Tool/prompt/resource handlers only getting added if collections aren't empty

[stephentoub]

We currently synthesize handlers for tools, prompts, resources if the relevant collection has been initialized, but only if it's not empty. If instead it's empty, with the idea that entries will be added later, no handlers are synthesized.

Repro:

using Microsoft.Extensions.DependencyInjection;
using Microsoft.Extensions.Hosting;
using Microsoft.Extensions.Logging;
using ModelContextProtocol.Client;
using ModelContextProtocol.Protocol;
using ModelContextProtocol.Server;
using System.IO.Pipelines;

Pipe clientToServerPipe = new(), serverToClientPipe = new();

McpServerResourceCollection resourceCollection = [];

// Uncomment this line and then both resources will work
// resourceCollection.Add(McpServerResource.Create(() => "something", new() { UriTemplate = "something://something/1" }));

var builder = Host.CreateApplicationBuilder(args);
builder.Logging.AddConsole(o => o.LogToStandardErrorThreshold = LogLevel.Trace);
builder.Services
    .AddMcpServer(options =>
    {
        options.ServerInfo = new Implementation
        {
            Name = "observable-game-state",
            Version = "1.0.0",
            Title = "Observable Game State Server"
        };

        options.ResourceCollection = resourceCollection;
    })
    .WithStreamServerTransport(clientToServerPipe.Reader.AsStream(), serverToClientPipe.Writer.AsStream());
_ = builder.Build().RunAsync();

await using McpClient client = await McpClient.CreateAsync(
    new StreamClientTransport(clientToServerPipe.Writer.AsStream(), serverToClientPipe.Reader.AsStream()));

Console.WriteLine("First:");
foreach (var resource in await client.ListResourcesAsync())
{
    Console.WriteLine($"Resource: {resource.Uri}");
}

resourceCollection.Add(McpServerResource.Create(() => "something", new() { UriTemplate = "something://something/2" }));

Console.WriteLine("Second:");
foreach (var resource in await client.ListResourcesAsync())
{
    Console.WriteLine($"Resource: {resource.Uri}");
}

That check is happening here:

csharp-sdk/src/ModelContextProtocol.Core/Server/McpServerImpl.cs

Line 298 in b40f06c

if (resources is { IsEmpty: false })

We should instead always synthesize these handlers if this collection has been initialized, even if it's empty.