It serves!

Writing a Working Web Server in Under 100 Lines of Code

UPDATE 12/30/12: You must run the compiled executable as an administrator on systems with UAC. Otherwise, the program will crash and exit.

I was reading some coding blogs last night and I happened across a guy who had written a minuscule web server in C# over his lunch break. I knew right away that I had to have one, too. A server like this one serves little practical purpose; it does not support any sort of server-side scripting (though adding a BASIC interpreter to it would be extremely cool). It does, however, serve a wide variety of documents (and will detect their MIME type prior to serving them.

The first thing I did was write a Log method to make console output look fancy:

static void Log(string message)
{
    Console.WriteLine("[{0}]: {1}", DateTime.Now, message);
}

That was quick. Next was to create an actual server method, which would continuously loop and be started asynchronously by the Main() method.

static void RunServer(int port)
{
    HttpListener listener = new HttpListener();
    listener.Prefixes.Add(string.Format("http://+:{0}/", port)); // Where to listen
    listener.Start(); // Fire up the server
    while (true)
    {
        HttpListenerContext context = listener.GetContext(); // Get a connection
        Log("Requested: " + context.Request.Url);
 
        if (context.Request.RawUrl.EndsWith("/")) // Just asking for index; redirect
        {
            context.Response.StatusCode = 301;
            context.Response.StatusDescription = "301 Moved Permanently";
            context.Response.RedirectLocation = context.Request.RawUrl + "index.html"; // Just send browser to index.html
            context.Response.Close();
            Log("Redirected client to /index.html.");
        }
        else
        {
            byte[] retVal;
            try
            {
                retVal = File.ReadAllBytes("www" + context.Request.RawUrl); // Give the client the requested page
            }
            catch
            {
                context.Response.StatusCode = 404; // File not found; tell the client.
                context.Response.StatusDescription = "404 Not Found";
                Log("404: " + context.Request.RawUrl);
                context.Response.Close();
                continue;
            }
            string mime = GetMimeType(context.Request.RawUrl);
            context.Response.Headers.Add(HttpResponseHeader.ContentType, mime); // Give response a MIME type
 
            Stream respStream = context.Response.OutputStream; // Response stream we'll write to
            respStream.Write(retVal, 0, retVal.Length);
 
            context.Response.StatusCode = 200;
            context.Response.StatusDescription = "200 OK"; // Let client know stuff is okay
 
            respStream.Close();
            context.Response.Close();
            Log("Served: " + context.Request.RawUrl + " as " + mime + ".");
        }
    }
}

There’s quite a bit going on here; I’ll try to explain it all. The beginning of this method configures an HttpListener to wait for connections and starts it. It then starts an infinite loop (the user can stop the server by closing the console window) of continuously waiting for and serving requests (listener.GetContext() blocks until something makes a request). A client can ask for three things in this simple server (since we’re not running server-side scripts):

  1. A page that exists (e.g. /index.html)
  2. A page that doesn’t exist (e.g. __++++_____-sdasfash1234.abcdf)
  3. A directory root that may or may not exist (e.g. /mysite/)

The first thing we check for is the directory root case. We check the URL to see if it ends in a slash. If it does, we redirect the client to whatever the path is, plus “index.html.” We could simply serve the “index.html” off the bat, but we’re (poorly) code golfing right now! That would take more code and accomplish essentially the same thing!

If the RawUrl does not end in a slash, we’re serving a page! Anyway, the server stores all its files in a “www” folder that lives in the same directory as the executable. Fetching “www” plus the raw path with File.ReadAllBytes gets the data of the file we are looking for. The try statement is used in the event we can’t find the file. If an exception does get raised, the server returns a “404 File Not Found” error to the client. If this server were fancy, it would also serve a 404 error page.

If the server can find the file, it determines the MIME type through a simplistic process I will detail in a moment, sets the MIME type in the header, writes the data to the client stream, gives the client a “200 OK” (I am not sure if the order matters for all this, or if all the data simply gets flushed when the HttpResponse gets closed, but this code worked fine in Chrome and IE 9), and closes the connection.

