Semantic Kernel for .NET

Trigger On

• adding AI-driven prompts, plugins, or orchestration to a .NET app
• reviewing kernel construction, service registration, or plugin usage
• building function-calling patterns with LLMs
• migrating older Semantic Kernel code to current APIs

Documentation

• Semantic Kernel Overview
• Plugins and Functions
• Agent Functions
• GitHub Repository
• Microsoft Agent Framework

References

• patterns.md - Plugin patterns, function calling patterns, multi-agent patterns, prompt templates, and RAG patterns
• anti-patterns.md - Common Semantic Kernel mistakes and how to avoid them

Core Concepts

Workflow

1. Build the Kernel with required services
2. Create Plugins with well-described functions
3. Configure Function Calling for automatic tool use
4. Handle Responses and manage conversation state
5. Test and Observe AI behavior with logging

Kernel Setup

Basic Configuration

var builder = Kernel.CreateBuilder();

builder.AddAzureOpenAIChatCompletion(
    deploymentName: "gpt-4",
    endpoint: config["AzureOpenAI:Endpoint"]!,
    apiKey: config["AzureOpenAI:ApiKey"]!);

// Or OpenAI
builder.AddOpenAIChatCompletion(
    modelId: "gpt-4",
    apiKey: config["OpenAI:ApiKey"]!);

var kernel = builder.Build();

With Dependency Injection

builder.Services.AddKernel()
    .AddAzureOpenAIChatCompletion(
        deploymentName: "gpt-4",
        endpoint: config["AzureOpenAI:Endpoint"]!,
        apiKey: config["AzureOpenAI:ApiKey"]!);

// Register plugins
builder.Services.AddSingleton<WeatherPlugin>();
builder.Services.AddSingleton<OrderPlugin>();

// In your service
public class AiService(Kernel kernel)
{
    public async Task<string> ChatAsync(string message)
    {
        var response = await kernel.InvokePromptAsync(message);
        return response.ToString();
    }
}

Plugin Patterns

Creating a Plugin

public class WeatherPlugin
{
    [KernelFunction]
    [Description("Gets the current weather for a specified city")]
    public async Task<string> GetWeather(
        [Description("The city name, e.g., 'Seattle'")] string city,
        [Description("Temperature unit: 'celsius' or 'fahrenheit'")] string unit = "celsius")
    {
        // Call actual weather API
        var weather = await _weatherService.GetCurrentAsync(city);
        return $"Weather in {city}: {weather.Temperature}° {unit}, {weather.Condition}";
    }

    [KernelFunction]
    [Description("Gets the weather forecast for the next N days")]
    public async Task<string> GetForecast(
        [Description("The city name")] string city,
        [Description("Number of days (1-7)")] int days = 3)
    {
        var forecast = await _weatherService.GetForecastAsync(city, days);
        return FormatForecast(forecast);
    }
}

Plugin Best Practices

[Description]

Function Calling

Automatic Function Calling

var settings = new OpenAIPromptExecutionSettings
{
    FunctionChoiceBehavior = FunctionChoiceBehavior.Auto()
};

kernel.Plugins.AddFromObject(new WeatherPlugin(), "Weather");
kernel.Plugins.AddFromObject(new OrderPlugin(), "Orders");

var result = await kernel.InvokePromptAsync(
    "What's the weather in Seattle and do I have any pending orders?",
    new KernelArguments(settings));

Manual Function Selection

var settings = new OpenAIPromptExecutionSettings
{
    FunctionChoiceBehavior = FunctionChoiceBehavior.Required(
        [kernel.Plugins["Weather"]["GetWeather"]])
};

Chat Completion Patterns

Multi-Turn Conversation

var chatService = kernel.GetRequiredService<IChatCompletionService>();
var history = new ChatHistory();

history.AddSystemMessage("You are a helpful assistant.");
history.AddUserMessage(userMessage);

var response = await chatService.GetChatMessageContentAsync(
    history,
    executionSettings: new OpenAIPromptExecutionSettings
    {
        FunctionChoiceBehavior = FunctionChoiceBehavior.Auto()
    },
    kernel: kernel);

history.AddAssistantMessage(response.Content!);

Streaming Response

await foreach (var chunk in chatService.GetStreamingChatMessageContentsAsync(
    history, executionSettings, kernel))
{
    Console.Write(chunk.Content);
}

Multi-Agent Plugin Isolation

// WRONG - agents share plugins
var sharedKernel = Kernel.CreateBuilder().Build();
sharedKernel.Plugins.AddFromObject(new AllPlugins());

var agent1 = new ChatCompletionAgent { Kernel = sharedKernel };
var agent2 = new ChatCompletionAgent { Kernel = sharedKernel };
// Both agents have same plugins!

// CORRECT - isolated kernels
var kernel1 = CreateKernelForAgent1();
kernel1.Plugins.AddFromObject(new WeatherPlugin());

var kernel2 = CreateKernelForAgent2();
kernel2.Plugins.AddFromObject(new OrderPlugin());

var agent1 = new ChatCompletionAgent { Kernel = kernel1 };
var agent2 = new ChatCompletionAgent { Kernel = kernel2 };

Anti-Patterns to Avoid

[Description]

FunctionChoiceBehavior

Error Handling

[KernelFunction]
[Description("Places an order for a product")]
public async Task<string> PlaceOrder(
    [Description("Product ID")] string productId,
    [Description("Quantity (1-100)")] int quantity)
{
    // Validate inputs
    if (string.IsNullOrEmpty(productId))
        return "Error: Product ID is required";

    if (quantity < 1 || quantity > 100)
        return "Error: Quantity must be between 1 and 100";

    try
    {
        var order = await _orderService.CreateAsync(productId, quantity);
        return $"Order {order.Id} placed successfully for {quantity} units";
    }
    catch (ProductNotFoundException)
    {
        return $"Error: Product '{productId}' not found";
    }
}

Testing Plugins

[Fact]
public async Task GetWeather_ReturnsFormattedWeather()
{
    var mockWeatherService = new Mock<IWeatherService>();
    mockWeatherService.Setup(w => w.GetCurrentAsync("Seattle"))
        .ReturnsAsync(new Weather { Temperature = 20, Condition = "Sunny" });

    var plugin = new WeatherPlugin(mockWeatherService.Object);

    var result = await plugin.GetWeather("Seattle", "celsius");

    Assert.Contains("20°", result);
    Assert.Contains("Sunny", result);
}

Microsoft Agent Framework

For complex multi-agent scenarios, consider

dotnet-microsoft-agent-framework

• Multi-agent orchestration
• Agent-to-agent communication
• Enterprise patterns

Deliver

• kernel setup with clear service and plugin composition
• AI features that fit naturally into the existing .NET app
• observable and testable function-calling behavior
• proper plugin isolation for multi-agent scenarios

Validate

• plugins have clear, specific descriptions
• function calling works as expected
• AI flows are logged and debuggable
• input validation prevents hallucination issues
• kernel instances are properly scoped
• deprecated APIs are not used