C# Lambdas: Do you know what you captured?
Posted a larger and more structured article on the (somewhat surprising) capture rules for outer variables inside C# lambdas.
May 23, 2010
May 17, 2010
Custom build step on file
BTW, in earlier versions of Visual Studio (definitely VC++ 6, but maybe later) it was possible to add an arbitrary custom compilation step for a file, that worked just fine – no need to modify projects and the like. They removed this capability in favor of “custom tools” that, in my opinion, is inferior: you no longer can use simple text transformation command-line utilities “as is”, you need some COM-bloated integration layer around them, to make them either MSBuild tasks, or custom tools, or add-ins, or whatever.
T4 – Too Troublesome to Tackle?
Text Template Transformation Toolkit (T4) is a code generator built into Visual Studio 2008 and 2010. Yes, you have it now on your machine 
It was proposed to me as a possible solution to the C# macros problem.
I played with it a little bit and my current verdict is it would be quite difficult to use in a serious project. Here’s why:
1. No automated build integration. This is a killer, especially if your templates include other templates. The only time template code is executed is when you save a template file. Not when you (re)build your solution. Not when you get your stuff from source control. If foobar.tt has changed and there are 50 files that include foobar.tt, you will have to hunt them down, open them in Visual Studio and save them to re-generate the code. That’s 51 files.
This is not T4′s fault per se – all custom tools suffer from this problem, but it does not make developer’s life any easier. There is some build integration in VS 2010, but “in order to configure a Visual Studio project for build-time template transformation, you have to manually modify the MSBuild definition in the project file
“. Not cool.
2. No proper source control integration. When you add your .tt file to source control, the generated file is added along with it, and every time you change anything, both files get checked out.
3. No ability to group all files of a class under one node. This is not a show stopper, but an inconvenience. I would like to have all code for class Foo to be under one node. Something like Foo.cs with Foo.props.tt below it, which generates Foo.props.cs. I am not sure how it can be accomplished with TT.
See also:
T4: Text Template Transformation Toolkit (Oleg Sych blog)
Understanding T4-MSBuild integration
May 14, 2010
C#: Trouble with Lambdas in For Loops
I had an interesting bug the other day. I wrote a foreach loop along these lines:
foreach (var entry in controlsByName)
{
entry.Key.SomeEvent += (sender,args)=>{ProcessControlName(entry.Value);}
}
Looks innocent enough, right? There is a big catch here. In functional languages we are accustomed to the fact that everything is immutable. I subconsciously transferred this notion to C# lambdas. I thought that the lambdas capture outer variables by value and carry this immutable value with them forever. This is wrong!.
C# language specification (found at C:\Program Files\Microsoft Visual Studio 9.0\VC#\Specifications if you have Visual Studio 2008) states in paragraph 7.14.4 that outer variables are captured by reference. They don’t use these exact words, but that’s the idea. If you change the value of the variable, all lambdas that captured it will be affected.
Even more surprisingly, there will be only one copy of the loop variable such as entry above, and all lambdas will share this single copy. This means that the code above has a bug. After the loop has finished, if SomeEvent is fired by one of the controls, the name passed to ProcessControlName() will always be the name of the last control, regardless of which control fired the event!
It turns out, you can even exploit this variable sharing making lambdas to communicate like this:
private void GetTangledLambdas(out Func<int> getter, out Action<int> setter)
{
int x = 0;
getter = () => x;
setter = v => {x = v;};
}
[TestMethod]
public void Lambads_Can_Communicate_Through_Captured_Variable()
{
Func<int> getter;
Action<int> setter;
GetTangledLambdas(out getter, out setter);
setter(10);
Assert.AreEqual(10, getter());
setter(20);
Assert.AreEqual(20, getter());
}
This, of course, is not even close to functional programming, even though it uses lambdas
May 6, 2010
Lost your thumbnails? Refresh Windows Components
For real. When you install a program that associates itself with image files, you may lose your thumbnails capability in Windows XP. It shows a giant icon of the program instead of the thumbnail. How to fix it? It’s elementary, Watson, Just go to Control Panel->Add or Remove Programs -> Windows components, don’t change anything and click “next” a couple of times. That’s it! The rumor goes, that if while doing that you three times say loudly “Microsoft rules”, it works even better.
May 4, 2010
Webdav – a nice alternative to FTP
Recently I needed to let remote users to transfer several hunderd megabytes of data to my machine. In the old days I would just go ahead and setup an FTP server, but it is so-o 1975. It is insecure, requires pinching wholes in the firewall, et cetera, et cetera.
So I googled for the alternatives and chose WebDav. I setup a WebDav folder on my Apache with relative ease, and then they used built-in Windows client (“My Network Places”) to access it. Of course, for it to be secure one needs to have https, and it is not very easy to setup, but fortunately I’ve already got it.
Of course, we used manual uploads. The user just opens my webdav folder in Windows Explorer and drags files to/from it. if automated uploads are required, webdav may be harder to tackle then FTP, since there are fewer command line clients. But you need only one that works, right?
Bottom line – if you have your own web server with HTTPS, webdav is highly recommended for secure file sharing.
BTW, I ran afoul of Windows data redirection again – Apache simply would not pick up my configuration changes, until I realized I am making them in a legacy editor (FAR) without elevated permissions, and all changes go into my local profile.
May 3, 2010
Live+Press: Victory!
I have downloaded Live+Press plugin for WordPress that can synchronize my WordPress-based blog and my LiveJournal. Of course, with my luck, nothing worked out of the box. After several debugging attempts I found that
1. PHP will automatically encode special characters in any posted form field, whereas “foo’bar” becomes “foo\’bar”.
2. This happens even to passwords.
3. The password hash then comes out wrong and LJ won’t accept it.
4. Needless to say, my password contained a special character.
Moral: never, ever do good deeds prematurely. If someone wants to write strings to the database, let the database worry about the special characters. Not everything POSTed is intended to a database, so there is no point (and it is even harmful) to automatically escape it.
PS. Why does the trackback link look so ugly? Well, I will deal with it later. Meanwhile, sorry for its appearance.