The last thing is to determine the MIME type of the file. Rather than using WIN32 interop calls, I decided to write a simplistic method of my own based on the file extension being served.

static string GetMimeType(string fileName)
{
    // Fast way of determining if the lowercase file name ends with something
    Func<string, bool> fendw = new Func<string, bool>(inp => fileName.ToLower().EndsWith(inp));
 
    // Use our function to figure out the mime type
    if (fendw(".htm") || fendw(".html")) return "text/html";
    else if (fendw(".ico")) return "image/vnd.microsoft.icon";
    else if (fendw(".png")) return "image/png";
    else if (fendw(".jpg") || fendw(".jpeg")) return "image/jpeg";
    else if (fendw(".gif")) return "image/gif";
    else if (fendw(".js")) return "text/javascript";
    else if (fendw(".css")) return "text/css";
    else return "application/octet-stream";
}

This method wouldn’t be very interesting were it not for the Lambda Expression (remember those things I love?). The Lambda Expression takes as a string as an argument and returns a Boolean of whether the lowercase version of the provided file name ends with the specified text. In other words, it checks to see if the file has whatever extension you pass to it and ignores case. The “fendw” name, although cryptic, is short for “file ends with;” it prevented me from having to type fileName.ToLower().EndsWith(…) over and over again!

That’s it. The web server works — we just need to add a Main() method:

static void Main(string[] args)
{
    Console.WriteLine("On what port should the server run?:");
    int port = int.Parse(Console.ReadLine());
 
    Task server = new Task(() => RunServer(port)); // Start the server async
    server.Start();
    Log("Server started on port " + port.ToString() + ". Close this window to stop it.");
 
    while (!server.IsCompleted) ; // Wait infinitely
}

The first part of this method gets the port the user wants to run the server on. I used 2013, since it’s almost New Year’s! Next, it creates an asynchronous task (using a Lambda Expression again!) to run the server on, then fires it up and blocks indefinitely. The use of the Lambda expression is unnecessary here (since the thread Main() runs on will just loop forever), but if you wanted to listen on more ports, you could create more tasks to run new instances of the server on. It makes the code more extensible, right?

It serves!

It serves!

Serving a real web page from a website I built for a charity organization last year!

Serving a real web page from a website I built for a charity organization last year!

If you desired, you could actually open a port in your router and let your friends connect to your cool new server. I should probably warn you that doing so would be a terribly risky idea, even if you only have the port open for a short while. You wouldn’t run through wolf-infested woods with a steak or swim at the Great Barrier Reef with a gaping leg wound for ten minutes, so why would you do the same on the Internet? In other words: this code is provided “AS-IS” with no express or implied warranty, especially since I do not really know how secure this server is. My guess is that it isn’t. Still, it’s hard to go wrong with a 100-line server!

Calculating Pi

One programming task that I had never taken on (up until this morning, anyway), was a pi calculator. Finding pi is not an overwhelmingly daunting task; one can get an extremely precise approximation from the System.Math.PI constant. However, that’s no fun. The real fun is in rolling your own pi calculator. As it turns out, this was no beginner project. It was math-heavy, theory-heavy and CPU-heavy. Oh, and it was a blast!

The trouble was that at first, I had no idea where to begin. In C#, there is no “arbitrary precision” decimal type; the decimal class can hold 28 or so significant figures (remember the calculator tutorial?). No self-respecting pi calculator would ever stop at 28 significant figures. 28,000 would be a bit better! The first challenge in coding the pi calculator would be coming up with a way to store an unlimited number of significant figures. I’d read about some pi calculator authors implementing “BigFraction” or “BigRational” classes based off of the fairly new BigInteger (arbitrarily-sized .NET integer) class, so I thought I’d take a crack at writing one of those. I also decided that before I wrote a single line of code, I needed to choose a pi calculation formula.

The requirements for a formula are simple: that it converges relatively quickly and requires relatively few calculations per iteration. Unfortunately, the use of a “BigRational” class adds another requirement: no fractional powers of numbers that have irrational fractional powers. That’s a real drag, because it means we can’t use the Chudnovsky Algorithm, which is the Holy Grail of fast pi calculation algorithms:

The trouble is with the 3k + 3/2 part; 640320 is not a perfect square (it is almost 800.2^2, but not quite).

The trouble is with the 3k + 3/2 part; 640320 is not a perfect square (it is almost 800.2^2, but not quite).

