A personal Blazor blog.
Showing posts from 2011.
" I have always wished for my computer to be as easy to use as my telephone; my wish has come true because I can no longer figure out how to use my telephone ."
Danish computer scientist Bjarne Stroustrup
One of my first posts on this blog was about the problem of using anonymous types when you want to send the data outside of your current class. At the time, the only way I knew to solve this was to create a simple class that had the structure of the anonymous type, and create one of these instead of the anonymous type. I do this regularly, although I have taken to naming such classes CustomerOverview, OrderOverview, etc, instead of CustomerTmp as I did in that blog post, as they aren't really temporary classes at all, they are slim, flattened overviews of the class.
This approach works well, but it can have its downsides. One, as mentioned in that post, is that it is easy to end up with a proliferation of classes, many of which may only be used in one location in the code. If the classes represent lightweight views of entities in the model (such as the two examples shown above), then I don't worry about this, as it is clear what they represent, and it's fairly certain that I'll end up using them somewhere else at some point.
I previously blogged about a seemingly innocent LINQ problem that had me baffled for ages, which was how you sort or filter an entity’s child collection in Linq. For example, if you want to pull a collection of customers, and include all of their orders from this year, but none from earlier, then there doesn’t seem to be a simple way to do this in Linq. You need to do the query in two stages, the first which builds an anonymous type, and the second which links the to parts of it together. See that post for more details.
I also blogged about the problem of Linq not including child entities when doing joins, which requires you to cast the query to an ObjectQuery so you can use the Include() method on it.
The problem comes when you want to combine the two methods, meaning that your query needs to be constructed in two stages to ensure that the sorting or filtering of the child collection is done correctly, but you also need to cast the final result to an ObjectQuery before you send it out over WCF. The problem arises because you need to enumerate the query before doing the Include(), as that is the only way to ensure that the sorting is done, but calling AsEnumerable() gives you an IEnumerable (reasonably enough), which can't be cast to an ObjectQuery! So what's a fellow to do? Good question, and one that had me going for ages.
Of course, we professional programmers never make mistakes, ahem. That’s why we never need to use debuggers, ahem.
Well suspend belief for a moment, and assume that I had a bug in the code I was developing. You know the feeling, you stare at it, you write unit tests, you stare at it some more, and still can’t work out why on earth Visual Studio is claiming that there is an error in your code, when it’s so obvious that there isn’t. You even get to the point of talking to your computer, pointing out the error of its ways...
I was underwhelmed at being awarded two separate recognitions for my contribution to the community.
Read on and be as underwhelmed as I was!
Sadly, whilst building a solution yesterday, my machine started behaving in a very weird manner, with applications not responding, the taskbar disappearing and so on, followed by the dreaded blue screen of death. When I checked the event log after pulling the plug out (I hate doing that!) and rebooting, I found lots of errors, which led me to a Microsoft Connect article (now sadly removed) where someone was reporting a very similar problem.
To my amazement, the very last comment by a Microsoft employee in response to this bus report was “This is known issue, this bug was resolved by mistake, we are already addressing this issue.â€
Surely they didn’t mean that did they? Someone tell me I read that wrong!
As the non-existent avid reader of this blog will know, I’m far more interested in learning new technologies than I probably should be, given the limited amount of time I have to learn them properly! With that in mind, I shouldn’t be looking at yet another, but this one does have a very immediate benefit (honest).
I have been an ASP.NET developer for quite a number of years, and can knock out a complex web site fairly quickly. However, as with most of my other programming until recently, this has always been along the “throw it all in the code-behind†anti-pattern. I’ve come a long way in the last six months or so, and am now very comfortable using MVVM in WPF, and separating out my code into logical classes as all the Good Programmers do.
Logically therefore, my web site development should follow the same lines. I read up on both MVC and MVP, and came to the conclusion that as an experienced ASP.NET developer, MVP made a lot more sense to me. I couldn’t honestly see any technical benefit of one over the other, mainly as the MVC proponents seem to raise the same old “webforms are evil†arguments, without any real justification (to me at least, they obviously feel justified). My own feeling was that MVP wins out because it completely isolates the view from any action, meaning that the view is as dumb as it can get, which makes unit testing a doddle. I was won over towards the Passive View version of MVP, as this has even less code in the view than the other flavours.
This is not to say there’s anything wrong with MVC, just that I feel more comfortable with MVP.
This post details my initial exploration of WebFormsMvp.
I had the need today to add my own property to a WPF user control, and have that property available in the Properties panel of the Visual Studio designer, so the property could be set at design-time. The purpose of this was that my user control had a toolbar, and I had come across the need to use the control, but not show the toolbar. Simple eh? Well, not quite!
I had been having some serious grief with Visual Studio’s unit testing tools. VS was complaining that some tests did not exist, and others called methods that didn’t exist. Both claims were total lies as all methods in question existed, and could be found by using the “Navigate to†feature in VS.
I had two basic errors when I tried to run tests. One was of the form "Method TestProject.SystemsRepositoryTest.CreateNewCamera does not exist" when the method did exist. I could right-click the test in the Test Results window and choose “Open test†and it would take me there. However, when trying to run the test, VS claimed it didn’t exist.
The other error I got was of the form "Test method TestProject.SystemsRepositoryTest.GetAllCameras threw exception: System.MissingMethodException: Method not found: 'System.Collections.ObjectModel.ObservableCollection`1 Repository.GetAllCameras()'" which was also a lie as the method being tested existed. Again, I could go to the test method, click on the name of the method being called, click f12 (Navigate to) and be taken to the code for the method.
Thanks to these problems, I have wasted loads of time debugging things that could have been fixed with unit testing. It has been frustrating to say the least!
Well, I finally found the answer…
I opened the bin/Debug folder in the test project in Windows Explorer and deleted everything in it. I then rebuilt the test project, and my tests ran fine.
For some odd reason, it looks like rebuilding the test project wasn't actually changing the DLLs in the folder, so it was using old versions, in which the methods didn't exist. Deleting them all forced VS to grab fresh copies of the referenced DLLs, and rebuild the test project's DLL.
I don’t know if this is a bug in Visual Studio 2010, but it doesn’t seem to be a feature that I would have added in by choice!
A common scenario is to have a button on a view that is bound to a command on the viewmodel. You can also have an ABCCommandCanExecute() method on the VM that tells the view whether or not the button can be clicked.
This is all fine until you want to ask the user to confirm the action that will be performed when they click the button. For example "Are you sure you want to reformat your hard disk, dial up all the people in your contact list and reformat their hard disks, and then destroy the world?" It's kind of rude to do this without confirmation first.
The problem is that when you use WPF binding to bind the VM's command method to the button's Command property, you lose control over the execution process, and so can't inject a message box into the flow.
When you use Linq to create a query against an entity framework model, a common scenario is to use the .Include() extension method to make sure that any child objects are also loaded. This is mainly useful when the results of the query are to be sent over WCF, where the client is disconnected from the source of the query, and so cannot go back to the database to pick up any child objects as needed.
This works fine for simple queries, but falls apart when you want to do anything clever, like joins or shaping.
I came across an innocent-looking Linq problem the other day that really had me baffled for some time.
It's easiest explained using the ubiquitous Northwind database (although just about any other relational database would probably do). Suppose you want a list of customers with their orders. Pretty easy eh?
Erm no, it wasn't!