Enable interface-wide command registration

README.md

@@ -167,7 +167,11 @@ app.Run();
 You can add (classic) Class-based style commands with the `AddCommands<T>` method.
 
 ```csharp
+// Adds a MyCommand class
 app.AddCommands<MyCommand>();
+
+// Adds all classes inheriting from the ICommands interface
+app.AddCommands<ICommands>();
 ```
 
 #### Public method as a command (Class-based style)

src/Cocona.Core/Builder/CoconaCommandsBuilderExtensions.cs

@@ -144,8 +144,21 @@ public static CommandTypeConventionBuilder AddCommands(this ICoconaCommandsBuild
         ThrowHelper.ThrowIfNull(commandType);
 
         var conventions = new List<Action<ICommandBuilder>>();
-        builder.Add(new TypeCommandDataSource(commandType, conventions, builder.GetFilters().ToArray()));
+        if (!commandType.IsInterface)
+        {
+            builder.Add(new TypeCommandDataSource(commandType, conventions, builder.GetFilters().ToArray()));
+            return new CommandTypeConventionBuilder(conventions);
+        }
+
+        var commandTypes = commandType.Assembly.DefinedTypes
+            .Where(x => x is { IsAbstract: false, IsInterface: false } && commandType.IsAssignableFrom(x))
+            .Select(x => x.AsType());
 
+        foreach (var type in commandTypes)
+        {
+            builder.Add(new TypeCommandDataSource(type, conventions, builder.GetFilters().ToArray()));
+        }
+
         return new CommandTypeConventionBuilder(conventions);
     }
 

test/Cocona.Test/Builder/CoconaCommandBuilderTest.cs

@@ -150,6 +150,27 @@ class MyCommands
     {
         public void Hello() => Console.WriteLine("Hello");
     }
+
+    [Fact]
+    public void AddCommands_Interface()
+    {
+        var builder = new CoconaCommandsBuilder();
+        builder.AddCommands<ICommands>();
+
+        var built = builder.Build();
+        built.Should().HaveCount(2);
+        built[0].Should().BeOfType<TypeCommandData>().Subject.Type.Should().Be<InterfaceCommands>();
+        built[1].Should().BeOfType<TypeCommandData>().Subject.Type.Should().Be<MoreInterfaceCommands>();
+    }
+
+    interface ICommands
+    { }
+
+    class InterfaceCommands : ICommands
+    { }
+
+    class MoreInterfaceCommands : ICommands
+    { }
 
     [Fact]
     public void Filter_Builder_UseFilter()
@@ -175,4 +196,4 @@ class UseFilter_Filter2 : IFilterFactory
     }
     class UseFilter_Filter3 : IFilterMetadata
     { }
-}
+}