How to use code generation to dynamically create C# methods?

Question

In order to define a method in C that is callable by Lua it has to match a given signature and use the Lua API to retrieve parameters and return results. I'm writing a C# wrapper of Lua and I'm interested in being able to call arbitrary C# methods without making them follow these conventions. When wrapping in something like D, one might use the template system to dynamically create this glue code for any given method. I was thinking this might be possible as well in C#, but by using dynamic code generation.

The C API looks something like this, and the generated code would manipulate this through a lower level part of my library which P/Invokes the Lua C library.

static int foo (lua_State *L)
{
    int n = lua_gettop(L);    /* number of arguments */
    lua_Number sum = 0;
    int i;
    for (i = 1; i <= n; i++)
    {
        if (!lua_isnumber(L, i)) 
        {
            lua_pushstring(L, "incorrect argument");
            lua_error(L);
        }
        sum += lua_tonumber(L, i);
    }
    lua_pushnumber(L, sum/n);        /* first result */
    lua_pushnumber(L, sum);         /* second result */
    return 2;                   /* number of results */
}

So basically the idea is to take a C# method, reflect its parameters and return values, generate (or retrieve from cache) a method that uses the Lua API like above to pass those parameters and return those return types and finally push that method to Lua. So when C# function is called from Lua it looks something like lua -> magic wrapper function -> ordinary C# function.

Thanks.

Answer 1

If I understand what you want, it seems you have 2 options:

1. use the CodeDOM to generate and dynamically compile code, at runtime.
2. emit actual C# source code, and dynamically compile it into a callable assembly at runtime.

CodeDom is sort of hairy, very low-level code to write. The idea is there's an object model for the C# language. You start by instantiating a CodeTypeDeclaration - this will generate a type or class. Then you add properties and fields - here you would likely add DllImport declarations for your p/invoke functions. Then you use different CodeDOM add methods to the type - this would be where you'd insert the generated method. You could make it public, static, whatever you like.

CodeDOM looks like this:

System.Type mt= a[0].GetType();

System.CodeDom.CodeTypeDeclaration class1 = new System.CodeDom.CodeTypeDeclaration(mt.Name);
class1.IsClass=true;
class1.TypeAttributes = System.Reflection.TypeAttributes.Public;
class1.Comments.Add(new System.CodeDom.CodeCommentStatement("Wrapper class for " + mt.Name));

System.CodeDom.CodeConstructor ctor;
ctor= new System.CodeDom.CodeConstructor();
ctor.Attributes = System.CodeDom.MemberAttributes.Public;
ctor.Comments.Add(new System.CodeDom.CodeCommentStatement("the null constructor"));
class1.Members.Add(ctor);
ctor.Statements.Add(new System.CodeDom.CodeAssignStatement(new System.CodeDom.CodeVariableReferenceExpression("m_wrapped"), new System.CodeDom.CodeObjectCreateExpression(mt)));

ctor= new System.CodeDom.CodeConstructor();
ctor.Attributes = System.CodeDom.MemberAttributes.Public;
ctor.Comments.Add(new System.CodeDom.CodeCommentStatement("the 'copy' constructor"));
class1.Members.Add(ctor);
ctor.Parameters.Add(new System.CodeDom.CodeParameterDeclarationExpression(mt,"X"));
ctor.Statements.Add(new System.CodeDom.CodeAssignStatement(new System.CodeDom.CodeVariableReferenceExpression("m_wrapped"), new System.CodeDom.CodeVariableReferenceExpression("X")));

// embed a local (private) copy of the wrapped type
System.CodeDom.CodeMemberField field1;
field1= new System.CodeDom.CodeMemberField();
field1.Attributes = System.CodeDom.MemberAttributes.Private;
field1.Name= "m_wrapped";
field1.Type=new System.CodeDom.CodeTypeReference(mt);
class1.Members.Add(field1);

...

it goes on. and on. As you can see, it gets pretty ugly. Then later you compile it, which I did not show. I'm assuming you're not gonna want to take this approach.

---

I found CodeDom to be pretty crufty to use; instead, now when I need dynamically-generated assemblies, I will emit actual C# code, normally via templates, into a string in memory, and compile that. It's much simpler for my purposes. The compilation looks like this:

var cp = new System.CodeDom.Compiler.CompilerParameters {
  ReferencedAssemblies.Add(filesystemLocation), // like /R: option on csc.exe
  GenerateInMemory = true,    // you will get a System.Reflection.Assembly back
  GenerateExecutable = false, // Dll
  IncludeDebugInformation = false,
  CompilerOptions = ""
};

var csharp = new Microsoft.CSharp.CSharpCodeProvider();

// this actually runs csc.exe:
System.CodeDom.Compiler.CompilerResults cr = 
      csharp.CompileAssemblyFromSource(cp, LiteralSource);


// cr.Output contains the output from the command

if (cr.Errors.Count != 0)
{
    // handle errors
}

System.Reflection.Assembly a = cr.CompiledAssembly;

// party on the type here, either via reflection...
System.Type t = a.GetType("TheDynamicallyGeneratedType");

// or via a wellknown interface

In the above code, LiteralSource contains the source code to be compiled. As I said, I generate this by reading a template and filling in the blanks.

Answer 2

I'm not sure that I'm interpreting your question correctly but you might want to have a look at Castle.Dynamic proxy. It allows you to create proxies for classes and interfaces and then intercept certain method calls (anything at all on an interface and anything virtual on a real class). When you intercept the call you can just look at the arguments and forward the call to the lua API via P-Invoke. There's a great tutorial here.