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When two threads try to update the same variable at the same time, you get **Race Conditions** and corrupted data. You must protect your shared state using Synchronization Primitives.
The simplest way. Only one thread can enter the code block at a time. **Pros:** Safe. **Cons:** It blocks threads entirely. You cannot `await` inside a `lock` block. For async code, use **SemaphoreSlim** instead.
The ultimate performance tool. `Interlocked.Increment(ref count)` updates a variable at the CPU hardware level without a lock. It is hundreds of times faster than a `lock` or `Semaphore`. Use it for counters and simple state flags.
Never use a `Dictionary` in a multi-threaded app; use **ConcurrentDictionary**. It is internally partitioned so that multiple threads can write to different 'buckets' of the dictionary without blocking each other. This is the foundation of high-performance caches.
Q: "What is a 'Deadlock' and how do you avoid it?"
Architect Answer: "A deadlock happens when Thread A is waiting for a Resource held by Thread B, while Thread B is waiting for a Resource held by Thread A. They both freeze forever. We avoid this by: 1) Always acquiring locks in the same order. 2) Using **Timeouts** (`WaitAsync(timeout)`). 3) Avoiding `.Result` or `.Wait()` on async tasks, which is the most common cause of deadlocks in ASP.NET applications."
Quizzes linked to this course—pass to earn certificates.
On this page
1. lock (Critical Sections) 2. Interlocked (Lock-Free) 3. Concurrent Collections 4. Interview Mastery