Finish implementation of command builders

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

@@ -0,0 +1,54 @@
+using System;
+using System.Linq;
+using System.Reflection;
+
+namespace Discord.Commands
+{
+    internal class ReflectionUtils
+    {
+        internal static T CreateObject<T>(TypeInfo typeInfo, CommandService service, IDependencyMap map = null)
+            => CreateBuilder<T>(typeInfo, service)(map);
+
+        internal static Func<IDependencyMap, T> CreateBuilder<T>(TypeInfo typeInfo, CommandService service)
+        {
+            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];
+            System.Reflection.ParameterInfo[] parameters = constructor.GetParameters();
+
+            return (map) =>
+            {
+                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($"Failed to create \"{typeInfo.FullName}\", dependency \"{parameter.ParameterType.Name}\" was not found.");
+                    }
+                    args[i] = arg;
+                }
+
+                try
+                {
+                    return (T)constructor.Invoke(args);
+                }
+                catch (Exception ex)
+                {
+                    throw new Exception($"Failed to create \"{typeInfo.FullName}\"", ex);
+                }
+            };
+        }
+    }
+}