I have not touched my layout for a long time. Today I tried to add a twitter to my blog my new twitter account. I realized that I can also add a twitter gadget from Blogger's layout. Actually, there a tons of gadgets available. That's really great! I like some of them.
One of gadgets is Label, something like tags you can define for your blog. While you create a new blog, you can add labels on your blog. This is a very convenient way to group blogs. With Label gadget, you will provide a way for you and other browsers or follows to get interested topics. I'll try to add my own labels to my blog. Hopefully, the Label gadget will be able to provide some visual information about those labels.
Thursday, October 16, 2008
Blogger Layout and Gadget
Saturday, October 11, 2008
Web Tool: Short URL
I read an article about web tool to make URL short long time ago (last year?). That is ShortURL. However, the site may not free soon as I recall a warning in the article. Now I think that I do need a tool to redirect long urls such as my blog entries to a short one. By gooling web, I found MetaMark.
The short URL generated by this site is not a user friendly one, but it works. According to the information of this site, if the url has not been used for more than two years, it may expire. For short term use, this web tool is great. For example, my previous post is at http://xrl.us/otgzf, comparing to http://davidchuprogramming.blogspot.com/2008/10/web-tools-firebug-and-smushit.html.
Here is the form I copied from the site to do the job:
I did several tests. If you use it to generate one and try the same long URL again, the one first time generated is returned back. Therefore, you just cannot change it. It does not check if the URL exists or not. I gave a none-existing one, and it still generated a new short name, which is not not-found-url. May be this is a way to change it (by making the previous existing one invalid). The web server just generates an alternative short URL, no matter the URL you provide is valid or none-short one. I tried to get a short URL for "http://google.ca". The new one is "http://xrl.us/bemw2" (it may exist or I just generate one for it):).
A related technique about URL is to implement ASP.Net's MVC framework. Its URL is based on controller, action and router. You can construct your own user-friendly URLs. The URLs based on ASP.Net MVC does not contain .aspx or .html suffix and not url parameters like ¶1=value1... You will see more short and user friendly URLs on web. The Metamark page mentioned that it is using MVC pattern. Maybe this server just use the pattern to build a very simple, random and short URL mapped to a long URL.
You can do the same thing for your company's web pages. Most of them are not-user-friendly URLs and hard to remember. By using ASP.Net and MVC framework, you can define a structure of URL and mapping user-friendly URLs to a existing ones without any change to the old ones. It should be very simple project.
Update: One of the great feature of Metamark web tool server is that you can provide a nickname and secret. The nickname must be unique one, not taken by others. Then the generated URL is [server_home]\[nickname]-[secret]. You could create a user-friendly URL! This blog is at http://xrl.us/shortURL1-WebTool. "shortURL" has been taken. I had to add "1" to it.
The generated short URL does not take any suffix like ".aspx" or ".html", which is not needed actually!
Web Tools: FireBug and Smushit
Two great tools for Web developers: FireBug and Smush.it.
FireBug provides a view panel on the bottom with a lots of options to view such as Console, HTML, CSS, Script, DOM and more. If you click on Inspect to toggle it on, then you can hover your web page to see selections with updates on FireBug panel. I think it will save a lots of time for Web-developers to debug their web-pages. You may find out more about what actually in your client side web page codes when you debug your ASP.Net applications.
Smush.it is a great tool by some Google developers. You simply upload your image and to see how much it can smush. For example, I posted a blog on my Mac log with png image. After smush.it, it reduces 37%. The logic is very simple. It tries to turn off some image bits which will not make much difference in view. I'll use this tool to reduce my images on web. It for-sure will make your web page much faster for your clients!
Thursday, October 09, 2008
Apple Programming: User Defaults
Just started to look at some Apple Xcode application examples. That's a whole new huge framework for OS X. One application is about user defaults. In Windows there are many ways to store application defaults or values such as Registry, XML file, Ini file or Active Directory.