With the really good algorithm out of the way, I started trying to find another good algorithm. My first thought took me back to my trig identity days a few years ago (okay, the past three years of high school):

So then if you take the deriv... actually, no.

So then if you take the derivat… actually, no.

This is actually a sound, correct thought. In fact it has a name: the Gregory-Leibniz formula. This will eventually converge to pi. However, after following a link off the handy, dandy Wikipedia page I used to decide which algorithm I would use in my program, I learned that it takes five billion iterations of arctan(1) (which I will get to in a moment) to produce ten correct digits of pi. YIKES! It was a good thought.

Anyway, I eventually decided on the original Machin formula, from 1706:

Although it was created in 1706, Wikipedia says it is still one of the fastest-converging pi-calculation algorithms!

Despite its age, it converges quickly and is relatively easy to compute.

Awesome. We have a formula. Now we need to figure out how an arctangent can be computed. It is actually rather simple, especially with programmatic help:

See. Not that bad.

See. Not that bad.

Alright! We have the formula for computing pi and we have the formula for computing arctangent, so now we’re done. Not so fast. Remember when I mentioned the BigRational class? As it turns out, writing a BigRational class is the bulk of this project. I won’t bore you with too many tiny details of the way I implemented it; I will tell you that it was designed for simplicity over speed. It definitely needs some refining, but it works.

The basic concept behind the BigRational class is simple: it is a fraction. It has a numerator and a denominator. The numerator and denominator are arbitrarily-precise, so the entire ratio is arbitrarily-precise. The class has a bunch of methods to help the numerator and denominator do their fraction-y duty:

As it turns out, a lot of help.

As it turns out, a lot of help.

Most of the methods listed in the class diagram are pretty self-explanatory; I won’t go into detail here (but you can download the source code to the project at the bottom of the page). All the operators help the BigRationals play well together, and the * operator has an overload that allows “scalar” multiplication. That is, it allows multiplication by a BigInteger.

The two methods that I will delve into are the Reduce method and the ToDecimalString method. The Reduce method reduces a BigRational into its simplest form. It implements Euclid’s Algorithm, which is a means of reducing a fraction. It works in steps:

  1. The larger of the numerator and the denominator is set as the “big number.” The smaller is set as the “small number.”
  2. The program finds the remainder when the big number is divided by the small number.
  3. If the remainder is not zero, the small number becomes the big number and the remainder from step two becomes the small number.
  4. If the remainder is zero, both the numerator and denominator may be divided by the small number. At this point, the small number is the greatest common factor of the numerator and denominator.
  5. Steps 2, 3 and 4 are repeated until step four terminates with the small number equal to one. At this point, the fraction is in simplest form.

In code, the above steps look like this:

public void Reduce()
{
    // Set up big and small number
    BigInteger bigNumber, smallNumber = 0;
    if (Numerator > Denominator)
    {
        bigNumber = Numerator;
        smallNumber = Denominator;
    }
    else
    {
        bigNumber = Denominator;
        smallNumber = Numerator;
    }
 
    // Now divide a bunch of times
    while (smallNumber != 1)
    {
        // Find the remainder
        BigInteger rem = (bigNumber % smallNumber);
        if (rem != 0)
        {
            bigNumber = smallNumber; // Set up the numbers for the next iteration
            smallNumber = rem;
        }
        else
        {
            // We can divide both the numerator and denominator by the previous remainder
            Numerator /= smallNumber;
            Denominator /= smallNumber;
 
            // Re-assign bigNumber and smallNumber
            if (Numerator > Denominator)
            {
                bigNumber = Numerator;
                smallNumber = Denominator;
            }
            else
            {
                bigNumber = Denominator;
                smallNumber = Numerator;
            }
        }
    }
 
    // One last thing -- if the numerator is positive and the denominator is negative, the numerator needs to become negative and the denominator needs to become positive.
    // If they're both negative, they should both be positive
    if (Denominator < 0 && Numerator > 0 || Numerator < 0 && Denominator < 0)
    {
        Numerator = -Numerator;
        Denominator = -Denominator;
    }
}

The only change between the pseudo-code and actual implementation was the addition of the small if-statement at the bottom. I noticed that the denominator would sometimes be negative instead of the numerator being negative. That would probably throw off some calculations, so I made sure that only the numerator of the BigRational could ever be negative.

