Author: kchristo Created: Wednesday, July 13, 2011
Sharing the Lights
By Kostas Christodoulou on Wednesday, February 6, 2013
In a previous post I suggested a way to override default LightSwitch add/edit behavior. In this post I will suggest an override to default delete behavior.

It has happened many times to delete entries, only to get an error message, when trying to save, informing me about reference constraints being violated and the action cannot be completed. This is not much of problem when you try to delete one record (apart from the fact this message is ugly and not one that I would allow my end users to deal with, for that matter). But when trying to delete many records you have no clue which was the one (or more) record(s) that cannot be deleted. In this case all you can do is delete-save until you find the record that cannot be deleted. And when you find it, you have to refresh your screen or undo the changes of the current record (a future article will suggest a way to do this).

What I needed...
By Kostas Christodoulou on Wednesday, November 7, 2012
If you have tried implementing something that required a mechanism more complex than the default one provided by LightSwitch regarding default screen selection, you already are familiar with the limitations.

For the ones that don’t get what I mean, a very common example is open the details screen of an object from a list containing items from a view (database view) that include the id of the original object. Imagine you have a Customer entity and a CustomerInfo entity that comes from a database view bringing aggregated data of the customer from various tables. Selecting and editing the CustomerInfo you want to edit the Customer not the CustomerInfo.

The above is only one fast example. Another would be having Add/Edit Screens i.e. screens that can handle both create or edit an existing object. In any case if you need to modify the default signature of a default screen (adding an extra parameter for example) causes this screen to be unselected as default screen because LightSwitch mechanism for default...
By Kostas Christodoulou on Friday, July 13, 2012
One of my favorite design patterns since the age of c++ is the Visitor Pattern. I will not explain here what the visitor pattern is. But, if you know how to use the design pattern this is a post worth reading. One may ask what the visitor pattern has to do with LightSwitch. Well, it doesn’t! I mean not exclusively. But the code below provides a full visitor pattern implementation background that can also be used in LS. Also, a part of the implementation is ideal for LS, since one of the challenges I had to face, when trying to widely use the visitor pattern, was make it work for “sealed” classes, classes that were not written by me and could not easily fit to my –reflection based- visitor pattern implementation. To solve this the best thing I could think of was “wrapping”. And working with LS, most of the classes (apart from the ones that belong to the domain/datasource) are actually “sealed” in the way described above.(I have...
By Kostas Christodoulou on Friday, July 13, 2012
One very common task when one is implementing the business logic in Common project is writing custom PropertyName_Changed partial methods. This “approach” has an undesired “side-effect” when the property is an entity property (as compared to screen properties). The property changes many times during the lifetime of the object. While loading, while deleting etc., whereas what we want to implement is handle the “actual” property changes caused either by the user or by some other piece of business logic. I found myself debugging code that shouldn’t be executing (according to my logic, obviously LS had another opinion) before ending up writing a small extension method to use in all PropertyName_Changed partial methods. I don’t claim this method will help you avoid ALL unwanted business logic code execution, but covers most of the cases. I would be glad to read any comments improving it in a generic way. Here is the code of the extension method: public static bool IgnorePropertyChange(this IEntityObject...
By Kostas Christodoulou on Friday, July 13, 2012
In the previous post I presented an extension method used mostly for overriding the edit and delete commands of a collection. One may ask “why do I want to do this?”. Apart from any other requirements/business logic dependent reason one might want to implement, for me there is one simple yet important reason: I don’t like at all (to be kind) the default add/edit modal windows when adding or editing an entry. It’s not a coincidence that the FIRST sample I wrote for LightSwitch and posted in the Samples of msdn.com/lightswitch was a set of extension methods and contracts to easily replace standard modal windows with custom ones. Most of the times when I have an editable grid screen, selecting Add or Edit I DON’T want the modal window to pop-up, I just want to edit in the grid. Or in list and details screen I want to edit the new or existing entry in the detail part of the screen. This is the main reason I most...
By Kostas Christodoulou on Friday, July 13, 2012
As soon as I started writing LightSwitch applications I noticed that many times I was repeating the same code over and over for trivial tasks. So after all this time I have collected a number of extension methods that I widely use in my apps. For me reusing code is a must and although the implementation of LS (IMHO) does not provide for this out of the box the underlying framework is ideal for writing extension classes and methods that are a major step towards code reusability. If you have downloaded any of my samples from msdn or have seen my Application Logo post, you already suspect I am an “extension method fanatic”. So I will present a small series (I don’t know how small) of posts with extension methods from my Base.LightSwitch.Client library. The first method is one of the first (if not the first one) extension methods I wrote. As soon as you want to override the code for default commands like Edit and Delete for a collection (let’s name it MyCollection) you have to write something like that: partial void MyCollectionDelete_CanExecute(ref bool result){ if (this.MyCollection is null || this.MyCollection.SelectedItem == null) result = false; else result = true;}...
By Kostas Christodoulou on Thursday, July 12, 2012
This is the most recent comment in the Application Logo Sample post: “Hi Kostas, I wanted to find out if there is a way of opening an active screen as soon as I click on the logo. I have tried but my attempt was in vain. If it is possible, could you please help me out? Thanks, Darryn” My first reaction was to ask for details (typical way of buying time Smile). But then I though I could do the changes required and post a new article. So simply put, this is the answer I propose to Darryn: public static void AddLogo(this Microsoft.LightSwitch.Client.IClientApplication application, System.Windows.HorizontalAlignment...
By Kostas Christodoulou on Friday, March 23, 2012
CLASS Extensions version 2 is now available. All the features demonstrated in the demo videos are included plus the AttachmentsListEx collection control that is demostrated in the sample application but was not mentioned in the previous post.