In Mac OS X, Apple provides a plist and bundle for user preferences and in Objective-C, there are some classes can be used to get user default values, NSUserDefaults for example:
NSUserDefaults *defaults;
// Get all the defautls for the current app
defaults = [NSUserDefaults standardUserDefaults];
// Register 2 defaults in case they are not available in user defaults
// Note, the registerDefaults does not change defaults.
[defaults registerDefaults:[NSDictionary dictionaryWithObjectsAndKeys:
@"Joe", @"first_name",
@"NO", @"is_married",
nil]];
//...
NSString firstName;
BOOL married;
// the following two calls will return the user's individual preferences,
// if they are available. Otherwise, it will just return the values we
// registered previously. Saves us some hassle!
firstName = [defaults stringForKey:@"first_name"];
married = [defaults boolForKey:@"is_married"];
That's quit difference way to do in Mac. Very interesting! Some good links about User Preference Defaults:
- O'Reily Mac DevCenter.com: Mac OS X's Preferences System (and More!)
- Apple Developer Center: NSUserDefaults Class Reference
- CocoaDev(WikiWikiWeb) article: NSUserDefaults
Friday, October 03, 2008
SQL Server Database Mail
I use msdb.dbo.sp_send_dbmail to send email notification in SQL Server. Recently I migrated our DB to a new server. The problem I encountered is my SQL SP does not working any more.
Finally, I find out this is the SQL server configuration issues. There are some configuration settings to be done on SQL server level.
First, I have to turn the mail feature on by using the command:
sp_configure 'Database Mail XPs', 1
reconfigure
Next, I have to configure the Database Mail. Here is the picture to access the configuration:
where you have to create a Database Mail account and profile. Basically, the configuration contains information about mail server, SMTP Authentication. The profile will be created for you after you add a new account.
One thing you have to do in the Database Mail configuration is that to set the default profile as you will use in
sp_send_dbmail so that you would not need to specify a profile.For my case, I use this sending mail feature to send file attachment in some cases. My notification report mail contains information from DB in html format. The content may be very big. If the content is very big, it will take time to open the mail. In case of very large size content, I'll send the report as an attachment file.
By default, the file size for Database Mail configuration is 1,000,000 bytes. It is very small in most cases. I have to change it to 10,000,000 or about 10MB. This can be done in Database Mail configuration.
After all these settings, my email notification runs with success.
Thursday, October 02, 2008
ASP.Net MVC
Just watched a training video Creating Task List on ASP.Net MVC. It is very impressive. MVC design pattern has been used for Windows application and now the framework for ASP.Net is available.
I have used this or MVP in several desktop applications before and I have not touched this type of design for several month. I'm very glad to see the new package available.
MVC pattern provides very good design infrastructure with separations of UI, Controller and Model. In addition to this model, I prefer to add a Repository level to separate data base and classes. The Repository model is responsible to provide data as objects and interface for updating objects back to DB.
Any good design pattern does not resolve all the issues. Still you have to keep good practice to use them. I understand that the tutorial video is just an instruction to MVC. My comments on the practice in this video is that there are too many client to server calls. To create a new task you have to make a call to server side to add a new task. However, to mark a task as completed, I don't think that the Complete action call to server for each edit is necessary. It could be done on client side to mark one or more tasks as completed. Add a Save button to save changes. This will avoid frequent calls to server.
In order to do that, I think client side script may be needed to handle these changes. In addition to that, you have to handle redirect change by client to another page. That means a warning message to indicate changes.
New design patterns would provide a gateway to design a better applications. You should remember that that does not resolve all the problems or make your application in a good structure. It'll take time to master skills to develop good applications.
Add Scripts to Client Side Page 3
In my first post on this topic, I mentioned that there some cases that prompts may not be desired. You can avoid prompt display for a control's click event by calling the base class's method BypassModifiedMethod().
This method actually disables the prompt display by setting the client side page level script var on click event:
m_needToConfirm = false;
However, in case of saving failure, such as exception raised from back-end DB, a postback call may be called back to the client side with some error message being displayed. This would cause a problem: if the user redirects to another page, the prompt would not be displayed and the changes may not be lost.
