Improve DI system

src/Discord.Net.Commands/ReflectionUtils.cs

@@ -1,4 +1,5 @@
 using System;
+using System.Collections.Generic;
 using System.Linq;
 using System.Reflection;
 
@@ -6,33 +7,39 @@ namespace Discord.Commands
 {
     internal class ReflectionUtils
     {
-        internal static object CreateObject(TypeInfo typeInfo, CommandService commands, IDependencyMap map = null)
+        internal static object CreateObject(TypeInfo typeInfo, CommandService service, IDependencyMap map = null)
         {
             if (typeInfo.DeclaredConstructors.Count() > 1)
                 throw new InvalidOperationException($"Found too many constructors for \"{typeInfo.FullName}\"");
+
             var constructor = typeInfo.DeclaredConstructors.FirstOrDefault();
+
+            if (constructor == null)
+                throw new InvalidOperationException($"Found no constructor for \"{typeInfo.FullName}\"");
+
+            object[] parameters;
             try
             {
-                if (constructor.GetParameters().Length == 0)
-                    return constructor.Invoke(null);
-                else if (constructor.GetParameters().Length > 1)
-                    throw new InvalidOperationException($"Could not find a valid constructor for \"{typeInfo.FullName}\" (Found too many parameters)");
-                var parameter = constructor.GetParameters().FirstOrDefault();
-                if (parameter == null)
-                    throw new InvalidOperationException($"Could not find a valid constructor for \"{typeInfo.FullName}\" (No valid parameters)");
-                if (parameter.ParameterType == typeof(CommandService))
-                    return constructor.Invoke(new object[1] { commands });
-                else if (parameter.ParameterType == typeof(IDependencyMap))
-                {
-                    if (map == null) throw new InvalidOperationException($"The constructor for \"{typeInfo.FullName}\" requires a Dependency Map.");
-                    return constructor.Invoke(new object[1] { map });
-                }
-                else
-                    throw new InvalidOperationException($"Could not find a valid constructor for \"{typeInfo.FullName}\" (Invalid Parameter Type: \"{parameter.ParameterType.FullName}\")");
+                // TODO: probably change this ternary into something sensible
+                parameters = constructor.GetParameters()
+                    .Select(x => x.ParameterType == typeof(CommandService) ? service : map.Get(x.ParameterType)).ToArray();
             }
-            catch (Exception e)
+            catch (KeyNotFoundException ex) // tried to inject an invalid dependency
             {
-                throw new InvalidOperationException($"Could not find a valid constructor for \"{typeInfo.FullName}\" (Error invoking constructor)");
+                throw new InvalidOperationException($"Could not find a valid constructor for \"{typeInfo.FullName}\" (could not provide parameter)", ex);
+            }
+            catch (NullReferenceException ex) // tried to find a dependency
+            {
+                throw new InvalidOperationException($"Could not find a valid constructor for \"{typeInfo.FullName}\" (type requires dependency injection)", ex);
+            }
+
+            try
+            {
+                return constructor.Invoke(parameters);
+            }
+            catch (Exception ex)
+            {
+                throw new InvalidOperationException($"Could not find a valid constructor for \"{typeInfo.FullName}\" (Error invoking constructor)", ex);
             }
         }
     }