessary to include additional lines or non-breaking spaces to ensure that the result looks good.
If you don’t overuse font size and don’t display in multiple fonts the RichBox should do the right thing most of the time.
Don’t overuse the RichBox to display colourful messages. There is a fine line between being informative and tasteless. Keep colours and formatting where it is useful.
I think that in most cases, the plain text version Box is more than enough.
How it works
The code makes extensive use of Win32 API calls.
Most of the hard work is done in the FomDialog class form. There is too much there to really go into the details but you are welcome to have a look at the commented code.
The code relies also on a utility function from Stephen Lebans used to calculate the size of of text. I have made some minor modification to that code so I would refer you to his original implementation if you are interested in calculating TextBox sizes for forms or reports.
In the code for the FormDialog, I re-implement some of the expected functionalities of the MsgBox: proper arrangement of the buttons, displaying of the appropriate icon, etc.
Once this is done, we calculate the size of the textbox needed to display the whole of the message.
In the case of RichText, we first use Application.PlainText() to convert the HTML into properly formatted plain text. We then calculate the Textbox size using a slightly larger font than needed as a way to ensure that the content of the RichText message will fit the box in most cases.
Once we know the size of the TextBox, we can easily resize the form to properly display the TextBox.
If there is too much text, we resize the form to its maximum permissible (70% or screen width and 90% of screen height) and change some of the visual cues to let the user know the text is overflowing.
One thing of note is the way the form is kept modal.
Rather than using DoCmd.OpenForm and DoCmd.Close I use the form as a class and create an instance manually (see the code in Dialog.Box and Dialog.Richbox). I keep this instance alive until I got the form’s result back.
If you are interested in knowing how the form is made modal, this is the code in FormDialog.ShowModal() what keeps the form open until the user clicks a button:
The Sleep() function is a Win32 API that stops the current process for the given number of milliseconds. This in effects hands back the control to the Operating System for a short time. That way the system is still responsive and does not consume resources when it’s just waiting for user input.
Sample database
You can download a sample database containing all the necessary code as well as a number of tests.
This version only contains the database in Microsoft Access 2007 accdb format.
Download the EnhancedMsgBox01.zip (107KB) containing the ACCDB database.
Resources
May 20th, 2008
In my previous article about changing the MS Access colour scheme I had the need to allow the user to restart the database after the colour scheme was changed.
Being able to cleanly restart the application is also useful in other instances:
- Changes made to the environment
- Recovering from errors (for instance after a network disconnection)
- Forcing the user to re-log cleanly into the application
- Automatically restarting a long-running application (for instance so that it may automatically compact on close and restart afresh with or without user intervention).
The problem is that you cannot -to the best of my knowledge- close and open again the same database from within MS Access itself.
Most executables cannot do that and the way to solve the issue is usually to pass the control to another boostrap programme, close the main application and let the bootstrap programme re-open the main application again.
I wanted a simple and clean way of using it. One that would not require shipping external programmes.
How to use it
Download the sample database below, copy the Utilities module or just the Restart sub defined in it into your own application.
To use it, just call the Restart sub and the application will close and re-open.
This will work for normal databases (accdb) and also compiled (accde) and runtime (accdr) databases as well.
How it works
If you’re curious about the technical details, here is how it was put together.
The main idea is that the MS Access database application has to be self-sufficient and restart itself by performing these steps:
- create a small batch file
- run the batch file, passing the path and extension of our database
- close the main application
- the running batch file would wait for the MS Access lock file to be removed
- once the lock file disappears, we open the database
The key point here is that the batch file cannot just reopen the database right away: if the application is big or if it’s compacting on close for instance, it may take several seconds to actually close.
The only moment we can be pretty sure that the database is effectively closed is when the lock file is deleted by MS Access.
The batch file is hard-wired in the Restart sub that does all the work:
When the application runs the batch file, it passes 2 arguments:
- the full path to the database without the extension
- the database file extension without the leading “.”