To resolve the problem, I added a method
ResetOnSubmit() in the base class. You should call this method on Page_Load event for postback case if you want to reset the flag variable back.protected void Page_Load(object sender, EventArgs e)
{
...
if (!IsPostBack)
{
base.ResetOnSubmit();
...
}
...
}
Another handy method I added to the client side base class is
DebugChanges():protected void DebugChanges(
WebControl wc,
string wcEventAttribute,
string clientIDForDisplay)
{
string script = string.Format("javascript: displayChanges('{0}')",
clientIDForDisplay);
wc.Attributes[wcEventAttribute] = script;
}
where
displayChanges() is a client side javascript function registered on the client side page level. This function will display the current monitored changes and original values on a WebControl specified by control's event. You call this method in Page_Load event to add this debug feature. You can decide when the control is visible. I used a URL request parameter to enable the visibility of a control in one case. See my previous post on Using Query String Parameters in ASP.Net Page.As I promised in my previous post, here is the source codes of ClientSidePage class.
Wednesday, October 01, 2008
SQL Server Project (5)
I have written several blogs on SQL Server Projects. Here is the list of links of previous posts:
Recently we moved our SQL server from one virtual machine to a real PC SQL server. DBAs created the new server and installed a blank SQL server. After that, we had to move our DBs to the new user.
Generally, DBAs don't give us sa password. What they did is to created an appsupport user with almost as same as settings as sa for us. We use this user to back up the existing DBs and copied to the new PC for restoring.
The restoring process went very well. Almost every thing has been set up as same as the existing DBs. However, for SQL database project, as I mentioned in previous articles, some Asymetric Keks for assembly keys and Log Permissions have to be set up in master db. Fortunately, I logged the steps to do that (as I wrote in my previous articles) and restored the keys and permissions in master DB.
One thing I realized that the master DB as created by sa and our other DBs with SQL database projects were restored by appsupport. The owners of these DBs are different. As a result, I could not deploy my project to DBs. The error messages say that the owners are different and they could be changed by ALTER... I used this command to change the DB(MyDB for example) owner:
ALERT AUTHORIZATION DATBASE::MyDB TO sa
I think that if DBs' owners are different, you cannot deploy your project from Visual Studio 2005. You have to make change to keep them in sync.
The second thing I had to do is to set DB TRUSTWORTH on:
ALTER DATABASE MyDB SET TRUSTWORTHY ON
since I referenced System.Web.dll framework assembly in my SQL database project. Otherwise an exception would be thrown out when Ssytem.Web.dll classes are called.
The last thing is to give permissions to Windows' users since I have to use VS to deploy my project remotely to the SQL server with Windows authentication. Instead of give permissions(alter and create assembly) to one user, I created a Role on SQL server DB called as db_developers and adding my user name as a member of the role. Then I set related permissions to the role.
After all these settings, I can deply my project remotely to the SQL server DB and run the deployed assembly based ST with success.
Friday, September 26, 2008
iPone Application: MoveMe (continued)
Finally, I finished my build of MoveMe application from scratch. That's the first demo application for beginner developers. I thought it is better if I build this application by myself. I added one piece by one piece. That's quite good experience. Since Object-C is new for me, it takes some time to gain experience.
Xcode is Microsfot VS like IDE. It is hard to compare two. My first impression is good. IB (Interface Builder) is something like .Net's new Expression Blend. As its name description, it is use for UI design and link UI components, actions(for events) to class codes. At first, I did not have any clue how to use it. I thought these connections or linking must be done through IB's UI such as property, tab or dropdown list. However, when I tried to link MoveMeView class to a component in IB's outlet, I was lost. Finally, by watching some YouTube demos on IB, I figured out the linking is done through drag with Control key and drop. It's quite innovative way. I have to understand IB or Mac's way to do it.
