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C# variable thread safety

A few questions about accessing a local variable from multiple threads at once:
I have multiple threads writing and reading the value of a variable, should i synchronize access to it or not?
The variable is being updated every few seconds from Thread1 and its being read and updated to the Database every few seconds from Thread2.
Which problems can occur if i don't hold any logic and don't have any concurrency issues?
Should i use volatile for this?
EDIT:
I would like to emphasize that i don't have any concurrency issues. Here's my specific scenarion:
a. My variable's name is pingLatency and it measures ping latency
b. Thread1 is sending a ping to 8.8.8.8 each 10 seconds and writes the latency to pingLatency
c. Thread2 updates a correcposing field with the value of pingLatency each 10 seconds.
d. Thread2 updates the same database row each time.
Now, i'm using this database field to monitor network connectivity. My question is - Can there be a situation where the variable is not updated or it would throw an exception due to thread safety issues? I want to avoid using lock because it just seems like an overkill.
What do you think?

Yes you should synchronize access to it, if it is a primitive type there are methods to do this for you without locks
no comment
not sure by what you mean by this... most likely you'll end up inserting the wrong value into the DB
Don't use volatile, per Eric Lippert, it's overly complicated and the semantics are very weird.
Be careful of breaking the memory model, C# by and large follows most other languages in using sequential consistency for data-race-free programs (SC-DRF). Volatile breaks this, so just use locks to prevent a data race.
As for lock it's not as heavy as one might imagine, in most cases the lock won't be contended in the scenario you imagine. So acquiring the lock should be painless in most cases.

If you want .NET managed parallelism use the built in good stuff. Task Parallelism. This will manage the threads for you and you can use the thread safe variables that are built in just as an array/list would be equal to ConcurrentBag, etc.

If access to your variable is atomic and there are no logical problems you are OK.
according to this you can know if you are using an atomic variable.

Why is the explicit management of threads a bad thing?

In a previous question, I made a bit of a faux pas. You see, I'd been reading about threads and had got the impression that they were the tastiest things since kiwi jello.
Imagine my confusion then, when I read stuff like this:
[T]hreads are A Very Bad Thing. Or, at least, explicit management of threads is a bad thing
and
Updating the UI across threads is usually a sign that you are abusing threads.
Since I kill a puppy every time something confuses me, consider this your chance get your karma back in the black...
How should I be using thread?