Features Color Business type and Controls. This business type is used to represent color values. The underlying CLR type is integer. After a request the change from the previous free version is that by default the pop-up window color picker control is used both for details and collections. Boolean property called “Drop Down” was added in the Appearance category...
By Kostas Christodoulou on Saturday, March 17, 2012
After a long time of preparation, my company Computer Life and me we are ready to publish the next –commercial- version of CLASS Extensions.

In this SkyDrive location you can find 2 videos briefly presenting capabilities of this new version. In less than a week a page will be started, where you would be able to buy the package you prefer. There will be 2 packages: no source code and with code and code documentation. Pricing will be announced along with the official release.

CLASS Extensions Demo Part I.swf

Part1

...
By Kostas Christodoulou on Tuesday, February 7, 2012
In previous posts I made reference to views (DB or RIA) as an alternative (amongst other well-known potentials of views) to search and sort against fields that would normally be relations (lookups or however you want to call them).

From the very beginning I had an issue with SQL Server views imported to LS. LightSwitch had a very strange way to define the primary keys of the views. These “inferred” primary keys are most of the time very complex combined keys. And this is not an issue actually until you decide to add a relationship between this view (which 99% of the times has only one field as actual primary key) and another table. Then you realize this, otherwise benign, complex primary key is an issue (to say the least).

It was only recently, in this thread,...
By Kostas Christodoulou on Friday, January 20, 2012
Reading this article in MSDN forums and taking the best (I hope) of what LR_ and Michael Washington offered as solutions I ended up with an extension class that allows you to put your application’s logo on the Ribbon Bar of any shell having one. You also have the choice to put it either on the left or the right. LR_’s original code was only modified in 2 points.

You can decide if you want you Application Logo on the left or on the right of your commands. You don’t need to pass a URL to the image. The application’s logo (defined in project properties) is bound inspired by the XAML provided by Michael Washington. ...
By Kostas Christodoulou on Monday, January 16, 2012
Currently being in the process of developing a business application, I came across an issue that looked harmless, but turned out to be what the title elegantly :-P implies.

Being very proud of what I can achieve in no time with LS, I had my product manager testing the application, when she ask me the simple: “How can I change something and save it as a new object with a new name. “It’s not implemented yet, but I am getting my hands on it right now and you will have it in no time”. What a typical developer answer!

“No time” turned out to be a full workday of experimenting trying to find out the best and most generic way to implement Save As functionality. Having an object being copied to another was something I had already implemented but I worked fine only on originals. Fetching an object to the client, modifying it somehow and then trying to copy to a new one, discard the changes of the original object and save the copy…well it was not a piece of cake, rather a pastry shop.

Creating a new...
By Kostas Christodoulou on Sunday, November 20, 2011
In MSDN forums I came across a post addressing an issue I have also faced. Auditing fields can cause concurrency issues in LightSwitch (not exclusively).

In general basic auditing includes keeping track of when an entity was created/modified and by whom. I say basic auditing because auditing is in general much more than this.

Anyhow, this basic auditing mechanism is very widely implemented (it’s a way for developers to be able to easily find a user to blame for their own bugs :-p), so let’s see what this can cause and why in LightSwitch.

In the aforementioned post but also in this one, I have clearly stated that IMHO the best way to handle concurrency issues is using RIA Services. If you don’t, read what follows.

