Using Lua with .Net C# part II
In the previous post I showed how the Lua source code can be built for use by .Net, and how to make a very simple hello Lua script, and execute it from .Net code.
Putting Lua to real use
To actually do something interesting Lua must be able to call into .Net functions that is registered with Lua.
This enables all sorts of exciting features, such as exposing your business / game objects to Lua and create Lua scripts for AI, custom business processing etc.
.Net function signature for Lua callbacks
Every .Net function that Lua should be able to call must match a specific function signature, and must also be registered with Lua.
This is how the function signature should look in .Net:
/* definition from lua.h source code file * typedef int (*lua_CFunction) (lua_State *L); */ public delegate int LuaFunction(IntPtr lua_State);
Every .Net function exposed to Lua must return an int and takes a pointer to the Lua state as its single argument.
The Lua state object is used to get the parameters for the function along with any other variable values you need for your logic to work.
Communicating with Lua and the Lua stack
You do not operate on the Lua state itself, instead you call Lua C library functions which all takes the Lua state as parameter.
Lua uses a stack for all communication from .Net to Lua (and from any other language to Lua)If you want to create a new object in Lua (called a Lua table) you use the stack available from the Lua state to create it and set values of any desired fields.
In short, you will need wrapper functions in .Net to operate on the Lua stack, and Lua exports a whole bunch of functions that operates on this stack, which I will show later.
A complete guide for the lua stack can be found in the Lua documentation, section 3 at http://www.lua.org/manual/5.1/manual.html
Lua wrappers and the Lua.h source code file
Lua has an API function for registering a function from another language, its quite simple and when wrapped in .Net I created one that looks like this:
public void RegisterLuaFunction(Lua.LuaFunction func, string luaFuncName)
{
//call the lua_register API function to register a .Net function with
//the name luaFuncName and a function pointer func
//the function pointer is defined using the delegate shown earlier
//when making the correct Lua function signature for .Net functions
Lua.lua_register(m_lua_State, luaFuncName, func);
//make sure the delegate callback is not collected by the garbage collector before
//unmanaged code has called back
m_refs.Add(func);
}
Dont worry too much about the m_refs.Add(func); call.
When I first started out creating a Lua wrapper I learned the hard way (debugging debugging debugging) that .Net garbage collects delegates, in use for Lua callbacks, before I intended .Net to do so.
When Lua calls back into .Net, the function pointer is suddenly no more valid, and a runtime exception occurs.
By maintaining a list of references to registered Lua callbacks in .Net, I make sure that the garbage collector wont collect the functions pointers before I want it to. The list Im using in my wrapper is defined as this:
private static List<Delegate> m_refs = new List<Delegate>();
When I close Lua I just empty the list of refs and they will be collected and I will be happy.
The wrapper for the register function looks like this:
//definition from lua.h source code file
//#define lua_register(L,n,f) (lua_pushcfunction(L, (f)), lua_setglobal(L, (n)))
public static void lua_register(IntPtr lua_State, string n, LuaFunction func)
{
lua_pushcfunction(lua_State, func);
lua_setglobal(lua_State, n);
}
You might wonder why theres no DLLImport Attribute.
Take a look at the definition from lua.h, its actually a macro using two other Lua API functions. These two functions must be wrapped as well.
Lua uses macro definitions here and there, actually lua_pushcclosure and lua_setglobal is also just macros:
//#define lua_pushcfunction(L,f) lua_pushcclosure(L, (f), 0)
public static void lua_pushcfunction(IntPtr lua_State, LuaFunction func)
{
lua_pushcclosure(lua_State, func, 0);
}
//#define lua_setglobal(L,s) lua_setfield(L, LUA_GLOBALSINDEX, (s))
public static void lua_setglobal(IntPtr lua_State, string s)
{
lua_setfield(lua_State, LUA_GLOBALSINDEX, s);
}
The lua_pushcclosure and lua_setfield functions is actual C API function and is lua.h defined and wrapped like this:
//LUA_API void (lua_pushcclosure) (lua_State *L, lua_CFunction fn, int n);
[DllImport("lua5.1.dll")]
public static extern void lua_pushcclosure(IntPtr lua_State, [MarshalAs(UnmanagedType.FunctionPtr)] LuaFunction func, int n);
//LUA_API void (lua_setfield) (lua_State *L, int idx, const char *k);
[DllImport("lua5.1.dll")]
public static extern void lua_setfield(IntPtr lua_State, int idx, string s);
This is getting a bit boring I hope I did not lose you completely there, if I did Im sorry 🙂
The point is to show how Lua API functions can be put in a .Net wrapper that emulates the complete Lua API.
Registering a .Net callback funtion with Lua
Lets imagine that we have created a game and in that game we have created a class in .Net which contains methods for manipulating a “NPC player” object.
Now we want to be able to create combat scripts, task scripts, AI scripts etc.
Furthermore we want users of the game to be able to create their own levels and define behaviour for NPC characters in that level.
Most users can learn to write a Lua script and we choose to expose some functionality from the “NPC player” class.
We fire up an instance of Lua by creating a Lua state, and then we register some .Net functions with Lua.
Somewhere in our ScriptEngine we have defined:
private static IntPtr m_lua_State = IntPtr.Zero;
And open Lua:
m_lua_State = Lua.lua_open(); //open Lua libraries Lua.luaL_openlibs(m_lua_State);
Our NPCPlayer class definition:
public class NPCPlayer
{
bool InCombat(string characterName)
{
//not implemented, always in combat
return true;
}
int Lua_InCombat(IntPtr m_lua_State)
{
//get the characterName from Lua
string cName = string.Empty;
//if theres a sring argument on top of the stack,
//LUA_TSTRING = 4 see lua.h
if (lua_type(m_lua_State, -1) == 4)
{
value = lua_tostring(m_lua_State, -1);
//pop value from stack
lua_pop(m_lua_State, 1);
//the stack is now 'balanced',
//we got 1 argument, we popped 1 argument
//now return 1 argument based on the result from InCombat(string)
//push the return value on the stack and return 1
Lua.lua_pushboolean(m_lua_State, InCombat(cName));
return 1;
}
else
{
//error, invalid argument or no argument on the Lua stack
//check Lua script for errors and optionally ask Lua for an error message
//clear the stack
lua_settop(m_lua_State, 0);
}
//sigmal no return value, something went wrong
return 0;
}
}
None of our methods match the Lua signature so we have to create Lua ‘trampolines’ for them and handle the Lua communication specifics in them. We register a Lua function callback for the NPCPlayer class like this:
RegisterLuaFunction(new Lua.LuaFunction(Lua_InCombat), "InCombat");
Calling the registered C# method from Lua
The Lua_InCombat method can now be used in a Lua script, Lua will pass the string parameter on its stack structure.
The Lua script could look something like this:
characterName = ‘Winnie the pooh’
winnieFighting = InCombat(characterName)
Its perfectly possible to mess up by calling the Lua function without any parameters or a parameter of a wrong type, there is no type checking, other than the defensive code you can and should implement in your .Net methods.
This concludes the post about registering and using c# methods from Lua.
Hope you found it usefull, please feel free to comment.
Thue Tuxen
Thue Tuxen 