Add AV1032: Consider a delegate instead of an interface with a single method (#312)

Author: dennisdoomen

_rules/1032.md

@@ -0,0 +1,39 @@
+---
+rule_id: 1032
+rule_category: class-design
+title: Consider a named delegate instead of an interface with a single method
+severity: 3
+---
+An interface with a single method is essentially a named function signature. Unless you know that this interface will gain more members in the near future, a delegate is simpler, creates a more loosely coupled abstraction, and you don't need mocking frameworks to provide a value in tests. 
+
+```csharp
+// Instead of this single-method interface...
+public interface IOrderFilter
+{
+    bool Matches(Order order);
+}
+
+// ...consider a delegate
+public delegate bool OrderFilter(Order order);
+
+// Or simply use Func<T, bool>
+public void Process(IEnumerable<Order> orders, OrderFilter filter)
+{
+    foreach (var order in orders.Where(filter))
+    {
+        // ...
+    }
+}
+
+// In a test you can pass any method or lambda that matches the signature.
+Process(orders, order => order.Value > 10000);
+```
+
+Prefer a single-method interface when:
+- Implementations need to carry state
+- The interface is supposed to provide a strong well-defined contract like `IEquatable`, `IDisposable`
+- You want to use Default Interface Methods
+- The interface is expected to gain additional members in the near future
+
+**Note:** A simple lambda is technically compatible with a named delegate, but a named delegate offers better discoverability: find-usages, go-to-definition, and DI registration all work the same as with an interface.
+