mono下c#和c交互，字符串处理

起因是ulua里，从luajit读字符串到c#里，做了编码转换，如下

public static string lua_tostring(IntPtr luaState, int index)
{
var kind = lua_type(luaState, index);
if (kind == LuaTypes.LUA_TNUMBER)
{
return lua_tonumber(luaState, index).ToString();
}
else if (kind == LuaTypes.LUA_TNIL || kind == LuaTypes.LUA_TNONE)
return "nil";
int strlen;

IntPtr str = lua_tolstring(luaState, index, out strlen);
if (str != IntPtr.Zero)
{
return Marshal.PtrToStringAnsi(str, strlen);
}
else
{
return null;
}
}

但是从c#到luajit里，没有做转换

[DllImport(LUADLL,CallingConvention=CallingConvention.Cdecl)]
public static extern void lua_pushstring(IntPtr luaState, string str);

原因是mono自动做了转换，从utf16到utf8