Normally in any application, updating the fields that implement Audit tracking would be a task completed in the business layer (or even Data layer in some cases and this could go as deep as a database trigger). So in LightSwitch the first place one would look into to put this logic would be EntityName_Inserting and EntityName_Updating  partial methods that run on the server. Which is right, but causes concurrency issues, since after saving the client instance of the entity is not updated by the changes made at the server and as soon as you try to save again this will cause concurrency error.

So, what can you do, apart from refreshing after every save which is not very appealing? Update at the client. Not appealing either but at least it can be done elegantly:

Let’s say all entities to implement auditing have 4 fields:

DateCreated CreatedBy DateModified ModifiedBy Add to the Common project a new interface called IAuditable like below:

 

namespace LightSwitchApplication{...
By Kostas Christodoulou on Tuesday, November 8, 2011
If you have installed CLASS Extensions you know that there are two different controls that are used to handle Color business type. The ColorPicker and the ColorViewer. If you have read the previous post of the series you know that behind the scenes, each one of these controls is a wrapper for 2 other controls. The ColorPicker is for editing whereas ColorViewer is used for viewing only. The reason of exposing 2 different controls was that I didn’t manage to correctly retrieve the information about the content being read-only and changing dynamically the behavior of the controls.

If you browse the code you will see in _Load and other places (be kind, I was desperate) the attempt to bind to IsReadOnly property of the DataContext.

What I wanted to achieve was to...
By Kostas Christodoulou on Saturday, October 29, 2011
Crashing your LightSwitch project with your extensions is a piece of cake. The first simplest thing I did was change the business type to handle integer values instead of string ones. But I changed “:String” to “:Int”. That was enough to do it. It should be “:Int32”.

So, after managing to create and use (declare as a type for an integer field in an entity) my business type, keeping my LightSwitch project in one piece, I had to work on my control. As I have already mentioned I had my custom control ready and tested, so, it should be easy.

I had used my custom control only in detail parts and although the expander used was “deforming” the layout when expanded (I have to fix this anyhow in the future) everything were going back to normal after collapsing the expander. This was not so for collections. Using my control in a grid the result after collapsing the color picker control was a grid row having a height of the expanded control. The reason is that the grid rows are auto-sized and have no specific...
By Kostas Christodoulou on Sunday, October 23, 2011
I haven’t been able to post during the last week. It’s been a very hectic week and I didn’t have time or when I did have the time I didn’t have the courage…

Anyhow, I am going to share with you issues I had to face creating my first LightSwitch Extensions project.

First thing was to create the Color business type. I already had custom Silverlight control and value converters I had already published in a sample in MSDN’s LightSwitch Developer Center. So half of the work was done, right? Well, not quite…

I must say here that many of the problems were a result of the fact that editing the lsml file is very error prone as there is nothing to help you while editing the file by hand. I mean it’s not like editing the XAML of a Silverlight control, with code completion, syntax checking and all the goodies. It’s raw XML editing. And the thing with errors in lsml is that the result is terrifying…a tag improperly...
By Kostas Christodoulou on Sunday, October 16, 2011
I had initially discarded LightSwitch from my deck of choices. At first glance (early Beta 1) I had misjudged it as poor relative of application builders that I hate altogether.

It was after watching Beth Massi’ s presentation in Tech-Ed USA 2011 regarding extensibility that made me take a second good look at it. And then…then it was love at second sight...
By Kostas Christodoulou on Friday, September 2, 2011
This is my approach to making screen authorization (show/hide) more effective and robust that hard-coding permission names in the “_CanRun” partial methods. The library and sample code is published here. As with my previous post the code is posted to msdn and here I will explain in detail how to use the library and what the sample does.

First let’s prepare. After downloading the sample and before you can run you must create a database I use for storing screen security information. Create a database named LSSecurity and then open and execute the script found in the zip at the path Files\LSSecurity.script.sql. Then you have to go to your ServerGenerated\Web.Config and change the connection string from localhost\clcadlocal to the name of your SQL server. This database is imported by the sample Lightswitch application in a datasource called LightSwitchSecurity. The entities from this datasource are used to implement...
By Kostas Christodoulou on Thursday, July 14, 2011
Immediately after using Excel Importer extension for the first time and trying to see how this would fit into our new “want to be” business application framework, I realized that defining the column mappings every time was definitely an issue.

So I decided to implement and integrate to the original Excel Importer a mechanism for using Column Mapping Templates as an optional capability. I decided to give it back to the LightSwitch community, where the original idea (msdn.com Lightswitch Samples) came from. You can download the the extension here. Feel free, no better yet, feel obliged Winking smile, ...
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