I make it a point to learn 2 new programming languages each year; this year I have learned Flex/ActionScript and Lua.

I already have a few blog posts about my work in Flex, specifically integration between Flex and .NET. I would now like to introduce you to Lua and demonstrate it’s embedding features in C#.

What is Lua?

In a nutshell, Lua is a fast, light-weight scripting language that is extremely portable (it can be built on any platform with an ANSI C compiler.) Lua is considered the fastest interpreted scripting language, and is highly embeddable, in fact; I first came into contact with Lua while working on an open source project that is primarily written in C++ but is currently being extended by Lua.

Getting started

Before we get our hands dirty writing code, we will need to get LuaInterface.dll, a library for integration between Lua and the CLR. You can get the LuaInteface here: http://luaforge.net/projects/luainterface/ or you can get it via SVN here: http://luainterface.googlecode.com/svn/trunk and compile it yourself.

Time to embed!

Step 1: Lets create a basic C# application, import the LuaInterface namespace, and create a Lua object interpreter object.

using LuaInterface;
class Foo
{
    static void Main(string[] args)
    {
        Lua lua = new Lua();
    }
}

At this point, we have a working lua interpreter embedded into our application; we can execute any lua string or lua file. Take this code for example:

string lua_loop = "for i=0,10 do print(i) end";
lua.DoString(lua_loop);

will print number 0-10, to the console, not super fascinating, but we are getting there…You can also execute entire Lua files by:

lua.DoFile("d:\bin\foo.lua");

Time to really embed!

So far I have shown you how to get a Lua interpreter running in a C# application and how to run strings of Lua in the embedded interpreter. Lets kick it up a notch and call C# functions via Lua.

Say we want to expose the following C# function that performs the SHA1 hash to Lua…

public string SHA1(sting text) {
    string hash = null;
    using(SHA1 sha = new SHA1CryptoServiceProvider()) {
        byte[] result  = sha.ComputeHash(Encoding.UTF8.GetBytes(text));
        hash = Convert.ToBase64String(result)
    }
    return hash;
}

All we need to do is hook the SHA1 function into Lua by the RegisterFunction function; whew, that was a mouthful. Here it is in code:

class Foo
{
    static void Main(string[] args)
    {
        Foo foo = new Foo();
        Lua lua = new Lua();

        // registers the C# function with the Lua interpreter.
        lua.RegisterFunction(”SHA1″,foo,foo.GetType().GetMethod(”SHA1″));

        // we can now call SHA1 from Lua.
        lua.DoString(”print SHA1(’gork’)”);
    }
    public string SHA1(sting text) {
        string hash = null;
        using(SHA1 sha = new SHA1CryptoServiceProvider()) {
            byte[] result  = sha.ComputeHash(Encoding.UTF8.GetBytes(text));
            hash = Convert.ToBase64String(result)
        }
        return hash;
    }
}

In this example I just called the function via the DoString function however, I could have just as easily execute SHA1 from a Lua script.

The Lua interpreter object can also be used as an indexer to pass data between Lua and C#, the following code snippets demonstrates:

// creates a global Lua variable 'foo' with value 'gork'
lua["foo"] = “gork”;
// converts Lua global variable back to C# variable.
string foo = (string)lua["foo"];

Putting it all together, we can perform the SHA1 hash in Lua and get a handle to our result:

// perform a SHA1 hash in Lua with a C# function.
lua.DoString("hash = SHA1('gork')");
string hash = (string)lua["hash"];
System.Console.WriteLine(hash);

There are certainly more things that can be done between Lua and C#, but I feel like this is a straightforward introduction that may spark some interest, so I am going to leave it at that. Until next time, happy hacking.