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Lua-CSharp

High performance Lua interpreter implemented in C# for .NET and Unity

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Overview

Lua-CSharp is a library that provides a Lua interpreter implemented in C#. By integrating Lua-CSharp, you can easily embed Lua scripts into your .NET applications.

Lua-CSharp leverages the latest C# features, designed with low allocation and high performance in mind. It is optimized to deliver maximum performance when used for interoperation between C# and Lua in C# applications. Below is a benchmark comparison with MoonSharp and NLua:

MoonSharp generally provides good speed but incurs significant allocations due to its design. NLua, being a C-binding implementation, is fast, but introduces substantial overhead when interacting with the C# layer. Lua-CSharp, fully implemented in C#, allows for seamless interaction with C# code without additional overhead. Moreover, it operates reliably in AOT environments since it does not rely on IL generation.

Features

• Lua 5.2 interpreter implemented in C#
• Easy-to-use API integrated with async/await
• Support for exception handling with try-catch
• High-performance implementation utilizing modern C#
• Unity support (works with both Mono and IL2CPP)

Installation

NuGet packages

To use Lua-CSharp, .NET Standard 2.1 or higher is required. The package can be obtained from NuGet.

.NET CLI

dotnet add package LuaCSharp

Package Manager

Install-Package LuaCSharp

Unity

Lua-CSharp can also be used in Unity (works with both Mono and IL2CPP).

Requirements

• Unity 2021.3 or higher

Installation

1. Install NugetForUnity.
2. Open the NuGet window by going to NuGet > Manage NuGet Packages, search for the LuaCSharp package, and install it.

Quick Start

By using the LuaState class, you can execute Lua scripts from C#. Below is a sample code that evaluates a simple calculation written in Lua.

using Lua;

// Create a LuaState
var state = LuaState.Create();

// Execute a Lua script string with DoStringAsync
var results = await state.DoStringAsync("return 1 + 1");

// 2
Console.WriteLine(results[0]);

[!WARNING] LuaState is not thread-safe. Do not access it from multiple threads simultaneously.

LuaValue

Values in Lua scripts are represented by the LuaValue type. The value of a LuaValue can be read using TryRead<T>(out T value) or Read<T>().

var results = await state.DoStringAsync("return 1 + 1");

// double
var value = results[0].Read<double>();

You can also get the type of the value from the Type property.

var isNil = results[0].Type == LuaValueType.Nil;

Below is a table showing the type mapping between Lua and C#.

When creating a LuaValue from the C# side, compatible types are implicitly converted into LuaValue.

LuaValue value;
value = 1.2;           // double   ->  LuaValue
value = "foo";         // string   ->  LuaValue
value = new LuaTable() // LuaTable ->  LuaValue

LuaTable

Lua tables are represented by the LuaTable type. They can be used similarly to LuaValue[] or Dictionary<LuaValue, LuaValue>.

// Create a table in Lua
var results = await state.DoStringAsync("return { a = 1, b = 2, c = 3 }");
var table1 = results[0].Read<LuaTable>();

// 1
Console.WriteLine(table1["a"]);

// Create a table in C#
results = await state.DoStringAsync("return { 1, 2, 3 }");
var table2 = results[0].Read<LuaTable>();

// 1 (Note: Lua arrays are 1-indexed)
Console.WriteLine(table2[1]);

Global Environment

You can access Lua's global environment through state.Environment. This table allows for easy value exchange between Lua and C#.

// Set a = 10
state.Environment["a"] = 10;

var results = await state.DoStringAsync("return a");

// 10
Console.WriteLine(results[0]);

Standard Libraries

You can use Lua's standard libraries as well. By calling state.OpenStandardLibraries(), the standard library tables are added to the LuaState.

using Lua;
using Lua.Standard;

var state = LuaState.Create();

// Add standard libraries
state.OpenStandardLibraries();

var results = await state.DoStringAsync("return math.pi");
Console.WriteLine(results[0]); // 3.141592653589793

For more details on standard libraries, refer to the Lua official manual.

[!WARNING] Lua-CSharp does not support all functions of the standard libraries. For details, refer to the Compatibility section.

Functions

Lua functions are represented by the LuaFunction type. With LuaFunction, you can call Lua functions from C#, or define functions in C# that can be called from Lua.