The second lesson I learned is that parameter names in a method has to be matched if it is a delegate method. For example, when I typed in the codes for MoveMeView.m codes, I missed i in the animation delegate method's parameter name "finished":
-(void) animationDidStop:(CAAnimation *) theAnimation finshed:(BOOL) flag
As a result, the animation of MoveMe button only works at the start and then no respond after I moved it. Finally, I found the mis-typing error. It is the signature of method not matched. In Objective-C, you can call a method on object that may not defined. It would not generate an error or cause compiling error. It is OK. That's very nice. However, it may cause some very hard find bug. After I corrected my mistake, the application finds it's matched animation delegate method, and works fine. The bug caused my animation never stopping.
Another good lesson I learned is the Frameworks in Xcode project. It is similar .Net project's References. Since I created a new iPhone project in Xcode from scratch. Xcode did not add animation framework class QuartzCore.framework, which is used in MoveMe example project. I typed in codes in MoveMeView.m file. It includes the QuartzCore's header files like this:
#import <QuartzCore/QuartzCore.h>
and when I typed animation class names and methods, Xcode does show them in intellisense. However, when I compile the application. I got some errors about references not found in xxx.o. I was confused and there was not further indication where in the codes where the error is. Eventually, I realized that it is the library files missing. I have to include animation's library into the build file. In Xcode, all the libraries are under Frameworks folder in the project.
It has been very productive period for me. I got my built MoveMe working. There are some similarities between Objective-C and .Net C#. With my knowledge and experience in .Net C#, it is not so hard to understand Objective-C. However, Objective-C does have its unique features and power. I would like to continue to learn it and to develop applications.
Sunday, September 21, 2008
iPone Application: MoveMe
I started to learn iPhone application development from Apple's iPhone Dev Center. The first application is MoveMe. The material on this application gives overall description on main items in the application, but it does provide hands-on information about how to build the application by steps.
I tried to create all the classes, resources files, and windows xib files from scratch. It is not so easy. XCode provides good IDE for coding, however, I could not find similar convenient ways as I have done .Net applications by VS.
I am still in the progress to get MoveMe build up and working. One thing I found out that the window xib file has to be specified in the Info.plist file (which is under Groups & Files->Resources):
I did not use MainWindow.xib file. Instead, I created MainMoveMe.xib file. When I first run it, I got crash exception. The problem was caused by this "Main nib file base name" in Info.plist.
After I got this window file, still I could not find a way to link delegate to MoveMeAppDelegate class in Interface Builder. Since I created this window file, I have link windows to classes through Interface Builder. Anyway, it is quite interesting experiences to learn and develop iPhone Application. I'll keep log my experiences.
Wednesday, September 17, 2008
iPhone, Android and Windows Mobile
It is getting very crowed in mobile phone. iPhone from Apple is mainly for hight-end users. It is very existing mobile phone. However, it has some limitations. For example, you can only run one application at one time and limited openness in iPhone SDK APIs.
Google announced Android early this year, and formed Open Handset Alliance a group of more than 30 technology and mobile companies. It is based on Java framework. It supports multi-processes and is expected to be run over a wide range of handsets from 12 keypads low-end ones to high-end ones. The first mobile phone supporting Android is coming in just a few weeks time.
Microsoft also put a lots of investment and efforts on Windows Mobile and a few mobile companies have already used it.
I think early next year we will see more interesting mobile phones with great applications available on the market!
Tuesday, September 16, 2008
Add Scripts to Client Side Page 2
I have changed the ClientSidePage class. In the following articles, I'll use my updated class as examples. It is very easy to use the class, first use the ClientSidePage class as base class, and then use the base class method MonitorChanges() to add a web control for monitoring any changes.
For example, here is the code to add a textbox control for monitoring:
protected void Page_Load(object sender, EventArgs e)
{
if (!IsPostBack)
{
base.MonitorChanges<TextBox>(MonitorChangeControl,
CONTROL_ID_MONITORCHANGECONTROL);
}
}
where
MonitorChanges is a TextBox control placed on aspx page. It is directly accessible in the server-side code class, and CONTROL_ID_MONITORCHANGECONTROL is a constant defined in the server-side class. I prefer to define control ID in a region in a class so that can be be easily maintained in once place instead spread all over the places.One change in the method is that it is a method with generic type T, where T is constrained with
WebControl class. The first parameter is a web control or the parent control where a specified control can be found by control's ID. In some cases, you may place some web controls within a GridView's templates. Those controls cannot be directly accessible. You have find them by their parent container such as GridViewRow control. In this case, you can pass in the GridViewRow control.With the method, I used another class to find a web control. This makes it easy to use. You don't need to add some codes to find the control. See my previous blog on Find a control in ASP.Net Page.
