How does simultaneous access of multiple threads to shared variables in an application affect it?

Concurrent access by multiple threads to shared variables without synchronization can lead to race conditions, where data is read or written incorrectly. This causes unpredictable program behavior, errors, and crashes.

For example, if two threads simultaneously increment a counter without locking, the final value may be less than expected due to conflicts during writing.

To avoid problems, synchronization mechanisms are used:

• lock (monitor)
• Mutex
• Interlocked operations
• volatile to indicate the need for direct memory read/write

Example with lock in C#:

private int counter = 0;
private readonly object lockObj = new object();

void Increment()
{
    lock(lockObj)
    {
        counter++;
    }
}