Enthusiam for learning about threading is great; don't get me wrong. Enthusiasm for using lots of threads, by contrast, is symptomatic of what I call Thread Happiness Disease.
Developers who have just learned about the power of threads start asking questions like "how many threads can I possible create in one program?" This is rather like an English major asking "how many words can I use in a sentence?" Typical advice for writers is to keep your sentences short and to the point, rather than trying to cram as many words and ideas into one sentence as possible. Threads are the same way; the right question is not "how many can I get away with creating?" but rather "how can I write this program so that the number of threads is the minimum necessary to get the job done?"
Threads solve a lot of problems, it's true, but they also introduce huge problems:
Performance analysis of multi-threaded programs is often extremely difficult and deeply counterintuitive. I've seen real-world examples in heavily multi-threaded programs in which making a function faster without slowing down any other function or using more memory makes the total throughput of the system smaller. Why? Because threads are often like streets downtown. Imagine taking every street and magically making it shorter without re-timing the traffic lights. Would traffic jams get better, or worse? Writing faster functions in multi-threaded programs drives the processors towards congestion faster.
What you want is for threads to be like interstate highways: no traffic lights, highly parallel, intersecting at a small number of very well-defined, carefully engineered points. That is very hard to do. Most heavily multi-threaded programs are more like dense urban cores with stoplights everywhere.
Writing your own custom management of threads is insanely difficult to get right. The reason is because when you are writing a regular single-threaded program in a well-designed program, the amount of "global state" you have to reason about is typically small. Ideally you write objects that have well-defined boundaries, and that do not care about the control flow that invokes their members. You want to invoke an object in a loop, or a switch, or whatever, you go right ahead.
Multi-threaded programs with custom thread management require global understanding of everything that a thread is going to do that could possibly affect data that is visible from another thread. You pretty much have to have the entire program in your head, and understand all the possible ways that two threads could be interacting in order to get it right and prevent deadlocks or data corruption. That is a large cost to pay, and highly prone to bugs.
Essentially, threads make your methods lie. Let me give you an example. Suppose you have:
if (!queue.IsEmpty) queue.RemoveWorkItem().Execute();
Is that code correct? If it is single threaded, probably. If it is multi-threaded, what is stopping another thread from removing the last remaining item after the call to IsEmpty is executed? Nothing, that's what. This code, which locally looks just fine, is a bomb waiting to go off in a multi-threaded program. Basically that code is actually:
if (queue.WasNotEmptyAtSomePointInThePast) ...
which obviously is pretty useless.
So suppose you decide to fix the problem by locking the queue. Is this right?
lock(queue) {if (!queue.IsEmpty) queue.RemoveWorkItem().Execute(); }
That's not right either, necessarily. Suppose the execution causes code to run which waits on a resource currently locked by another thread, but that thread is waiting on the lock for queue - what happens? Both threads wait forever. Putting a lock around a hunk of code requires you to know everything that code could possibly do with any shared resource, so that you can work out whether there will be any deadlocks. Again, that is an extremely heavy burden to put on someone writing what ought to be very simple code. (The right thing to do here is probably to extract the work item in the lock and then execute it outside the lock. But... what if the items are in a queue because they have to be executed in a particular order? Now that code is wrong too because other threads can then execute later jobs first.)
It gets worse. The C# language spec guarantees that a single-threaded program will have observable behaviour that is exactly as the program is specified. That is, if you have something like "if (M(ref x)) b = 10;" then you know that the code generated will behave as though x is accessed by M before b is written. Now, the compiler, jitter and CPU are all free to optimize that. If one of them can determine that M is going to be true and if we know that on this thread, the value of b is not read after the call to M, then b can be assigned before x is accessed. All that is guaranteed is that the single-threaded program seems to work like it was written.
Multi-threaded programs do not make that guarantee. If you are examining b and x on a different thread while this one is running then you can see b change before x is accessed, if that optimization is performed. Reads and writes can logically be moved forwards and backwards in time with respect to each other in single threaded programs, and those moves can be observed in multi-threaded programs.
This means that in order to write multi-threaded programs where there is a dependency in the logic on things being observed to happen in the same order as the code is actually written, you have to have a detailed understanding of the "memory model" of the language and the runtime. You have to know precisely what guarantees are made about how accesses can move around in time. And you cannot simply test on your x86 box and hope for the best; the x86 chips have pretty conservative optimizations compared to some other chips out there.
That's just a brief overview of just a few of the problems you run into when writing your own multithreaded logic. There are plenty more. So, some advice:
Do learn about threading.
Do not attempt to write your own thread management in production code.
Use higher-level libraries written by experts to solve problems with threads. If you have a bunch of work that needs to be done in the background and want to farm it out to worker threads, use a thread pool rather than writing your own thread creation logic. If you have a problem that is amenable to solution by multiple processors at once, use the Task Parallel Library. If you want to lazily initialize a resource, use the lazy initialization class rather than trying to write lock free code yourself.
Avoid shared state.
If you can't avoid shared state, share immutable state.
If you have to share mutable state, prefer using locks to lock-free techniques.

Explicit management of threads is not intrinsically a bad thing, but it's frought with dangers and shouldn't be done unless absolutely necessary.
Saying threads are absolutely a good thing would be like saying a propeller is absolutely a good thing: propellers work great on airplanes (when jet engines aren't a better alternative), but wouldn't be a good idea on a car.

You cannot appreciate what kind of problems threading can cause unless you've debugged a three-way deadlock. Or spent a month chasing a race condition that happens only once a day. So, go ahead and jump in with both feet and make all the kind of mistakes you need to make to learn to fear the Beast and what to do to stay out of trouble.