You can monitor as many web controls as you want. However, from my experiences, I found there are too many problems to directly monitor each editable web controls. Some times you don't know how many they. For example, a
GridView control may be bound to a database with dynamical rows of data. As a result, it is very hard to maintain.The second issue is you have to understand when to initialize these controls on the client side script. As I mentioned in my previous article, the ClientSidePage class registers two groups of scripts on client side page. One is the startup scripts, where the method
assignInitialValuesForMonitorChanges() is called when the web page is loaded. However, some web controls may not have data values available in the first loading process if the control is bound to an object data source and the object will be created on server side codes. The control may be late postcalled back to set values. In this case, the client side array for initial values would be nulls. What I find out is that I have to call MonitorChanges at the time the control is bound to data on server side. Then I use another overloaded method to pass initial values to the client side script so that the array could be initialized with correct values.protected void GridView_RowDataBound(object sender,
GridViewRowEventArgs e)
{
if (e.Row.RowType == DataControlRowType.DataRow)
{
GridViewRow row = e.Row;
base.MonitorChanges<CheckBox>(row,
CONTROL_ID_ISEMPLOYEECONTROL, true);
}
}
What I find out is that I don't need to monitor each web controls. What I need is to monitor only one
TextBox control. It is initialized as empty. If any other control values are changed, this monitored control's value will be something like "Some data may have been changed. Click on Save button to confirm the changes". In order to achieve this, I added two methods on ClientSidePage class. The first one is
SetScriptOnClientSide(). This method will register two groups of scripts to client side in the same way as CliendSidePage does: a group script for starup and a group script for script block. These two groups of scripts are customized scripts with a customer provided registry key. This will make sure ClientSidePage registered scripts will not be conflict with customized scripts. You add whatever scripts you want to the client side.SetControlEventScript(). This method takes three parameters, one for WelControl object, a string for an event attribute, a string for script, and a position to indicate where to add the script to the controls' event attribute if it has some scripts defined already.Here is one example I use these methods in one aspx web page server side codes:
private void SetScriptsForClientSidePage()
{
string script;
/* Monitor one text box control: MonitorChangeControl with its control ID*/
base.MonitorChanges<TextBox>(MonitorChangeControl,
CONTROL_ID_MONITORCHANGECONTROL);
script = @"
<script language=""JavaScript"">
function {0}
{
var v_elem = document.getElementById('{1}');
if ( v_elem )
{
v_elem.value = '{2}';
}
}
</script>
".Replace("{0}", SCRIPT_FUNC_SomeValueChanged) /* constant for function name: "SOmeValueChaned()" */
.Replace("{1}", MonitorChangeControl.ClientID)
.Replace("{2}", SCRIPT_CHANGE_MESSAGE); /* Message like "Some data may have been changed. Click on Save button to confirm changes or click on Cancel to revoke changes." */
base.SetScriptOnClientSide("script", null, script);
/*Add scripts for one text box control: CurrentSales */
script = string.Format("{0};",
SCRIPT_FUNC_SomeValueChanged);
string eventOnChange = "onchange";
ScriptInsertPosition pos =
ScriptInsertPosition.BeforeExistingScrit;
base.SetControlEventScript(CurrentSales,
eventOnChange, script, pos);
}
This private method is called in the Page_Load() event when the page is first time loaded: monitoring one text box control, adding client side script with a script function definition, and adding event script for a web control. After done this, I always to view the client side source to verify my scripts.
The next article will provide the codes of ClientSidePage class.