The ToDecimalString method converts a BigRational into its decimal equivalent — a handy helper when it comes to calculating pi (since it’s no fun unless it starts with 3.14159…). I considered two different implementation styles:

  1. Multiplying the entire fraction by 10^however many digits I wanted.
  2. Creating my own long division algorithm that would return a string of digits of a specified length.

Contrary to what seems sane, I chose the second method. I suppose it was partially because I had always wanted to write an actual long division algorithm, but by any means, it took some head-scratching at first. Long division isn’t something I have to do very much, so I started by doing out a few long division problems. As it turns out, there is a nice pattern. The non-decimal part of the number may be calculated in one fell swoop by using integer division. It may then be committed to a string. After that, a decimal point should be committed to the string. Then, a “new numerator” should be created. Its value is the remainder from the previous iteration multiplied by ten. The next decimal digit is equal to the “new numerator” minus the remainder when it is divided by the denominator of the fraction, all divided by the denominator of the fraction. This process can be repeated until the desired number of digits is reached.

Okay, it makes more sense in code. Wow.

public string ToDecimalString(BigInteger decimalDigits)
{
    StringBuilder rv = new StringBuilder();
    this.Reduce(); // Go as fast as possible
    BigInteger remainder = Numerator % Denominator;
 
    // Get the non-decimal part
    rv.Append(((Numerator - remainder) / Denominator).ToString() + ".");
    remainder = Numerator % Denominator;
 
    BigInteger newNum = remainder * 10;
 
    // Now get the decimal part
    for (BigInteger i = 0; i < decimalDigits; i++)
    {
        rv.Append(((newNum - (newNum % Denominator)) / Denominator).ToString()); // This literally just does long division
        newNum = (newNum % Denominator) * 10;
    }
 
    return rv.ToString();
}

The only real advantage this method presents is the lack of any string parsing. It is all basic use of a StringBuilder. Plus, it seems to work very well!

With the BigRational class done, the next step was the actual pi calculation. The first step was to implement an arctangent method, in accordance with the formula I mentioned earlier.

public static BigRational ArcTangent(BigRational input, BigInteger iterations)
{
    BigRational retVal = input;
    for (BigInteger i = 1; i < iterations; i++)
    {
        // arctan(x) = x - x^3/3 + x^5/5 ...
        // = summation(n->infinity) (-1)^(n) * x^(2n+1)/(2n+1)
        BigRational powRat = input.Pow((2 * i) + 1);
        retVal += new BigRational(powRat.Numerator * (BigInteger)Math.Pow(-1d, (double)(i)), ((2 * i) + 1) * powRat.Denominator);
        if (i % 100 == 0)
        {
            Console.WriteLine("ArcTangent {0}: {1}/{2} iterations complete.", input, i, iterations); // Status update.
        }
    }
 
    return retVal;
}

Wait. There’s a parameter called “iterations.” How do we figure out how many iterations are necessary? This page provided the answer. As it turns out…

I find it's a good idea to add twenty or so digits to the end to make sure everything is correct.

I find it’s a good idea to add twenty or so digits to the end to make sure everything is correct.

In other words…

public static BigInteger GetConversionIterations(BigInteger digits, BigRational q)
{
    return (BigInteger)((double)digits / (2 * Math.Log10((double)q.GetReciprocal())));
}

That was quick. That also finished laying the foundation for actually calculating pi. Phew. That was a lot of keystrokes.

public static BigRational GetPi(BigInteger numDigits)
{
    // pi = 16 arctan(1/5) − 4 arctan(1/239)
    BigRational oneFifth = new BigRational(1,5);
    BigRational oneTwoThirtyNine = new BigRational(1, 239);
    BigRational arcTanOneFifth = PiCalc.ArcTangent(oneFifth, PiCalc.GetConversionIterations(numDigits + 1, oneFifth)); // Start computing
    BigRational arcTanOneTwoThirtyNine = PiCalc.ArcTangent(oneTwoThirtyNine, PiCalc.GetConversionIterations(numDigits + 1, oneTwoThirtyNine));
 
    return (arcTanOneFifth * 16) - (arcTanOneTwoThirtyNine * 4);
}

This code block implements Machin’s formula as discussed above. An improved version of this program could take advantage of multi-core computing by calculating one arctangent on one thread and the other arctangent on another. More formulas should also be tested to determine which is the fastest; this program really slows down as the iteration count gets higher because the fractions get really, really, really huge. Of course, this program was never going to break any records anyway because it was written in C#. JIT-compiled, super high-level languages aren’t exactly speed demons. Then again, my Phenom II x4 (overclocked to 3.7 gHz) is also beginning to show its age. I’d be curious to see how performance compares on one of the latest-generation i7s.

