What Are Interceptors in C# 12

Introduction

C# 12 introduces Interceptors, an experimental compiler feature that empowers developers to reroute specific method calls to alternative code paths at compile time. This capability, primarily intended for use with source generators, expands programming possibilities by enabling modifications to existing code without directly altering it. In this article, we will explore the concept of interceptors in C# 12 through an example, demonstrating how they can be employed to enhance the functionality of a simple application.

// Interceptor to log method calls and their arguments
[SourceInterceptor("MyNamespace.Interceptors")]
public static class LoggingInterceptor
{
    [OnMethodCall]
    public static void BeforeCall(object instance, ref ReadOnlySpan<object> arguments)
    {
        var methodName = instance.GetType().GetMethod(new MethodIdentifier(arguments[0])).Name;
        Console.WriteLine($"Logging before call to {methodName} with arguments:");
        for (int i = 1; i < arguments.Length; i++)
        {
            Console.WriteLine($"- {arguments[i]}");
        }
    }

    [OnMethodReturn]
    public static object AfterCall(object instance, ref ReadOnlySpan<object> arguments, object returnValue)
    {
        var methodName = instance.GetType().GetMethod(new MethodIdentifier(arguments[0])).Name;
        Console.WriteLine($"Logging after call to {methodName} with return value: {returnValue}");
        return returnValue;
    }
}

public class MyClass
{
    public int Add(int x, int y)
    {
        return x + y;
    }
}

public static void Main(string[] args)
{
    var obj = new MyClass();
    int result = obj.Add(5, 3);
    Console.WriteLine($"Result: {result}");
}

Explanation

1. Interceptor Declaration: LoggingInterceptor is declared with [SourceInterceptor], specifying the namespace containing its definitions.
2. Method Interception: BeforeCall and AfterCall are invoked before and after the Add method call, respectively.
3. Call Context Access: Inside BeforeCall, instance, and arguments provide access to the calling instance and arguments.
4. Code Modifications: Interceptor methods can log, modify arguments, or even replace the original call entirely. In this example, they simply log details.
5. Return Value: AfterCall receives the original return value, allowing potential modifications before returning.

// Interceptor to cache expensive method results
[SourceInterceptor("MyNamespace.Interceptors")]
public static class CachingInterceptor
{
    private static readonly ConcurrentDictionary<(Type Instance, string MethodName, object[] Arguments), object> Cache = new();

    [OnMethodCall]
    public static object BeforeCall(object instance, ref ReadOnlySpan<object> arguments)
    {
        var key = (instance.GetType(), instance.GetType().GetMethod(new MethodIdentifier(arguments[0])).Name, arguments.ToArray());
        if (Cache.TryGetValue(key, out object cachedValue))
        {
            Console.WriteLine("Returning cached value");
            return cachedValue;
        }
        return null; // Proceed with the original method call
    }

    [OnMethodReturn]
    public static object AfterCall(object instance, ref ReadOnlySpan<object> arguments, object returnValue)
    {
        var key = (instance.GetType(), instance.GetType().GetMethod(new MethodIdentifier(arguments[0])).Name, arguments.ToArray());
        Cache.TryAdd(key, returnValue);
        return returnValue;
    }
}

public class MyExpensiveClass
{
    public int CalculateIntensiveResult(int input)
    {
        // Perform time-consuming calculations...
        Thread.Sleep(2000); // Simulate computation time
        return input * 2;
    }
}

Explanation

1. Caching Interceptor: The CachingInterceptor stores the results of method calls to avoid redundant computations.
2. Cache: The Cache dictionary stores method calls and their results, keyed by method details and arguments.
3. Check for Cached Value: BeforeCall checks the cache for a matching result and returns it if found, avoiding the original method call.
4. Store Result: AfterCall adds the method call's result to the cache for future reuse.
5. Performance Benefits: Subsequent calls to CalculateIntensiveResult with the same arguments will retrieve cached results, improving performance significantly.

This example illustrates how Interceptors can be applied for real-world performance optimization scenarios, potentially reducing execution time and resource consumption.

Conclusion

Interceptors in C# 12 offer an intriguing path for advanced code transformation at compile time. they demonstrate potential for cross-cutting concerns, performance optimizations, and dynamic code adaptation. It's crucial to approach them cautiously due to their experimental nature and limitations. Stay informed about their evolution as they mature and potentially become a more prevalent tool in the C# developer's toolkit.