## Why is this an issue?

The recommended way to access Azure Durable Entities is through generated proxy objects with the help of interfaces.

The following restrictions, during interface design, are enforced:

- Entity interfaces must be defined in the same assembly as the entity class. This is not detected by the rule.
- Entity interfaces must only define methods.
- Entity interfaces must not contain generic parameters.
- Entity interface methods must not have more than one parameter.
- Entity interface methods must return void, Task, or Task<T>.

If any of these rules are violated, an `InvalidOperationException` is thrown at runtime when the interface is used as a type argument to `IDurableEntityContext.SignalEntity<TEntityInterface>`, `IDurableEntityClient.SignalEntityAsync<TEntityInterface>` or `IDurableOrchestrationContext.CreateEntityProxy<TEntityInterface>`. The exception message explains which rule was broken.

This rule raises an issue in case any of the restrictions above is not respected.

### Noncompliant code example

    namespace Foo // Noncompliant, must be defined in the same assembly as the entity class that implements it
    {
        public interface ICounter<T> // Noncompliant, interfaces cannot contain generic parameters
        {
            string Name { get; set; } // Noncompliant, interface must only define methods
            void Add(int amount, int secondParameter); // Noncompliant, methods must not have more than one parameter
            int Get(); // Noncompliant, methods must return void, Task, or Task<T>
        }
    }

    namespace Bar
    {
        public class Counter : ICounter
        {
            // do stuff
        }

        public static class AddToCounterFromQueue
        {
            [FunctionName("AddToCounterFromQueue")]
            public static Task Run(
                [QueueTrigger("durable-function-trigger")] string input,
                [DurableClient] IDurableEntityClient client)
            {
                var entityId = new EntityId("Counter", "myCounter");
                int amount = int.Parse(input);
                return client.SignalEntityAsync<ICounter>(entityId, proxy => proxy.Add(amount, 10));
            }
        }
    }

### Compliant solution

    namespace Bar
    {
        public interface ICounter
        {
            void Add(int amount);
            Task<int> Get();
        }
    }

    namespace Bar
    {
        public class Counter : ICounter
        {
            // do stuff
        }

        public static class AddToCounterFromQueue
        {
            [FunctionName("AddToCounterFromQueue")]
            public static Task Run(
                [QueueTrigger("durable-function-trigger")] string input,
                [DurableClient] IDurableEntityClient client)
            {
                var entityId = new EntityId("Counter", "myCounter");
                int amount = int.Parse(input);
                return client.SignalEntityAsync<ICounter>(entityId, proxy => proxy.Add(amount));
            }
        }
    }

## Resources

- Restrictions on Entity Interfaces
- Durable Entities
- IDurableEntityContext.SignalEntity
- IDurableEntityClient.SignalEntityAsync
- IDurableOrchestrationContext.CreateEntity