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I understand the main function of the lock key word from MSDN
lock Statement (C# Reference)
The lock keyword marks a statement block as a critical section by obtaining the mutual-exclusion lock for a given object, executing a statement, and then releasing the lock.
When should the lock be used?
For instance it makes sense with multi-threaded applications because it protects the data. But is it necessary when the application does not spin off any other threads?
Is there performance issues with using lock?
I have just inherited an application that is using lock everywhere, and it is single threaded and I want to know should I leave them in, are they even necessary?
Please note this is more of a general knowledge question, the application speed is fine, I want to know if that is a good design pattern to follow in the future or should this be avoided unless absolutely needed.
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The lock statement acquires the mutual-exclusion lock for a given object, executes a statement block, and then releases the lock. While a lock is held, the thread that holds the lock can again acquire and release the lock. Any other thread is blocked from acquiring the lock and waits until the lock is released.
Acquiring a lock allows a thread to have exclusive access to the data guarded by that lock, forcing other threads to block — as long as those threads are also trying to acquire that same lock. The monitor pattern guards the rep of a datatype with a single lock that is acquired by every method.
Avoid using 'lock keyword' on string object String object: Avoid using lock statements on string objects, because the interned strings are essentially global in nature and may be blocked by other threads without your knowledge, which can cause a deadlock.
What is the difference between Monitor and Lock in C#? Both Monitor and lock provides a mechanism that synchronizes access to objects. lock is the shortcut for Monitor.
When should the lock be used?
A lock should be used to protect shared resources in multithreaded code. Not for anything else.
But is it necessary when the application does not spin off any other threads?
Absolutely not. It's just a time waster. However do be sure that you're not implicitly using system threads. For example if you use asynchronous I/O you may receive callbacks from a random thread, not your original thread.
Is there performance issues with using lock?
Yes. They're not very big in a single-threaded application, but why make calls you don't need?
...if that is a good design pattern to follow in the future[?]
Locking everything willy-nilly is a terrible design pattern. If your code is cluttered with random locking and then you do decide to use a background thread for some work, you're likely to run into deadlocks. Sharing a resource between multiple threads requires careful design, and the more you can isolate the tricky part, the better.
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All the answers here seem right: locks' usefulness is to block threads from acessing locked code concurrently. However, there are many subtleties in this field, one of which is that locked blocks of code are automatically marked as critical regions by the Common Language Runtime.
The effect of code being marked as critical is that, if the entire region cannot be entirely executed, the runtime may consider that your entire Application Domain is potentially jeopardized and, therefore, unload it from memory. To quote MSDN:
For example, consider a task that attempts to allocate memory while holding a lock. If the memory allocation fails, aborting the current task is not sufficient to ensure stability of the AppDomain, because there can be other tasks in the domain waiting for the same lock. If the current task is terminated, other tasks could be deadlocked.
Therefore, even though your application is single-threaded, this may be a hazard for you. Consider that one method in a locked block throws an exception that is eventually not handled within the block. Even if the exception is dealt as it bubbles up through the call stack, your critical region of code didn't finish normally. And who knows how the CLR will react?
For more info, read this article on the perils of Thread.Abort().
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