Introduce ICommand

CliFx/Command.cs

@@ -4,7 +4,7 @@ using CliFx.Models;
 
 namespace CliFx
 {
-    public abstract class Command
+    public abstract class Command : ICommand
     {
         public virtual ExitCode Execute() => throw new InvalidOperationException(
             "Can't execute command because its execution method is not defined. " +

CliFx/ICommand.cs

@@ -0,0 +1,10 @@
+using System.Threading.Tasks;
+using CliFx.Models;
+
+namespace CliFx
+{
+    public interface ICommand
+    {
+        Task<ExitCode> ExecuteAsync();
+    }
+}

CliFx/Internal/CommandType.cs

@@ -33,9 +33,7 @@ namespace CliFx.Internal
     internal partial class CommandType
     {
         public static bool IsValid(Type type) =>
-            // Derives from Command
+            type.GetInterfaces().Contains(typeof(ICommand)) &&
-            type.IsDerivedFrom(typeof(Command)) &&
-            // Marked with DefaultCommandAttribute or CommandAttribute
             type.IsDefined(typeof(CommandAttribute));
 
         public static CommandType Initialize(Type type)

CliFx/Services/CommandResolver.cs

@@ -74,7 +74,7 @@ namespace CliFx.Services
                 $"Apply {nameof(CommandAttribute)} to give command a name.");
         }
 
-        public Command ResolveCommand(CommandOptionSet optionSet)
+        public ICommand ResolveCommand(CommandOptionSet optionSet)
         {
             // Get command type
             var commandType = !optionSet.CommandName.IsNullOrWhiteSpace()

CliFx/Services/ICommandResolver.cs

@@ -4,6 +4,6 @@ namespace CliFx.Services
 {
     public interface ICommandResolver
     {
-        Command ResolveCommand(CommandOptionSet optionSet);
+        ICommand ResolveCommand(CommandOptionSet optionSet);
     }
 }