Although it is sluggish, it does seem to compute pi properly. Here are the first 10,001 digits:

Although it looks like a lot of pi, it's not a lot of pi.

Although it looks like a lot of pi, it’s not a lot of pi.

So that’s it. It’s done. It works. Now it just needs to run for a few days so I can brag to my friends! The funny thing about calculating pi is how although we believe pi is completely irrational and will never end, we’ve calculated enough digits that it might as well end. If my calculations are correct, it only takes about 80 digits of pi to calculate the volume of the observable universe to the nearest cubic meter. Thus, ten thousand would calculate it to the nearest 1/10^9920 cubic meter, whatever that would be. That’s some mind-blowing precision. Imagine what a million digits would do.

Download this Project (MIT License)

A Brainf**k Interpreter in JavaScript

Brainf**k (I bet you can guess what the asterisks are covering…) is a tiny, esoteric programming language. It is classified as Turing-complete, but its practical use is very limited (if not nonexistent). BF is an interesting challenge because it is so simple. Many devoted developers spend lots of time creating the smallest, fastest BF interpreters and compilers they can. One user on Codegolf created a BF implementation that was only 106 bytes long. That’s pretty impressive.

BF has eight commands (which will be discussed in a moment). Every program has access to a series of memory blocks that can have any range of values. When a BF program loads, all the “cells” in the memory (really just an array) are initialized in value to zero. BF programs can switch between different blocks of memory (change the array index that they are accessing), increment the values of specific blocks, decrement the values of specific blocks, print a character representation of a block to the screen, or put a character’s value into a block. Programs can also loop. They are executed command-by-command from left to right.

There is no official language specification for BF, so many details are left up to the individual implementation. However, the following standards are pretty common (and were implemented in my BF interpreter):

  • Using bytes for memory. My BF interpreter limits values in cells to [0, 255].
  • Allowing overflows. Blocks overflow when their value goes outside the permissible range of [0, 255]. That is, when a program tries to make a block’s value -1, its value becomes 255, and when a program tries to make a block’s value 256, its value becomes zero.
  • 30,000 blocks (29.3kb) of available memory.
  • Non-command characters are ignored.

The eight commands:

  •  + increases the value of the current block by one.
  • - decreases the value of the current block by one.
  • > increases the block index by one.
  • < decreases the block index by one.
  • . prints the block to the screen as an ASCII character.
  • , gets an ASCII character and puts it in the current block.
  • [ opens a loop. It will jump to the end of the loop if the current block’s value is zero.
  • ] closes a loop. It will jump to the beginning of the loop if the current block’s value is not zero. If it is zero, the program will continue.

I’m not going to teach you how to write BF programs in this post; I’m hardly a maestro of it myself. If you want to learn more about BF, Google is an awesome resource.

My interpreter is pretty simple. The first task was to create a crash-reporting function, since I figured that would probably happen a lot. For future reference, “output” is the ID of the part of the page where BF script output goes.

function crash(reason, at){ // When the program dies
	$("#output").text("CRASHED: " + reason + " ( at char index " + at + ")");
}

The actual interpreter starts out with the declaration of a run function and all sorts of BF-related variables.

function run(data){ // Runs the BF program. Data: program to run.
	var mem = []; // Program memory
 
	// Initialize all the memory
	for(var i = 0; i < 30000; i++){
		mem[i] = 0;
	}
 
	var pointer = 0; // Pointer to memory
 
	$("#output").text(""); // Clear output field

Next, it loops through each character in the program (specified to the function as data) and if it is BF-legal, the required action is performed. The simple ones (that aren’t loops) are self-explanatory.