Monday, September 15, 2008
Add Scripts to Client Side Page 1
I have read an excellent article on how to add client side scripts to monitor changes and prompt user if a redirection call is fired by Scott Mitchell. He later updated the class to disable prompt in case there is no need such as Save and leaving the page.
I have used this class in my project. I found there are some features missing and then further updated the class. In this article and later on articles, I'll discuss how to use this class.
Basically, the class ClientSidePage is used as a base class for an ASP.Net Web Page (server side codes). You can use this class to monitor changes made by client on the page for editable web controls by the method of MonitorChanges().
Here is the way how it works. This method will register two script array variables (page level) on client side, one for monitored control's IDs and another for initial values for those controls.
In order to make this to work, the ClientSidePage class registers two groups of scripts on the client side: startup script and script block. The startup script is a group of scripts that will be called when the client side web page is loaded. This is for initialization purpose, for example, setting the monitored array with initial values by a script function assignInitialValuesForMonitorChanges(), which is defined in the second script group. This function initializes the array of monitored values as original values, and set a page level variable, m_needToConfirm, to true. This is a flag for confirming changes.
The second group of script, script block, contains some client side function declarations, page level events, and variables such as variable m_needToConfirm, and function assignInitialValuesForMonitorChanges() as mentioned above. Another example is to set page event window.onbeforeunload is set to a script function confirmClose(). When a unload page request is made on client side, the window.onbeforeunload event is called. The function confirmClose() checks if the flag variable is set and any monitored control' values are changed or not by comparing control's value to the original values. If so, a prompt is displayed with a message to confirm leaving the page with two options: OK for leaving the page, and Cancel for staying.
By the way, in Google's Chrome beta version browser, the OK button is "Leave this page" and "Stay on this page" for Cancel.
Most browsers have view source option. You can view those groups of scripts by this option. I like Chrome's viewing source. It opens a new tab with page's source codes, and it is very nice to search. For example, you can search for the variables, arrays, or functions as mentioned above. The searched strings are highlighted with yellow color and there are marks on scroll bar to indicate their positions. You can easily location them and know if they exist right away. Maybe that's Google's strength: search everywhere and know where they are.
There are some cases that the prompt are not desired. For example, a user makes some changes on a page, and then click on a Save button to save changes. Normally the saving request is a postback call to the server side, where changes are saved and new/refreshed data are sent back to the client side. In this case, the prompt is not needed. Scott updated the class to handle this case. Read his articles for details.
Saturday, September 13, 2008
Mac Software Development
I have have done my software development mainly in Windows Platform, some in Unix long time ago. I have got an iMac in March this year. Since then I spend a lot of my time using it at home. I really enjoy it. Recently I have installed XCode on my Mac and watched training videos about iPhone Application Development API kit.
It is very interesting to learn Objective-C and Cocoa framework even they are very different from .Net languages and framework. iMac computer is very easy to use and very stable. From what I have seen from Apple's training materials, the Objective-C and Cocoa are also very straight forward in syntax. Objective-C for Mac applications uses MVC pattern. That's the one I have seen on Microsoft's P&P team as well. I think there are a lots similarities between both.
I'll spend some time to learn Mac application development. Maybe I'll get more and more involved in Mac software development. Enjoy the new adventure!
Tuesday, September 02, 2008
Google Launches Chrome Web Browser Today
It is getting more crowded in the web browser war. I have IE that is a part of Windows, Firefox, an excellent open source browser with multi-tabs and add-ins, Safari for Mac, and others rarely used. Now Google enters the web browser competition with Chrome!
It is a positive thing for most end users for sure. I think Chrome will have some special offers for us. First, it is built based on IE but it is open source. Soon it will be available for Mac. That means that we could have alternative browser for IE in Mac with IE special features. Secondly, its infrastructure is sandbox so that each process has its own rights which mean more privacy. Another related benefit is that one process crashing would not affect others like tabs and mails.
