Discord.Net/src/Discord.Net.Commands/ReflectionUtils.cs

using System;
using System.Linq;
using System.Reflection;

namespace Discord.Commands
{
    internal class ReflectionUtils
    {
        internal static object CreateObject(TypeInfo typeInfo, CommandService service, IDependencyMap map = null)
        {
            var constructors = typeInfo.DeclaredConstructors.Where(x => !x.IsStatic).ToArray();
            if (constructors.Length == 0)
                throw new InvalidOperationException($"No constructor found for \"{typeInfo.FullName}\"");
            else if (constructors.Length > 1)
                throw new InvalidOperationException($"Multiple constructors found for \"{typeInfo.FullName}\"");

            var constructor = constructors[0];            
            ParameterInfo[] parameters = constructor.GetParameters();
            object[] args = new object[parameters.Length];

            for (int i = 0; i < parameters.Length; i++)
            {
                var parameter = parameters[i];
                object arg;
                if (map == null || !map.TryGet(parameter.ParameterType, out arg))
                {
                    if (parameter.ParameterType == typeof(CommandService))
                        arg = service;
                    else if (parameter.ParameterType == typeof(IDependencyMap))
                        arg = map;
                    else
                        throw new InvalidOperationException($"Failed to create \"{typeInfo.FullName}\", dependency \"{parameter.ParameterType.Name}\" was not found.");
                }
                args[i] = arg;
            }

            try
            {
                return constructor.Invoke(args);
            }
            catch (Exception ex)
            {
                throw new Exception($"Failed to create \"{typeInfo.FullName}\"", ex);
            }
        }
    }
}