for(var i = 0; i < data.length; i++){ 	
        if(data[i] === ">"){ // Right one cell
		if(pointer < 30000) pointer++;
		else{
			crash("There are only 30,000 usable cells of memory. Sorry.", i); // Trying to use too many cells.
			return;
		}
	}
	else if(data[i] === "<"){ // Left one cell 		
                if (pointer > 0) pointer--;
		else {
			crash("Cannot decrement pointer when pointer is at first cell.", i); // Trying to go below cell zero.
			return;
		}
	}
	else if(data[i] === "+"){ // Increment cell value
		mem[pointer]++;
		if(mem[pointer] > 255) mem[pointer] = 0; // Overflow
	}
	else if(data[i] === "-"){ // Decrement cell value
		if (mem[pointer] > 0) mem[pointer]--;
		else {
			mem[pointer] = 255; // Overflow back to 255
		}
	}
	else if(data[i] == "."){ // Put character to screen
		var memChar = String.fromCharCode(mem[pointer]);
		if (memChar == "\n") memChar = ""; // Turn newlines into breaks
		$("#output").append(String.fromCharCode(mem[pointer])); // Log the correct character from its code
	}
	else if(data[i] == ","){
		mem[pointer] = window.prompt("Please enter one character.").charCodeAt(0); // Set memory to char code
	}

Next come loops. There are loads of elegant ways of going about loops; a stack storing the start indices of loops could be used to avoid all the searching my implementation does. However, simplicity was on my mind when I wrote this program: it suffices to say I took the slow, not-so-elegant way out. When the interpreter encounters the start of a loop (a [), it checks to see if the value of the current cell is zero. If it is not, it just continues along. If it is, things get more interesting. The interpreter loops through the program until it finds the corresponding end of the loop so that the program can continue executing properly.

The task is not as simple as incrementing i (the “instruction pointer,” so to speak) until data[i] is a ]. Other loops could be in the way. The solution is shockingly simple: keep track of the number of newly open loops as the program searches for the end of the original loop. I used a counter to keep track of the loop openings. Every [ increments it; every ] decrements it. When the counter gets to zero and the program finds a ], the index has been found. All that is left is to change the “instruction pointer” to the new location in the program.

The ] routine works in the exact opposite way. It keeps track of ]s instead of [s but is otherwise very similar. Here’s what it looks like:

else if(data[i] == "["){
	if(mem[pointer] != 0) continue;
	else{ // Search for corresponding ]
		var openCount = 0; // # of open loops
		for(var j = i; j < data.length;j++){ // Loop through more characters
			if(data[j] === "[") openCount++; // Another open loop
			else if(data[j] === "]"){ // A closing of a loop
				openCount--; // Decrement open count
				if(openCount === 0){ // If we're at zero, we're done.
					i = j; // Move the program forward
					break; // Stop looping
				}
			}
		}
		if(openCount != 0){
			crash("Open loop.", i);
			return;
		}
	}
}
else if(data[i] === "]"){
	// Same deal as [ except going backwards
	if(mem[pointer] != 0){
		var closeCount = 0; // We use close count on this one because it makes more sense (since we're doing the opposite from before!)
		for(var j = i; j >= 0; j--){
			if(data[j] === "]") closeCount++;
			else if(data[j] === "["){
				closeCount--;
				if(closeCount === 0){
					i = j;
					break;
				}
			}
		}
		if(closeCount != 0) {
			crash("Too many loop closings.", i);
			return;
		}
	}
}

That’s all there is to it. The rest of the page that contains the interpreter, which you can find here, is instructions and a textbox to let you put a program in.

Here’s what the finished product looks like:

It's not as pretty as TextRacer but it gets the job done.

The interpreter running a ROT13 cipher example I found on Wikipedia.

Overall, I was surprised with how quickly I managed to get my interpreter working. I was also glad I went with the architecture I did; though it would have been possible to convert the BF to JavaScript and then eval() it, the program would have lost most of its charm. This program could certainly use some refining but overall, it gets the job done! Happy BFing!

I LOVE Lambda Expressions

Lambda Expressions essentially add anonymous methods (methods that are attached to a piece of code, have no name and cannot really be explicitly called) to C#. I have not used them very much in my time as a developer, partly because they are newish and partly because I have never really had a need to use them. Yesterday, I decided to do some research into them and I really liked what I saw. Using Lambda Expressions in C# code adds a real JavaScript feel. It’s like the inline, more organized beauty of JavaScript without all sorts of annoying asynchronousness (though asynchronous delegates are possible in C#).