Calling Lua Functions from C#

-- lua2cs.lua

local function add(a, b)
    return a + b
end

return add;

var results = await state.DoFileAsync("lua2cs.lua");
var func = results[0].Read<LuaFunction>();

// Execute the function with arguments
var funcResults = await func.InvokeAsync(state, new[] { 1, 2 });

// 3
Console.WriteLine(funcResults[0]);

Calling C# Functions from Lua

It is possible to create a LuaFunction from a lambda expression.

// Add the function to the global environment
state.Environment["add"] = new LuaFunction((context, buffer, ct) =>
{
    // Get the arguments using context.GetArgument<T>()
    var arg0 = context.GetArgument<double>(0);
    var arg1 = context.GetArgument<double>(1);

    // Write the return value to the buffer
    buffer.Span[0] = arg0 + arg1;

    // Return the number of values
    return new(1);
});

// Execute a Lua script
var results = await state.DoFileAsync("cs2lua.lua");

// 3
Console.WriteLine(results[i]);

-- cs2lua.lua

return add(1, 2)

[!TIP]
Defining functions with LuaFunction can be somewhat verbose. When adding multiple functions, it is recommended to use the Source Generator with the [LuaObject] attribute. For more details, see the LuaObject section.

Integration with async/await

LuaFunction operates asynchronously. Therefore, you can define a function that waits for an operation in Lua, such as the example below:

// Define a function that waits for the given number of seconds using Task.Delay
state.Environment["wait"] = new LuaFunction(async (context, buffer, ct) =>
{
    var sec = context.GetArgument<double>(0);
    await Task.Delay(TimeSpan.FromSeconds(sec));
    return 0;
});

await state.DoFileAsync("sample.lua");

-- sample.lua

print "hello!"

wait(1.0) -- wait 1 sec

print "how are you?"

wait(1.0) -- wait 1 sec

print "goodbye!"

This code can resume the execution of the Lua script after waiting with await, as shown in the following figure. This is very useful when writing scripts to be incorporated into games.

Coroutines

Lua coroutines are represented by the LuaThread type.

Coroutines can not only be used within Lua scripts, but you can also await Lua-created coroutines from C#.

-- coroutine.lua

local co = coroutine.create(function()
    for i = 1, 10 do
        print(i)
        coroutine.yield()
    end
end)

return co

var results = await state.DoFileAsync("coroutine.lua");
var co = results[0].Read<LuaThread>();

for (int i = 0; i < 10; i++)
{
    var resumeResults = await co.ResumeAsync(state);

    // Similar to coroutine.resume(), returns true on success and the return values afterward
    // 1, 2, 3, 4, ...
    Console.WriteLine(resumeResults[1]);
}

LuaObject

By applying the [LuaObject] attribute, you can create custom classes that run within Lua. Adding this attribute to a class that you wish to use in Lua allows the Source Generator to automatically generate the code required for interaction from Lua.

The following is an example implementation of a wrapper class for System.Numerics.Vector3 that can be used in Lua:

using System.Numerics;
using Lua;

var state = LuaState.Create();

// Add an instance of the defined LuaObject as a global variable
// (Implicit conversion to LuaValue is automatically defined for classes with the LuaObject attribute)
state.Environment["Vector3"] = new LuaVector3();

await state.DoFileAsync("vector3_sample.lua");

// Add LuaObject attribute and partial keyword
[LuaObject]
public partial class LuaVector3
{
    Vector3 vector;

    // Add LuaMember attribute to members that will be used in Lua
    // The argument specifies the name used in Lua (if omitted, the member name is used)
    [LuaMember("x")]
    public float X
    {
        get => vector.X;
        set => vector = vector with { X = value };
    }

    [LuaMember("y")]
    public float Y
    {
        get => vector.Y;
        set => vector = vector with { Y = value };
    }

    [LuaMember("z")]
    public float Z
    {
        get => vector.Z;
        set => vector = vector with { Z = value };
    }

    // Static methods are treated as regular Lua functions
    [LuaMember("create")]
    public static LuaVector3 Create(float x, float y, float z)
    {
        return new LuaVector3()
        {
            vector = new Vector3(x, y, z)
        };
    }

    // Instance methods implicitly receive the instance (this) as the first argument
    // In Lua, this is accessed with instance:method() syntax
    [LuaMember("normalized")]
    public LuaVector3 Normalized()
    {
        return new LuaVector3()
        {
            vector = Vector3.Normalize(vector)
        };
    }
}

-- vector3_sample.lua

local v1 = Vector3.create(1, 2, 3)
-- 1  2  3
print(v1.x, v1.y, v1.z)

local v2 = v1:normalized()
-- 0.26726123690605164  0.5345224738121033  0.8017836809158325
print(v2.x, v2.y, v2.z)

The types of fields/properties with the [LuaMember] attribute, as well as the argument and return types of methods, must be either LuaValue or convertible to/from LuaValue.