I think the biggest thing why Google decides to have its own browser is that the current browsers have too many dependencies. Many of Google's products and great features are depending on browser and end user decisions. For example, Google released Google Gears which provides off-line browsing. That's a great feature for most end users. The browsing process would more smooth and faster. The real-time data can be updated by the back-end database store and processes when the Internet connection is available and there is need to fresh the data. However, it is an add-in. It will be up to the end user to install it. Without the installation, it would not work.
jQuery is another great feature for web browsing. It loads additional JavaScript libraries. There are already many jQuery add-ins or plug-ins available and Google shows great interest in it. However, many features will depend on end users to install them.
I guess that Chrome will install Google Gears and many other add-ins as components by default. This will make Chrome very unique and outstanding. Many advanced features will be available right away. That will shine Chrome for sure.
Welcome Chrome to the web browser world!
Monday, September 01, 2008
jQuery
Today I read David Hayden's blog on jQuery. It is a quite interesting JavaScript library. I tried some tutorial examples and get some tastes on how to use and its initial power.
I downloaded the library jQuery-1.2.6.js source code. Then I created a test html file. I followed the instruction to save the js file in the same folder as the html file. Then I tried some example codes to hide a reference link <a...>, add a style class to the link, add a click event to the link, and some chained queries. Basically, this library provides API functions to manipulate DOM objects or elements, handle events, perform animation, and add Ajax to your web project. As David recommended, jQuary in Action is a good book for beginners.
I'll spend some time to try out and learn this great tool. Hopefully I can use this in my web projects.
Here is a training video program by a 12 years old child in google engEdu:
Here is very good reference to jQuery API libraries: Visual jQuery 1.1
50+ Amazing jQuery Examples. Unfortunately, some links are not available. However, some are really amazing web pages.
Sunday, August 31, 2008
Collection Filter Function
Let's continue the topic of my previous blog on .Net Generic and Value Data Type with an example of collection filter function for generic value data types. This function takes two collections, the first one as input collection and another one as filter collection. The returned collection will filter out all the items in the filter collection from the first collection. In this case, in order to filter items, the value data type makes sense when comparing equalities for all the items in a collection to an item. For reference data types, however, it is hard to tell equality or not, by reference or property members? You even can expect multiple-levels if some property members are reference types.
public IEnumerable<T> FilerCollections<T>(
IEnumerable<T> list,
IEnumerable<T> filterList)
where T: struct
{
if ( list != null )
{
foreach (T item in list)
{
bool found = false;
if (filterList != null)
{
foreach (T filterItem in filterList)
{
if (filterItem.Equals(item))
{
found = true;
break;
}
}
}
if (!found)
{
yield return item;
}
}
}
}
Here is the test program:
Test t = new Test();
IEnumerable<int> list;
List<int> list1 = new List<int>(new int [] {1, 2, 3, 4});
List<int> list2 = new List<int>(new int[] {3});
list = t.FilerCollections<int>(list1, list2);
if (list != null)
{
foreach (int item in list)
{
Console.WriteLine("item: {0}", item);
}
}
else
{
Console.WriteLine("List is empty");
}
and the result screen:
.Net Generics and Value Data Type
I use .Net Generics a lot. The great benefit is that you can make your codes reusable even you may use it only for one data type. You may extract the generic class or method to a library to a library for reuse. Secondly, it still keeps type safe or strong type integrity. Any attempts to use wrong type will be caught at compile time instead of run time.
The generics can be specified by where clause. In most cases I use generics for reference data types, i.e., classes. If you say a generic as class, that means the generics data type is a reference type. You can use new() to further indicate the class can be instantiated by the default constructor with no parameters. If you specify the class a specified class, that means any class which inherits from the class can be used for the generics class or method.
You cannot use more than one classes in where clause, however, you can use interfaces to specify that the generic class should implement some interfaces. Then in your generic class or method, you can safely call the implemented interfaces or methods.