C# and the .NET Framework have always had very slick event handling. I have heard horror stories about listeners and hundreds of lines of code in Java, but since I am not a Java developer, I cannot relay a firsthand account of just how scary it gets. I can tell you that C# event handling started out great and became even better.

For the purposes of this blog post, I made a Windows application with three buttons (aptly named button1, button2 and button3).

Lambda Expressions at Work

Hey look, a sneak preview! Yippee!

In early versions of the language, if you wanted to do something when one of the buttons got clicked, you had to attach an event handler method like this:

button1.Click += new EventHandler(button1_Click);
 
void button1_Click(object sender, EventArgs e)
{
    MessageBox.Show("You clicked button1!");
}

Granted, this is awesome, but for quick snippets to run in event handlers, it’s not a very efficient use of space. Additionally, it’s hard to keep track of your code flow because you have to scroll down from where the method is attached to see what the event handler does. I am not advocating for cluttering your methods with tons of anonymous method code. I am saying that sometimes, you just want to see what is happening in your program without looking all over the place.

That’s where a delegate method comes in:

button2.Click += delegate(object sender, EventArgs e)
{
    MessageBox.Show("You clicked button2!")
};

Whoa! That’s way better! But wait, there’s more! Enter the Lambda Expression! To use Lambda Expressions, you need to use the Lambda operator. Since λ and Λ aren’t standard keyboard characters, C# uses => (not to be confused with >=) as the lambda operator. On the left side of your expression are the parameters. They do not need to be strongly-typed (scary, I know); you only have to provide a name for each and the type is inferred. On the right side of the expression is what you want to do with the code. Ready, set…

button3.Click += (sender, e) => MessageBox.Show("You clicked button3!");

WOW! That saved a lot of code! Unfortunately, it was kind of a silly example. Let’s try a slightly more complicated, relevant one. Let’s say you wanted to return the first name in a list that started with “B”. Ordinarily, you’d use a foreach loop:

List names = new List();
names.AddRange(new string[] { "John", "Bob", "Steve", "Mike", "Bill", "Emily", "Jenny", "Morgan" });
 
foreach (string name in names)
{
    if (name.StartsWith("B"))
    {
        MessageBox.Show(name);
        break;
    }
}

That’s seven lines of code for the loop. With a Lambda Expression, you can cut that done to one:

List names = new List();
names.AddRange(new string[] { "John", "Bob", "Steve", "Mike", "Bill", "Emily", "Jenny", "Morgan" });
 
MessageBox.Show(names.First(name => name.StartsWith("B")));

Uhhh… okay. What the heck. Maybe that example is worth dissecting a little bit!

Just as before, we create a list of strings and add a bunch of names to it. Then, instead of manually looping through the list, we use a LINQ extension method. The LINQ extension will return a string (because names is a list of strings) and takes a Lambda Expression as an argument. The Lambda expression takes an argument of its own, which will be a string. It will return true if name.StartsWith(“B”) is true (note that you do not use the “return” keyword with it), and names.First will return whatever string causes the Lambda Expression to return true. Basically you end up with the first name that starts with a capital letter B, which in this case is “Bob.” Try it!

Bob Message Box

Success.

Of course, Lambda expressions have an associated type: System.Func. System.Func is a generic type; the List<string>.First method we’ve been using actually takes a parameter of the type System.Func<string, bool> where string is the parameter and bool is the return value of the Lambda Expression with which it is associated. Thus, you can actually declare Lambda expressions as variables!

Func fd = st => st.StartsWith("B"); // Declare Lambda Expression
 
MessageBox.Show(names.First(fd)); // Pass the Lambda Expression variable as an argument.
 
MessageBox.Show(fd("Steve").ToString()); // Use the Lambda Expression just like any other method (displays "False" here).

I have barely scratched the surface of Lambda Expressions in this post. You can use them asynchronously and do countless, really cool things with them with the help of LINQ. As I am hoping to get a Macbook for Christmas so I can learn Objective-C, I won’t get to use Lambda Expressions very much but I already know I am going to miss them.

If you want to learn more about Lambda Expressions, go to the MSDN page about them here or this really good StackOverflow post.

Happy expressing!