Return types such as void, Task/Task<T>, ValueTask/ValueTask<T>, UniTask/UniTask<T>, and Awaitable/Awaitable<T> are also supported.

If the type is not supported, the Source Generator will output a compile-time error.

LuaMetamethod

By adding the [LuaMetamethod] attribute, you can designate a C# method to be used as a Lua metamethod.

Here is an example that adds the __add, __sub, and __tostring metamethods to the LuaVector3 class:

[LuaObject]
public partial class LuaVector3
{
    // The previous implementation is omitted

    [LuaMetamethod(LuaObjectMetamethod.Add)]
    public static LuaVector3 Add(LuaVector3 a, LuaVector3 b)
    {
        return new LuaVector3()
        {
            vector = a.vector + b.vector
        };
    }
    
    [LuaMetamethod(LuaObjectMetamethod.Sub)]
    public static LuaVector3 Sub(LuaVector3 a, LuaVector3 b)
    {
        return new LuaVector3()
        {
            vector = a.vector - b.vector
        };
    }

    [LuaMetamethod(LuaObjectMetamethod.ToString)]
    public override string ToString()
    {
        return vector.ToString();
    }
}

local v1 = Vector3.create(1, 1, 1)
local v2 = Vector3.create(2, 2, 2)

print(v1) -- <1, 1, 1>
print(v2) -- <2, 2, 2>

print(v1 + v2) -- <3, 3, 3>
print(v1 - v2) -- <-1, -1, -1>

[!NOTE]
__index and __newindex cannot be set as they are used internally by the code generated by [LuaObject].

Module Loading

In Lua, you can load modules using the require function. In regular Lua, modules are managed by searchers within the package.searchers function list. In Lua-CSharp, this is replaced by the ILuaModuleLoader interface.

public interface ILuaModuleLoader
{
    bool Exists(string moduleName);
    ValueTask<LuaModule> LoadAsync(string moduleName, CancellationToken cancellationToken = default);
}

You can set the LuaState.ModuleLoader to change how modules are loaded. By default, the FileModuleLoader is set to load modules from Lua files.

You can also combine multiple loaders using CompositeModuleLoader.Create(loader1, loader2, ...).

state.ModuleLoader = CompositeModuleLoader.Create(
    new FileModuleLoader(),
    new CustomModuleLoader()
);

Loaded modules are cached in the package.loaded table, just like regular Lua. This can be accessed via LuaState.LoadedModules.

Exception Handling

Lua script parsing errors and runtime exceptions throw exceptions that inherit from LuaException. You can catch these to handle errors during execution.

try
{
    await state.DoFileAsync("filename.lua");
}
catch (LuaParseException)
{
    // Handle parsing errors
}
catch (LuaRuntimeException)
{
    // Handle runtime exceptions
}

Compatibility

Lua-CSharp is designed with integration into .NET in mind, so there are several differences from the C implementation.

Binary

Lua-CSharp does not support Lua bytecode (tools like luac cannot be used). Only Lua source code can be executed.

Character Encoding

The character encoding used in Lua-CSharp is UTF-16. Since standard Lua assumes a single-byte character encoding, string behavior differs significantly.

For example, in regular Lua, the following code outputs 15, but in Lua-CSharp, it outputs 5.

local l = string.len("あいうえお")
print(l)

All string library functions handle strings as UTF-16.

Garbage Collection

Since Lua-CSharp is implemented in C#, it relies on .NET's garbage collector. As a result, memory management behavior differs from regular Lua.

While collectgarbage() is available, it simply calls the corresponding .NET garbage collection method and may not exhibit the same behavior as C's Lua garbage collector.

License

Lua-CSharp is licensed under the MIT License.