Add IDurableEntityContext.DispatchAsync() directly to interface (#1581)

In a similar vein to PR #1422, this PR aims to transfer an extension
method to a direct interface implementation. This allows the method to
be directly mockable via tooling like Moq.

Addresses #1419

Author: Connor McMahon

src/WebJobs.Extensions.DurableTask/ContextImplementations/DurableEntityContext.cs

@@ -4,7 +4,9 @@
 using System;
 using System.Collections.Generic;
 using System.Linq;
+using System.Reflection;
 using System.Runtime.ExceptionServices;
+using System.Threading.Tasks;
 using DurableTask.Core;
 using Microsoft.Azure.WebJobs.Host.Bindings;
 using Newtonsoft.Json;
@@ -415,12 +417,95 @@ string IDurableEntityContext.StartNewOrchestration(string functionName, object i
             return instanceId;
         }
 
+        async Task IDurableEntityContext.DispatchAsync<T>(params object[] constructorParameters)
+        {
+            IDurableEntityContext context = (IDurableEntityContext)this;
+            MethodInfo method = FindMethodForContext<T>(context);
+
+            if (method == null)
+            {
+                // We support a default delete operation even if the interface does not explicitly have a Delete method.
+                if (string.Equals("delete", context.OperationName, StringComparison.InvariantCultureIgnoreCase))
+                {
+                    Entity.Current.DeleteState();
+                    return;
+                }
+                else
+                {
+                    throw new InvalidOperationException($"No operation named '{context.OperationName}' was found.");
+                }
+            }
+
+            // check that the number of arguments is zero or one
+            ParameterInfo[] parameters = method.GetParameters();
+            if (parameters.Length > 1)
+            {
+                throw new InvalidOperationException("Only a single argument can be used for operation input.");
+            }
+
+            object[] args;
+            if (parameters.Length == 1)
+            {
+                // determine the expected type of the operation input and deserialize
+                Type inputType = method.GetParameters()[0].ParameterType;
+                object input = context.GetInput(inputType);
+                args = new object[1] { input };
+            }
+            else
+            {
+                args = Array.Empty<object>();
+            }
+
+#if !FUNCTIONS_V1
+            T Constructor() => (T)context.FunctionBindingContext.CreateObjectInstance(typeof(T), constructorParameters);
+#else
+            T Constructor() => (T)Activator.CreateInstance(typeof(T), constructorParameters);
+#endif
+
+            var state = ((Extensions.DurableTask.DurableEntityContext)context).GetStateWithInjectedDependencies(Constructor);
+
+            object result = method.Invoke(state, args);
+
+            if (method.ReturnType != typeof(void))
+            {
+                if (result is Task task)
+                {
+                    await task;
+
+                    if (task.GetType().IsGenericType)
+                    {
+                        context.Return(task.GetType().GetProperty("Result").GetValue(task));
+                    }
+                }
+                else
+                {
+                    context.Return(result);
+                }
+            }
+        }
+
         void IDurableEntityContext.Return(object result)
         {
             this.ThrowIfInvalidAccess();
             this.CurrentOperationResponse.SetResult(result, this.messageDataConverter);
         }
 
+        internal static MethodInfo FindMethodForContext<T>(IDurableEntityContext context)
+        {
+            var type = typeof(T);
+
+            var interfaces = type.GetInterfaces();
+            const BindingFlags bindingFlags = BindingFlags.IgnoreCase | BindingFlags.Public | BindingFlags.NonPublic | BindingFlags.Instance;
+
+            var method = type.GetMethod(context.OperationName, bindingFlags);
+            if (interfaces.Length == 0 || method != null)
+            {
+                return method;
+            }
+
+            return interfaces.Select(i => i.GetMethod(context.OperationName, bindingFlags)).FirstOrDefault(m => m != null);
+        }
+
         private void ThrowIfInvalidAccess()
         {
             if (this.CurrentOperation == null)

src/WebJobs.Extensions.DurableTask/ContextInterfaces/IDurableEntityContext.cs

@@ -2,6 +2,8 @@
 // Licensed under the MIT License. See LICENSE in the project root for license information.
 
 using System;
+using System.Reflection;
+using System.Threading.Tasks;
 using Microsoft.Azure.WebJobs.Host.Bindings;
 
 namespace Microsoft.Azure.WebJobs.Extensions.DurableTask
@@ -169,5 +171,25 @@ public interface IDurableEntityContext
         /// </exception>
         /// <returns>The instance id of the new orchestration.</returns>
         string StartNewOrchestration(string functionName, object input, string instanceId = null);
+
+        /// <summary>
+        /// Dynamically dispatches the incoming entity operation using reflection.
+        /// </summary>
+        /// <typeparam name="T">The class to use for entity instances.</typeparam>
+        /// <returns>A task that completes when the dispatched operation has finished.</returns>
+        /// <exception cref="AmbiguousMatchException">If there is more than one method with the given operation name.</exception>
+        /// <exception cref="MissingMethodException">If there is no method with the given operation name.</exception>
+        /// <exception cref="InvalidOperationException">If the method has more than one argument.</exception>
+        /// <remarks>
+        /// If the entity's state is null, an object of type <typeparamref name="T"/> is created first. Then, reflection
+        /// is used to try to find a matching method. This match is based on the method name
+        /// (which is the operation name) and the argument list (which is the operation content, deserialized into
+        /// an object array).
+        /// </remarks>
+        /// <param name="constructorParameters">Parameters to feed to the entity constructor. Should be primarily used for
+        /// output bindings. Parameters must match the order in the constructor after ignoring parameters populated on
+        /// constructor via dependency injection.</param>
+        Task DispatchAsync<T>(params object[] constructorParameters)
+            where T : class;
     }
 }