C# plugin architecture question

Question

I'm working on a system monitoring application similar to Nagios in C#. I have a plugin interface defined as:

public interface IPlugin
{
    PluginResult Execute();
}

Each plugin, depending on its functionality, will have a variable number of arguments. As an example, a ping plugin might take a hostname, # of packets, timeout value, etc. I want the user to be able to define these arguments per service in my user interface, but obviously these arguments won't be known until the application discovers which plugins are available. I'm curious as to how others might design a plugin such that these variable arguments would be discoverable by the application.

Right now, as an example, I've got a ping plugin:

public class PingPlugin : IPlugin
{
    private const string RESULT_MESSAGE = "Average ms: {0}; Packet loss: {1}";

    private string _hostname;
    private int _packets;
    private int _timeout;
    private int _warningTimeThreshold;
    private int _warningLossThreshold;
    private int _errorTimeThreshold;
    private int _errorLossThreshold;

    public PingPlugin(
        string hostname,
        int packets,
        int timeout,
        int warningTimeThreshold,
        int warningLossThreshold,
        int errorTimeThreshold,
        int errorLossThreshold)
    {
        _hostname = hostname;
        _packets = packets;
        _timeout = timeout;
        _warningTimeThreshold = warningTimeThreshold;
        _warningLossThreshold = warningLossThreshold;
        _errorTimeThreshold = errorTimeThreshold;
        _errorLossThreshold = errorLossThreshold;
    }

    public PluginResult Execute()
    {
        // execute the plugin
    }
}

I thought I might be able to discover the constructor parameters using reflection and present the user with a property grid to allow the configuration of the plugin, but I'm not sure the best way to provide a set of default values with this design. What might some alternatives be?

Answer 1

Have you considered looking at the Managed Extensibility Framework?

Answer 2

You can apply the [DefaultValue] attribute to the parameters.

In C# for, you can use new syntax for this: int warningLossThreshold = 30,

Answer 3

I voted +1 for the MEF answer too, it will solve many of your problems.

However, if you want to do it without MEF, it seems to me that you are missing some way to have the plugins tell your application via metadata, about the parameters it require.

One possible design could be this: Have an IPluginProvider interface, which your application can discover. This should have a parameterless constructor, so you can easily new up an instance. It should then have methods that return whatever metadata is needed (such as "pretty names" for the parameters, which are required, what are some sensible defaults, and so on). It should then include CreateInstance method, which takes the actual parameters as IDictionary<string,object> and returns the actual IPlugin instance.

Answer 4

Rather than have a Plugin constructor determine the parameters, you might consider something like this:

public interface IPlugin
{
    PluginResult Execute(Object parameters);
}

public class PingParameters
{
    //Various parameters here, including [Description] and [DisplayName] attributes if you wish
}

public class ParametersTypeAttribute : Attribute
{
    public Type Type { get; private set; }

    public ParametersTypeAttribute(Type type)
    {
        Type = type;
    }
}

[ParametersType(typeof(PingParameters))]
public class PingPlugin : IPlugin
{
    public PluginResult Execute(Object parameters)
    {
        return Execute((PingParameters) parameters);
    }

    private PluginResult Execute(PingParameters parameters)
    {
        //Your execution code here
    }
}

This gives you more flexibility for the parameters, as you can add attributes, provide setter validation and even specify designer/converter integration for the property grid. The property grid hooks up directly to the parameters object.

Answer 5

I haven't looked at the MEF (will do now).

I had a problem almost identical to yours, I solved it with Attributes.
I have a UI which (calls BL which) uses reflection to show all the available "services" (nothing more than appropriately decorated classes).

When the user selects a "service" further attributes drive the UI. The attribute "schema" is fairly straight forward, and allows for any number of parameters with any name. By introducing constants (with the attribute definition) you can standardise common things like "name" so that your services are consistent.

All the data is then stored in a Key-Value pair table.

The great thing about this is that you can just dump new / modified "service" assemblies in teh bin dir - no extra work required. The only dependency is the attribute definitions assembly - so keep this lean.

Source code is at CodePlex if you want to "steal" some :)