There's no way I could offer a better answer than what's already here. But I can offer a concrete example of some multithreaded code that we actually had at my work that was disastrous.
One of my coworkers, like you, was very enthusiastic about threads when he first learned about them. So there started to be code like this throughout the program:
Thread t = new Thread(LongRunningMethod);
t.Start(GetThreadParameters());
Basically, he was creating threads all over the place.
So eventually another coworker discovered this and told the developer responsible: don't do that! Creating threads is expensive, you should use the thread pool, etc. etc. So a lot of places in the code that originally looked like the above snippet started getting rewritten as:
ThreadPool.QueueUserWorkItem(LongRunningMethod, GetThreadParameters());
Big improvement, right? Everything's sane again?
Well, except that there was a particular call in that LongRunningMethod that could potentially block -- for a long time. Suddenly every now and then we started seeing it happen that something our software should have reacted to right away... it just didn't. In fact, it might not have reacted for several seconds (clarification: I work for a trading firm, so this was a complete catastrophe).
What had ended up happening was that the thread pool was actually filling up with long-blocking calls, leading to other code that was supposed to happen very quickly getting queued up and not running until significantly later than it should have.
The moral of this story is not, of course, that the first approach of creating your own threads is the right thing to do (it isn't). It's really just that using threads is tough, and error-prone, and that, as others have already said, you should be very careful when you use them.
In our particular situation, many mistakes were made:
Creating new threads in the first place was wrong because it was far more costly than the developer realized.
Queuing all background work on the thread pool was wrong because it treated all background tasks indiscriminately and did not account for the possibility of asynchronous calls actually being blocked.
Having a long-blocking method by itself was the result of some careless and very lazy use of the lock keyword.
Insufficient attention was given to ensuring that the code that was being run on background threads was thread-safe (it wasn't).
Insufficient thought was given to the question of whether making a lot of the affected code multithreaded was even worth doing to begin with. In plenty of cases, the answer was no: multithreading just introduced complexity and bugs, made the code less comprehensible, and (here's the kicker): hurt performance.
I'm happy to say that today, we're still alive and our code is in a much healthier state than it once was. And we do use multithreading in plenty of places where we've decided it's appropriate and have measured performance gains (such as reduced latency between receiving a market data tick and having an outgoing quote confirmed by the exchange). But we learned some pretty important lessons the hard way. Chances are, if you ever work on a large, highly multithreaded system, you will too.

Unless you are on the level of being able to write a fully-fledged kernel scheduler, you will get explicit thread management always wrong.
Threads can be the most awesome thing since hot chocolate, but parallel programming is incredibly complex. However, if you design your threads to be independent then you can't shoot yourself in the foot.
As fore rule of the thumb, if a problem is decomposed into threads, they should be as independent as possible, with as few but well defined shared resources as possible, with the most minimalistic management concept.

I think the first statement is best explained as such: with the many advanced APIs now available, manually writing your own thread code is almost never necessary. The new APIs are a lot easier to use, and a lot harder to mess up!. Whereas, with the old-style threading, you have to be quite good to not mess up. The old-style APIs (Thread et. al.) are still available, but the new APIs (Task Parallel Library, Parallel LINQ, and Reactive Extensions) are the way of the future.
The second statement is from more of a design perspective, IMO. In a design that has a clean separation of concerns, a background task should not really be reaching directly into the UI to report updates. There should be some separation there, using a pattern like MVVM or MVC.

I would start by questioning this perception:
I'd been reading about threads and had got the impression that they were the tastiest things since kiwi jello.
Don’t get me wrong – threads are a very versatile tool – but this degree of enthusiasm seems weird. In particular, it indicates that you might be using threads in a lot of situations where they simply don’t make sense (but then again, I might just mistake your enthusiasm).
As others have indicated, thread handling is additionally quite complex and complicated. Wrappers for threads exist and only in rare occasions do they have to be handled explicitly. For most applications, threads can be implied.
For example, if you just want to push a computation to the background while leaving the GUI responsive, a better solution is often to either use callback (that makes it seem as though the computation is done in the background while really being executed on the same thread), or by using a convenience wrapper such as the BackgroundWorker that takes and hides all the explicit thread handling.
A last thing, creating a thread is actually very expensive. Using a thread pool mitigates this cost because here, the runtime creates a number of threads that are subsequently reused. When people say that explicit management of threads is bad, this is all they might be referring to.

Many advanced GUI Applications usually consist of two threads, one for the UI, one (or sometimes more) for Processing of data (copying files, making heavy calculations, loading data from a database, etc).
The processing threads shouldn't update the UI directly, the UI should be a black box to them (check Wikipedia for Encapsulation).
They just say "I'm done processing" or "I completed task 7 of 9" and call an Event or other callback method. The UI subscribes to the event, checks what has changed and updates the UI accordingly.
If you update the UI from the Processing Thread you won't be able to reuse your code and you will have bigger problems if you want to change parts of your code.