How about value data types such as int, string or double as examples. You can use struct in the where clause. If you want to use the nullable value data types, you can simply use ? after the generic type, such as T?.
class Test
{
public T GetValueOrDefault<T>(T? item)
where T: struct
{
return item.GetValueOrDefault();
}
public T? GetValue<T>(T? item)
where T: struct
{
if (item.HasValue)
{
return item.Value;
}
else
{
return null;
}
}
}
here are codes to call the methods:
Test t = new Test();
int? a = 2;
a = t.GetValueOrDefault<int>(a);
//t.GetValueOrDefault<Test>(t); //Error: t is not none-nullable data type
Console.WriteLine("GetValueOrDefault<int>(a) = {0}; GetValue<int>(a) = {1}",
t.GetValueOrDefault<int>(a), t.GetValue<int>(a));
a = null;
Console.WriteLine("GetValueOrDefault<int>(a) = {0}; GetValue<int>(a) = {1}",
t.GetValueOrDefault<int>(a), t.GetValue<int>(a));
Console.WriteLine("GetValueOrDefault<int>(a) = {0}; GetValue<int>(a) = {1}",
t.GetValueOrDefault<int>(3), t.GetValue<int>(3));
The result of the above codes in a console is:
Here is an interest posting on .Net Generics and a question about enum data type.
Tuesday, August 26, 2008
SQL Nested SELECT Statement in FROM Clause
You can create a view by using SELECT statement. For example, a View called as AllEmployeeInfo by SQL statement: SELECT * FROM Employees. Employees is a table. That's quite simple. Then you many list all employee names hired after '2005-01-01' from the view:
SELECT FirstName, LastName
FROM AllEmployeeInfo
Is there any way to use SQL SELECT statement as a subquery directly in FROM clause? For example:
SELECT FirstName, LastName
FROM SELECT * FROM Employees
This does not work in Microsoft SQL 2005. However, if you name the subquery SELECT as an alias, you can do it. Here is the example:
SELECT FirstName, LastName
FROM (SELECT * FROM Employees
WHERE HireDate > '2005-01-01') AS Temp
-- or
SELECT FirstName, LastName
FROM (SELECT * FROM Employees) AS Temp
WHERE HireDate > '2005-01-01'
It looks like that SQL will create a temporary or dynamic view for the alias, then you could SELECT columns from there. Quite cool? I tried this in PL/SQL Oracle 9.0 but it does not work. Anyway, I like this quick way to get data. Sometimes you may need this kind of nested SELECTs so that you can filter data by several levels and avoid to create unnecessary views.
More on nested SQL queries, read this blog: Using a Subquery in a T-SQL statement.
Notes and Updates on this blog: Actually, the nested SELECT statement is available in Oracle or PL/SQL as well, but you don't need to AS in the statement. I got this correction from my StackOverFlow Q&A. Thanks for the correction!
Sunday, August 24, 2008
ASP.Net and Data Source Controls
In an ASP.Net page, many asp controls can be bound with a data source control, such as SqlDataSource and ObjectDataSource. I prefer to use ObjectDataSource than SqlDataSource. Here are my two main reasons.
First SqlDataSource includes a SQL statement in the control. If you change different data DB source from Microsoft SQL to Oracle DB, the SQL statement may be different. Therefore, you may have to make change in the UI aspx page or server side .Net class codes.
ObjectDataSource is binding data source from a a class's methods to get or update data. The class hides the way how data are retrieved or saved. I call it as business logic layer class (BLL). The class can be defined in another library assembly. This will make unit test much easier.
Second reason is that you don't have control how many times to call SqlDataSource to get data. For example, if you have a combo box in a GridView for product types and link it to a SqlDataSource to get product types. Each combo box will call the SqlDataSource to get product types when GridView is initialized. If there are tens or hundreds of rows in the GridView, there will be tens or hundreds call to database to get the same data. Even worse, in case of post-back calls, the GridView will be refreshed again. This may make the front UI very slow if the retrieving data process is slow.
In BLL class, you can easily cache data to avoid unnecessary calls to database. Simple define your data collection as a static List in the class. Only the first time when the page is initialized the static data member is populated with data from database. The subsequent calls will get data from the cache. You may need to define a reset method in the BLL class so that in the Page_Load event, if it is not post-back call, call this reset method to reset the static member to null. Then when the GridView is initialized, the data will be obtained from database for the first time.
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