Using a generic command-line runner for utility tasks

23 Nov 2011

Most enterprise projects have one or more console applications for utility tasks such as cleaning up or importing data into the database. These utilities tend to be project-specific and small in terms of code size, and instead of several smaller assemblies, it makes sense to combine these into a single assembly. The generic runner would read the utility, called the command, and arguments from the command-line and use the command pattern to create and execute it.

For the generic runner to work, each command has to fulfill the contract.

public enum ExitCode {
    Success = 0,
    Failure
};

public interface ICommand {
    string Usage { get; }
    string Description { get; }
    ExitCode Execute(string[] args);
}

I want the runner to adhere to the open/closed principle. For its behavior to be modified without altering its core delegation logic. This requires the use of reflection to retrieve and instantiate a command based on command-line arguments.

class Program {
    static IEnumerable<ICommand> GetCommands() {
        var iCommand = typeof (ICommand);
        return System.Reflection.Assembly.GetExecutingAssembly().GetTypes().ToList()
            .Where(t => iCommand.IsAssignableFrom(t) && t != iCommand)
            .Select(t => Activator.CreateInstance(t) as ICommand);
    }

    static void DisplayHelp() {
        Console.WriteLine("Console [Command] [Arg1] [Arg2] [ArgN]\n\n");
        GetCommands().ToList().ForEach(command => 
            Console.WriteLine(command.Usage + "\n" + command.Description + "\n\n"));
    }

    static int Main(string[] args) {
        if (args.Length == 0) {
            DisplayHelp();
            return (int)ExitCode.Failure;
        }

        var commandName = args[0];
        var command = GetCommands().SingleOrDefault(t => t.GetType().Name == commandName);
        if (command == null)
            throw new ArgumentException(string.Format("Command '{0}' not found", commandName));

        var executeArguments = new List<string>(args);
        executeArguments.RemoveAt(0);

        var exitCode = command.Execute(executeArguments.ToArray());
        return (int)exitCode;
    }
}

A trivial example of a command that adds two numbers would be the following:

// $> GenericRunner.exe Calculator 2 3 => 2 + 3 = 5
public class Calculator : ICommand {
    public string Usage {
        get { return "Calculator [Op1] [Op2]"; }
    }

    public string Description {
        get { return "World's simplest calculator"; }
    }
        
    public ExitCode Execute(string[] args) {
        try {
            Console.WriteLine(              
                "{0} + {1} = {2}",
                args[0], args[1], int.Parse(args[0]) + int.Parse(args[1]));
            return ExitCode.Success;
        } catch (Exception e) {
            Console.WriteLine(e.ToString());
            return ExitCode.Failure;
        }
    }
}

Now multiple smaller assemblies can be grouped into one, with a description of all commands automatically being assembled, and without commands interfering (too much) with each other.

Have comments or questions? Please drop me an email or tweet to @ronnieholm.