I think you should experiement as much as possible with Threads and get to know the benefits and pitfalls of using them. Only by experimentation and usage will your understanding of them grow. Read as much as you can on the subject.
When it comes to C# and the userinterface (which is single threaded and you can only modify userinterface elements on code executed on the UI thread). I use the following utility to keep myself sane and sleep soundly at night.
public static class UIThreadSafe {
public static void Perform(Control c, MethodInvoker inv) {
if(c == null)
return;
if(c.InvokeRequired) {
c.Invoke(inv, null);
}
else {
inv();
}
}
}
You can use this in any thread that needs to change a UI element, like thus:
UIThreadSafe.Perform(myForm, delegate() {
myForm.Title = "I Love Threads!";
});

A huge reason to try to keep the UI thread and the processing thread as independent as possible is that if the UI thread freezes, the user will notice and be unhappy. Having the UI thread be blazing fast is important. If you start moving UI stuff out of the UI thread or moving processing stuff into the UI thread, you run a higher risk of having your application become unresponsive.
Also, a lot of the framework code is deliberately written with the expectation that you will separate the UI and processing; programs will just work better when you separate the two out, and will hit errors and problems when you don't. I don't recall any specifics issues that I encountered as a result of this, though I have vague recollections of in the past trying to set certain properties of stuff the UI was responsible for outside of the UI and having the code refuse to work; I don't recall whether it didn't compile or it threw an exception.

Threads are a very good thing, I think. But, working with them is very hard and needs a lot of knowledge and training. The main problem is when we want to access shared resources from two other threads which can cause undesirable effects.
Consider classic example: you have a two threads which get some items from a shared list and after doing something they remove the item from the list.
The thread method that is called periodically could look like this:
void Thread()
{
if (list.Count > 0)
{
/// Do stuff
list.RemoveAt(0);
}
}
Remember that the threads, in theory, can switch at any line of your code that is not synchronized. So if the list contains only one item, one thread could pass the list.Count condition, just before list.Remove the threads switch and another thread passes the list.Count (list still contains one item). Now the first thread continues to list.Remove and after that second thread continues to list.Remove, but the last item already has been removed by the first thread, so the second one crashes. That's why it would have to be synchronized using lock statement, so that there can't be a situation where two threads are inside the if statement.
So that is the reason why UI which is not synchronized must always run in a single thread and no other thread should interfere with UI.
In previous versions of .NET if you wanted to update UI in another thread, you would have to synchronize using Invoke methods, but as it was hard enough to implement, new versions of .NET come with BackgroundWorker class which simplifies a thing by wrapping all the stuff and letting you do the asynchronous stuff in a DoWork event and updating UI in ProgressChanged event.

A couple of things are important to note when updating the UI from a non-UI thread:
If you use "Invoke" frequently, the performance of your non-UI thread may be severely adversely affected if other stuff makes the UI thread run sluggishly. I prefer to avoid using "Invoke" unless the non-UI thread needs to wait for the UI-thread action to be performed before it continues.
If you use "BeginInvoke" recklessly for things like control updates, an excessive number of invocation delegates may get queued, some of which may well be pretty useless by the time they actually occur.
My preferred style in many cases is to have each control's state encapsulated in an immutable class, and then have a flag which indicates whether an update is not needed, pending, or needed but not pending (the latter situation may occur if a request is made to update a control before it is fully created). The control's update routine should, if an update is needed, start by clearing the update flag, grabbing the state, and drawing the control. If the update flag is set, it should re-loop. To request another thread, a routine should use Interlocked.Exchange to set the update flag to update pending and--if it wasn't pending--try to BeginInvoke the update routine; if the BeginInvoke fails, set the update flag to "needed but not pending".
If an attempt to control occurs just after the control's update routine checks and clears its update flag, it may well happen that the first update will reflect the new value but the update flag will have been set anyway, forcing an extra screen redraw. On the occasions when this happens, it will be relatively harmless. The important thing is that the control will end up being drawn in the correct state, without there ever having been more than one BeginInvoke pending.

