Allow call from native contract

src/neo/SmartContract/ApplicationEngine.cs

@@ -17,6 +17,12 @@ namespace Neo.SmartContract
 {
     public partial class ApplicationEngine : ExecutionEngine
     {
+        private class InvocationState
+        {
+            public Type ReturnType;
+            public Delegate Callback;
+        }
+
         public static event EventHandler<NotifyEventArgs> Notify;
         public static event EventHandler<LogEventArgs> Log;
 
@@ -28,6 +34,7 @@ public partial class ApplicationEngine : ExecutionEngine
         private readonly List<NotifyEventArgs> notifications = new List<NotifyEventArgs>();
         private readonly List<IDisposable> disposables = new List<IDisposable>();
         private readonly Dictionary<UInt160, int> invocationCounter = new Dictionary<UInt160, int>();
+        private readonly Dictionary<ExecutionContext, InvocationState> invocationStates = new Dictionary<ExecutionContext, InvocationState>();
 
         public static IEnumerable<InteropDescriptor> Services => services.Values;
         public TriggerType Trigger { get; }
@@ -55,6 +62,38 @@ internal bool AddGas(long gas)
             return testMode || GasConsumed <= gas_amount;
         }
 
+        internal void CallFromNativeContract(Action onComplete, UInt160 hash, string method, params StackItem[] args)
+        {
+            invocationStates.Add(CurrentContext, new InvocationState
+            {
+                ReturnType = typeof(void),
+                Callback = onComplete
+            });
+            CallContract(hash, method, new VMArray(args));
+        }
+
+        internal void CallFromNativeContract<T>(Action<T> onComplete, UInt160 hash, string method, params StackItem[] args)
+        {
+            invocationStates.Add(CurrentContext, new InvocationState
+            {
+                ReturnType = typeof(T),
+                Callback = onComplete
+            });
+            CallContract(hash, method, new VMArray(args));
+        }
+
+        protected override void ContextUnloaded(ExecutionContext context)
+        {
+            base.ContextUnloaded(context);
+            if (!(UncaughtException is null)) return;
+            if (invocationStates.Count == 0) return;
+            if (!invocationStates.TryGetValue(CurrentContext, out InvocationState state)) return;
+            if (state.Callback is Action action)
+                action();
+            else
+                state.Callback.DynamicInvoke(Convert(Pop(), new InteropParameterDescriptor(state.ReturnType)));
+        }
+
         protected override void LoadContext(ExecutionContext context)
         {
             // Set default execution context state

src/neo/SmartContract/InteropParameterDescriptor.cs

@@ -39,20 +39,25 @@ internal class InteropParameterDescriptor
         };
 
         public InteropParameterDescriptor(ParameterInfo parameterInfo)
+            : this(parameterInfo.ParameterType)
         {
-            Name = parameterInfo.Name;
-            Type = parameterInfo.ParameterType;
+            this.Name = parameterInfo.Name;
+        }
+
+        public InteropParameterDescriptor(Type type)
+        {
+            this.Type = type;
             if (IsEnum)
             {
-                Converter = converters[Type.GetEnumUnderlyingType()];
+                Converter = converters[type.GetEnumUnderlyingType()];
             }
             else if (IsArray)
             {
-                Converter = converters[Type.GetElementType()];
+                Converter = converters[type.GetElementType()];
             }
             else
             {
-                IsInterface = !converters.TryGetValue(Type, out var converter);
+                IsInterface = !converters.TryGetValue(type, out var converter);
                 if (IsInterface)
                     Converter = converters[typeof(InteropInterface)];
                 else