Microsoft.Extensions.AI: Unified AI Abstractions for .NET

using System.Runtime.CompilerServices;
using Microsoft.Extensions.AI;

public sealed class FakeChatClient(params string[] replies) : IChatClient
{
    private readonly Queue<string> _replies = new(replies);

    public List<ChatMessage[]> Requests { get; } = [];

    public Task<ChatResponse> GetResponseAsync(IEnumerable<ChatMessage> messages,
        ChatOptions? options = null, CancellationToken cancellationToken = default)
    {
        Requests.Add([.. messages]);
        string reply = _replies.Count > 0 ? _replies.Dequeue() : "";
        return Task.FromResult(new ChatResponse(new ChatMessage(ChatRole.Assistant, reply)));
    }

    public async IAsyncEnumerable<ChatResponseUpdate> GetStreamingResponseAsync(
        IEnumerable<ChatMessage> messages, ChatOptions? options = null,
        [EnumeratorCancellation] CancellationToken cancellationToken = default)
    {
        ChatResponse response = await GetResponseAsync(messages, options, cancellationToken);
        foreach (ChatResponseUpdate update in response.ToChatResponseUpdates())
        {
            yield return update;
        }
    }

    public object? GetService(Type serviceType, object? serviceKey = null) =>
        serviceKey is null && serviceType.IsInstanceOfType(this) ? this : null;

    public void Dispose() { }
}

public sealed class TicketClassifier(IChatClient chatClient)
{
    public async Task<TicketCategory> ClassifyAsync(string body, CancellationToken ct = default)
    {
        var response = await chatClient.GetResponseAsync<TicketCategory>(
            $"Classify this support ticket:\n{body}", cancellationToken: ct);

        return response.TryGetResult(out TicketCategory? category)
            ? category
            : throw new InvalidOperationException("The model returned an unexpected format.");
    }
}

public sealed class TicketClassifierTests
{
    [Fact]
    public async Task Classifies_double_charge_as_billing()
    {
        var fake = new FakeChatClient("""{"area":"Billing","urgent":true}""");
        var classifier = new TicketClassifier(fake);

        TicketCategory category = await classifier.ClassifyAsync("I was charged twice!");

        Assert.Equal("Billing", category.Area);
        Assert.Contains("charged twice", fake.Requests[0][^1].Text);
    }
}