LuaDotNet: a thin wrapper around Lua and Luabind for .NET

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Introduction

This article presents a thin wrapper around Lua, see [1], and Luabind, see [2], for .NET: with it you can embed a scripting engine with a C, C++ backbone into your .NET applications.

If you are not familiar with Lua and Luabind, a small introductory example is also given below.

Lua, an embeddable C scripting language

Here are some quotes extracted from the Lua "about" page:

"Lua is a powerful light-weight programming language designed for extending C, C++ applications."

"Lua is a language engine that you can embed into your application. This means that, besides syntax and semantics, Lua has an API that allows the application to exchange data with Lua programs and also to extend Lua with C functions. In this sense, Lua can be regarded as a language framework for building domain-specific languages."

Usually, using Lua in an application is done with the following steps:

1. Create a Lua state:
2. Bind methods to Lua
3. Execute Lua scripts
4. Clean and deallocate state

Create a Lua state

"The Lua library is fully reentrant: it has no global variables. The whole state of the Lua interpreter (global variables, stack, etc.) is stored in a dynamically allocated structure of type lua_State. A pointer to this state must be passed as the first argument to every function in the library, except to lua_open, which creates a Lua state from scratch."

As mentioned above, lua_open is used to allocate a Lua state:

lua_State* L = lua_open();

Bind methods to Lua

Suppose that the following method need to bound:

void my_print( const char* str )
{
    printf( str );
}

First, we need to make a function wrapper for my_print that receives a Lua state and returns an integer, the number of values it wants to return to Lua:

int my_print_lua(lua_State *L)
{
   /* get the number of arguments (n) and
    check that the stack contains at least 1*/
   int n = lua_gettop(L);
   if (n<1)
   {
       lua_pushstring(L, "not enough arguments");
       lua_error(L);
   }

   /* try to cast argument to string and call my_print,
    - remember that Lua is dynamically typed
    - we take the first element in the stack
   */
   my_print( lua_checkedstring( L, 1) );

   /* my_print does not return values */   
   return 0;
};

At last, the method is registered in Lua using:

lua_register(L, "pr", my_print_lua);

Remark: Handling the Lua stack can become tedious and error-prone. Hopefully, Luabind is here to simplify (a lot) the wrapping task.

Executing Lua scripts

Consider the following script:

s = 'this is a test';
pr( s );

As one can see, Lua syntax is quite straightforward. In the script, we see that the method we bound (pr) is called. To execute this script in C, lua_dostring is used:

const char* str = "s = 'this is a test';pr( s );";
lua_dostring(L, str);

The program will output:

this is a test

Clean and deallocate state

When you are finished, do not forget to call lua_close to deallocate the Lua state:

lua_close(L);

Luabind, simplifying things

As mentioned before, handling the Lua state is a tedious work that we would like to avoid. Luabind uses template meta-programming to simplify things for us.

Using Luabind, the previous steps become:

1. Create a state:

   using namespace luabind; // luabind namespace
   
   lua_State* L =lua_open(L);
   luabind::open(L); // init luabind

2. Bind method:

   module(L)
   [
       def("pr", &my_print);
   ];

Remarks:

• We did need to write my_print_lua wrapper, Luabind does this for us.
• Luabind can also bind classes.
• Check the Luabind page for more information as their documentation is really well done.

LuaNET, the wrapper

So what about executing Lua script in .NET applications? This should not be a major problem, just the matter of writing a managed C++ wrapper.

Remarks:

• A detailed class documentation can be built by running the Documentation tool on the LuaNET project.
• All the classes live in the Lua namespace.

State, wrapping lua_State

The managed class State wraps up the lua_State structure. It handles the calls to lua_open and lua_close.

Here's a small example that creates a state, sets some variables and executes a script:

• C#:

  Lua.State L = new Lua.State(); // creating a lua state
  
  L.set_Global("a",1);          // equivalent to a=1 in Lua
  
  L.DoString("b=a * 2;")        // executing a Lua script
  
  Double b = L.get_Global("b").ToDouble(); // retreive the value of b
  
  

Note that get_Global returns a LuaObject which is then cast to a double using ToDouble.

LuaObject, wrapping luabind::object

LuaObject enables you to set and retrieve values from Lua. Supported values are:

• string
• double, int
• bool
• table

Since Lua is dynamically typed, you need to check the type of the LuaObject before casting it. Otherwise, if cast fails, exceptions will be raised:

L.DoString("s='string';");
L.DoString("print('s: ' .. tostring(s));");

LuaObject s=state.get_Global("s");
s.ToString(); // ok

s.ToDouble(); // fails, s is a string

If the object is a table, LuaObject implements the IDictionnary interface on the table and TableEnumerator implements the IDictionnaryEnumerator interface:

L.DoString("t={1,'string'};");
LuaObject t=L.get_Global("t");

System.Collections.IDictionaryEnumerator te = t.GetEnumerator();
while( te.MoveNext() )
{
    System.Console.WriteLine("t...(" + te.Key + "," + te.Value +")");
};

Loading libraries

Lua comes with a set of default APIs to handle strings, table, IO, files etc...To load these APIs, use DefaultLibrary.Load:

Lua.Libraries.DefaultLibrary.Load( L );

Luabind is loaded using LuabindLibrary.Load:

Lua.Libraries.LuabindLibrary.Load( L );

You can wrap up your API and load them in .NET using the same method as DefaultLibrary or LuabindLibrary.

Using the demo

The demo features Lua 5.0 and Luabind. You need to recompile all the projects and launch LuaNetTest.

History

• 8/8/2004: Initial release

References

• [1] Lua programming language,
• [2] Luabind