This allows us to easily construct the path to the lock file at line 30 (we add the missing “L” in the extension).
The funny use of PING is actually a simple way to wait for 100ms or so before we check if the lock file is still there or not. There is not SLEEP or WAIT function provided by default in Windows so we have to be a bit creative and use the time-out option of the PING command trying to ping an inexistent, but valid, IP address.
Once the lock file has disappeared, we open the database at line 40 and then delete the batch file itself so we leave no leftovers.
Download the ColorScheme.zip (31KB) containing the ACCDB database (same as the Color Scheme sample).
May 6th, 2008
Microsoft Office 2007 comes with 3 colour (color) schemes.
Users can easily change it but when you deploy an Access application under the Runtime your users have no way to set the colour scheme as the application’s options are not available.
Luckily for us, Office 2007 stores the global colour scheme setting in the registry under:
HKEY_CURRENT_USER\Software\Microsoft\Office\12.0\Common\Theme
The values being stored under that key are:
- Blue scheme
- Silver scheme
- Black scheme
With this information, we can easily both read and set the colour scheme.
The only caveat is that I could not find a way to notify Access to reload the setting automatically once it is changed, so users will have to restart the application before the change becomes active.
A small price to pay but if anyone has a better idea, please let me know.
To write the new value to the registry I use a set of WIN 32 APIs that are more flexible than the default ones provided in VBA.
You can download the sample database as it contains all necessary files, including the definition for the Win32 API functions.
Download the ColorScheme.zip (31KB) containing the ACCDB database.
The sample also contains some code to restart the database. It will probably be the subject of a next post.
Improvements/uses:
- Find a way for Access to reload the settings without having to restart the application.
- Use the knowledge about the current scheme to change the other colour settings in the application (or even adapt the form’s theme).
References:
May 3rd, 2008
Rob Cooper, of the Microsoft Access Team made an interesting post and a follow-up on how to add a transparent layer that cover the screen to focus the attention of the user to a login form or other important popup window.
The trick is to use some WIN 32 API calls to modify the transparency of a standard MS Access form made to cover the screen.
The effect is quite neat and I thought I would try it and make a sample database for others to tinker with it.
My version allows you to chose between covering the whole screen or just the main Access window and it will test if it’s running under a Remote Desktop Terminal and disable the layer in that case.
Update 07MAY2008
Following Rob’s improvements I made another sample database that incorporates his code with a few improvements:
- I added the
LightBoxForm.LayerToFullScreen property so users can choose explicitly how they want the layer to be shown.
- I moved the code to hide the layer into a Hide() sub so you can just show/hide the layer using
LightboxForm.Show and LightboxForm.Hide.
- I changed the Form’s
Resize event code in the LightBoxForm class to avoid flickering: resizing the form within its Resize event actually trigger the Resize event again a second time which causes flickering.
I simply modified the code to make the form totally transparent (opacity of 0) the first time the event is fired and assign it the expected opacity when the event handler in re-entered.
Samples
There are now 2 sample databases. Ech zip contains a Microsoft Access 2007 ACCDB file and its conversion to Access 2000-2003 MDB but please note that I have not been been able to test those in older version of Access and that form transparency doesn’t work in Operating Systems older than Windows 2000.
Download TransparentLayer02.zip (95KB), recommended version
(improved, more flexible version, based on Rob’s updated article).
Download TransparentLayer01.zip (82KB), original version
(simple code, based Rob’s original article).
Troubleshooting
- If you are getting security warnings: make sure that you open the database from a Trusted Location or you will receive a security prompt.
If you don’t know how to do that, check these steps.
- If the layer appears on top of the login form instead of behind: make sure that the top-most form has ist
Modal properties set to Yes and the frmLightBox form has its modal property set to No.
If you improve on it, please let me know and I’ll post it here for all to find.
May 1st, 2008
