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Mastering Concurrency with SemaphoreSlim in C#

Abstract representation of multiple threads accessing a shared resource, controlled by a semaphore gate.

Explore the power of SemaphoreSlim for efficient resource management and thread synchronization in C# applications, preventing race conditions and optimizing performance.

In the realm of concurrent programming, managing access to shared resources is a critical challenge. Uncontrolled access can lead to race conditions, data corruption, and unpredictable application behavior. C#'s SemaphoreSlim provides a lightweight, efficient mechanism to control the number of threads that can concurrently access a limited resource or a pool of resources. Unlike its heavier counterpart, System.Threading.Semaphore, SemaphoreSlim is optimized for in-process synchronization and is often preferred in modern asynchronous C# applications.

What is SemaphoreSlim?

SemaphoreSlim is a synchronization primitive that limits the number of threads that can access a resource or a section of code concurrently. It maintains a count representing the number of available slots. When a thread wants to access the resource, it calls Wait() or WaitAsync(), which decrements the count. If the count is zero, subsequent calls will block until another thread releases a slot by calling Release(), which increments the count.

flowchart TD
    A[Thread A wants access] --> B{SemaphoreSlim.Wait()};
    B -->|Count > 0| C[Decrement Count];
    C --> D[Access Resource];
    D --> E[SemaphoreSlim.Release() - Increment Count];
    E --> F[Thread A Done];
    B -->|Count = 0| G[Thread A Blocks];
    G --> H{Another Thread Calls Release()};
    H --> C;

Flowchart illustrating how SemaphoreSlim controls resource access.

Basic Usage: Limiting Concurrent Operations

A common use case for SemaphoreSlim is to limit the number of concurrent operations, such as network requests, database connections, or CPU-intensive tasks. This prevents resource exhaustion and improves the stability of your application. Let's consider an example where we want to limit concurrent downloads to three at a time.

using System;
using System.Collections.Generic;
using System.Diagnostics;
using System.Linq;
using System.Threading;
using System.Threading.Tasks;

public class SemaphoreSlimExample
{
    private static readonly SemaphoreSlim _semaphore = new SemaphoreSlim(3); // Allow 3 concurrent operations
    private static readonly Random _random = new Random();

    public static async Task RunExample()
    {
        Console.WriteLine("Starting concurrent operations...");
        var tasks = new List<Task>();

        for (int i = 0; i < 10; i++)
        {
            int operationId = i + 1;
            tasks.Add(PerformOperation(operationId));
        }

        await Task.WhenAll(tasks);
        Console.WriteLine("All operations completed.");
    }

    private static async Task PerformOperation(int id)
    {
        Console.WriteLine($"Operation {id}: Waiting to acquire semaphore...");
        await _semaphore.WaitAsync(); // Asynchronously wait for a slot

        try
        {
            Console.WriteLine($"Operation {id}: Acquired semaphore. Performing work...");
            int delay = _random.Next(1000, 3000); // Simulate work
            await Task.Delay(delay);
            Console.WriteLine($"Operation {id}: Work completed in {delay}ms.");
        }
        finally
        {
            _semaphore.Release(); // Release the slot
            Console.WriteLine($"Operation {id}: Released semaphore.");
        }
    }

    public static void Main(string[] args)
    {
        RunExample().GetAwaiter().GetResult();
    }
}

Advanced Scenarios and Considerations

SemaphoreSlim offers additional constructors and methods for more complex scenarios. You can specify an initial count different from the maximum count, allowing you to start with fewer available slots and increase them later if needed. You can also use Wait(int millisecondsTimeout) or WaitAsync(int millisecondsTimeout) to wait for a slot with a timeout, which is useful for preventing indefinite blocking.

using System;
using System.Threading;
using System.Threading.Tasks;

public class SemaphoreSlimAdvancedExample
{
    // Initial count of 0, max count of 2. No one can enter until Release() is called.
    private static readonly SemaphoreSlim _gate = new SemaphoreSlim(0, 2);

    public static async Task RunExample()
    {
        Console.WriteLine("Gate is closed. No one can enter yet.");

        // Task 1 will try to enter immediately
        Task t1 = Task.Run(async () =>
        {
            Console.WriteLine("Task 1: Waiting for gate...");
            await _gate.WaitAsync();
            Console.WriteLine("Task 1: Entered gate.");
            await Task.Delay(1000);
            _gate.Release();
            Console.WriteLine("Task 1: Exited gate.");
        });

        // Task 2 will try to enter immediately
        Task t2 = Task.Run(async () =>
        {
            Console.WriteLine("Task 2: Waiting for gate...");
            await _gate.WaitAsync();
            Console.WriteLine("Task 2: Entered gate.");
            await Task.Delay(1500);
            _gate.Release();
            Console.WriteLine("Task 2: Exited gate.");
        });

        // Task 3 will try to enter with a timeout
        Task t3 = Task.Run(async () =>
        {
            Console.WriteLine("Task 3: Waiting for gate with timeout...");
            bool entered = await _gate.WaitAsync(TimeSpan.FromSeconds(2));
            if (entered)
            {
                Console.WriteLine("Task 3: Entered gate.");
                await Task.Delay(500);
                _gate.Release();
                Console.WriteLine("Task 3: Exited gate.");
            }
            else
            {
                Console.WriteLine("Task 3: Timed out waiting for gate.");
            }
        });

        await Task.Delay(500); // Give tasks a moment to start waiting
        Console.WriteLine("Opening gate for 1 slot...");
        _gate.Release(); // Open one slot
        await Task.Delay(500); // Give a moment for one task to enter
        Console.WriteLine("Opening gate for another slot...");
        _gate.Release(); // Open another slot

        await Task.WhenAll(t1, t2, t3);
        Console.WriteLine("All tasks completed.");
    }

    public static void Main(string[] args)
    {
        RunExample().GetAwaiter().GetResult();
    }
}

When to Use SemaphoreSlim

SemaphoreSlim is particularly well-suited for scenarios such as:

  • Throttling concurrent requests: Limiting the number of API calls to an external service.
  • Managing connection pools: Controlling the number of active database or network connections.
  • Resource pooling: Restricting access to a limited set of reusable objects.
  • Controlling parallel processing: Ensuring that only a certain number of CPU-intensive tasks run simultaneously.

It's a versatile tool for managing concurrency when you need to limit the degree of parallelism rather than enforce strict mutual exclusion.