What methods do you use to verify the correctness of multithreaded code?

To verify the correctness of multithreaded code, I use several approaches:

• Unit tests with multithreading simulation — checking the logic correctness during parallel execution.
• Using tools to detect data races and deadlocks, such as Visual Studio Concurrency Visualizer or Thread Sanitizer.
• Logging and thread tracing — analyzing the execution sequence.
• Stress testing — running the code under high load to identify unstable behavior.

An example of a simple test using Task and lock:

object _lock = new object();
int counter = 0;

Parallel.For(0, 1000, i => {
    lock(_lock) {
        counter++;
    }
});

Console.WriteLine(counter); // Expected 1000