.Net: New Feature: Add support for adding [KernelFunction] attribute to an interface methods

[ruslany]

---

name: Add support for adding [KernelFunction] attribute to an interface methods
about: Need this for testing the AI without invoking the actual plugin code

For testing and prompt engineering purposes I want to define a plugin as an interface and then use a mock class to test the AI prompts and interaction. However it is not possible to add this attribute to interface methods. I am getting an error "Unhandled exception. System.ArgumentException: The sklearn.SubscriptionPlugin instance doesn't implement any [KernelFunction]-attributed methods."

Example code that does not work:

using Microsoft.SemanticKernel;
using System.ComponentModel;

namespace sklearn
{
    [Description("The id and display name of an Azure subscription")]
    public sealed record SubscriptionDescriptor(
        string Id,
        string DisplayName);

    internal interface ISubscriptionPlugin
    {
        [KernelFunction("list_subscriptions")]
        [Description("Lists all Azure subscriptions that user has access to")]
        public Task<IReadOnlyList<SubscriptionDescriptor>> ListSubscriptionsAsync();

        [KernelFunction("get_subscription")]
        [Description("Gets the details of a specific Azure subscription id")]
        public Task<SubscriptionDescriptor?> GetSubscriptionAsync(string id);
    }

    public class SubscriptionPlugin : ISubscriptionPlugin
    {
        private readonly List<SubscriptionDescriptor> _subscriptions = new()
        {
            new SubscriptionDescriptor("31cad2f7-1ec4-4491-b69f-d0360ec2b723", "Subscription 1"),
            new SubscriptionDescriptor("7f07aade-c4d8-416c-a3d9-06c76acacf71", "Subscription 2"),
            new SubscriptionDescriptor("62b38d30-c1ab-4f92-b26f-9fb4c85a01a2", "Subscription 3")
        };

        public Task<IReadOnlyList<SubscriptionDescriptor>> ListSubscriptionsAsync()
        {
            return Task.FromResult<IReadOnlyList<SubscriptionDescriptor>>(_subscriptions);
        }

        public Task<SubscriptionDescriptor?> GetSubscriptionAsync(string id)
        {
            return Task.FromResult(_subscriptions.FirstOrDefault(subscription => subscription.Id == id));
        }
    }
}

[ruslany]

Worked around by using this approach:

using System.ComponentModel;

namespace sklearn
{
    [Description("The id and display name of an Azure subscription")]
    public sealed record SubscriptionDescriptor(
        string Id,
        string DisplayName);

    public interface ISubscriptionPlugin
    {
        public Task<IReadOnlyList<SubscriptionDescriptor>> ListSubscriptionsAsync();

        public Task<SubscriptionDescriptor?> GetSubscriptionAsync(string id);
    }

    public class SubscriptionPluginDefinition
    {
        private readonly ISubscriptionPlugin _plugin;

        public class SubscriptionPluginDefinition(ISubscriptionPlugin plugin)
        {
            _plugin = plugin;
        }

        [KernelFunction("list_subscriptions")]
        [Description("Lists all Azure subscriptions that user has access to")]
        public Task<IReadOnlyList<SubscriptionDescriptor>> ListSubscriptionsAsync()
        {
            return _plugin.ListSubscriptionsAsync();
        }

        [KernelFunction("get_subscription")]
        [Description("Gets the details of a specific Azure subscription id")]
        public Task<SubscriptionDescriptor?> GetSubscriptionAsync(string id)
        {
            return _plugin.GetSubscriptionAsync(id);
        }
    }

    public class SubscriptionPlugin : ISubscriptionPlugin
    {
        private readonly List<SubscriptionDescriptor> _subscriptions = new()
        {
            new SubscriptionDescriptor("31cad2f7-1ec4-4491-b69f-d0360ec2b723", "Subscription 1"),
            new SubscriptionDescriptor("7f07aade-c4d8-416c-a3d9-06c76acacf71", "Subscription 2"),
            new SubscriptionDescriptor("62b38d30-c1ab-4f92-b26f-9fb4c85a01a2", "Subscription 3")
        };

        public Task<IReadOnlyList<SubscriptionDescriptor>> ListSubscriptionsAsync()
        {
            return Task.FromResult<IReadOnlyList<SubscriptionDescriptor>>(_subscriptions);
        }

        public Task<SubscriptionDescriptor?> GetSubscriptionAsync(string id)
        {
            return Task.FromResult(_subscriptions.FirstOrDefault(subscription => subscription.Id == id));
        }
    }
}

Then instantiate this and add to the kernel

serviceCollection.AddScoped<ISubscriptionPlugin, SubscriptionPlugin>();
serviceCollection.AddScoped<SubscriptionPluginDefinition>();

var subscriptionPluginDefinition = sp.GetRequiredService<SubscriptionPluginDefinition>();
kernelBuilder.Plugins.AddFromObject(subscriptionPluginDefinition, "SubscriptionPlugin");