ReaderWriterLock vs lock{}

Please explain what are the main differences and when should I use what.
The focus on web multi-threaded applications.

lock allows only one thread to execute the code at the same time. ReaderWriterLock may allow multiple threads to read at the same time or have exclusive access for writing, so it might be more efficient. If you are using .NET 3.5 ReaderWriterLockSlim is even faster. So if your shared resource is being read more often than being written, use ReaderWriterLockSlim. A good example for using it is a file that you read very often (on each request) and you update the contents of the file rarely. So when you read from the file you enter a read lock so that many requests can open it for reading and when you decide to write you enter a write lock. Using a lock on the file will basically mean that you can serve one request at a time.

Consider using ReaderWriterLock if you have lots of threads that only need to read the data and these threads are getting blocked waiting for the lock and and you don’t often need to change the data.
However ReaderWriterLock may block a thread that is waiting to write for a long time.
Therefore only use ReaderWriterLock after you have confirmed you get high contention for the lock in “real life” and you have confirmed you can’t redesign your locking design to reduce how long the lock is held for.
Also consider if you can't rather store the shared data in a database and let it take care of all the locking, as this is a lot less likely to give you a hard time tracking down bugs, iff a database is fast enough for your application.
In some cases you may also be able to use the Aps.net cache to handle shared data, and just remove the item from the cache when the data changes. The next read can put a fresh copy in the cache.
Remember
"The best kind of locking is the
locking you don't need (i.e. don't
share data between threads)."

Monitor and the underlying "syncblock" that can be associated with any reference object—the underlying mechanism under C#'s lock—support exclusive execution. Only one thread can ever have the lock. This is simple and efficient.
ReaderWriterLock (or, in V3.5, the better ReaderWriterLockSlim) provide a more complex model. Avoid unless you know it will be more efficient (i.e. have performance measurements to support yourself).
The best kind of locking is the locking you don't need (i.e. don't share data between threads).

ReaderWriterLock allows you to have multiple threads hold the ReadLock at the same time... so that your shared data can be consumed by many threads at once. As soon as a WriteLock is requested no more ReadLocks are granted and the code waiting for the WriteLock is blocked until all the threads with ReadLocks have released them.
The WriteLock can only ever be held by one thread, allow your 'data updates' to appear atomic from the point of view of the consuming parts of your code.
The Lock on the other hand only allows one thread to enter at a time, with no allowance for threads that are simply trying to consume the shared data.
ReaderWriterLockSlim is a new more performant version of ReaderWriterLock with better support for recursion and the ability to have a thread move from a Lock that is essentially a ReadLock to the WriteLock smoothly (UpgradeableReadLock).

ReaderWriterLock/Slim is specifically designed to help you efficiently lock in a multiple consumer/ single producer scenario. Doing so with the lock statement is possible, but not efficient. RWL/S gets the upper hand by being able to aggressively spinlock to acquire the lock. That also helps you avoid lock convoys, a problem with the lock statement where a thread relinquishes its thread quantum when it cannot acquire the lock, making it fall behind because it won't be rescheduled for a while.

It is true that ReaderWriterLockSlim is FASTER than ReaderWriterLock. But the memory consumption by ReaderWriterLockSlim is outright outrageous. Try attaching a memory profiler and see for yourself. I would pick ReaderWriterLock anyday over ReaderWriterLockSlim.

I would suggest looking through http://www.albahari.com/threading/part4.aspx#_Reader_Writer_Locks. It talks about ReaderWriterLockSlim (which you want to use instead of ReaderWriterLock).

What is the meant by 'thread safe' object?

I have used generic queue in C# collection and everyone says that it is better to use the object of System.Collection.Generic.Queue because of thread safety.
Please advise on the right decision to use Queue object, and how it is thread safe?

"Thread safe" is a bit of an unfortunate term because it doesn't really have a solid definition. Basically it means that certain operations on the object are guaranteed to behave sensibly when the object is being operated on via multiple threads.
Consider the simplest example: a counter. Suppose you have two threads that are incrementing a counter. If the sequence of events goes:
Thread one reads from counter, gets
zero.
Thread two reads from counter, gets
zero.
Thread one increments zero, writes
one to counter.
Thread two increments zero, writes
one to counter.
Then notice how the counter has "lost" one of the increments. Simple increment operations on counters are not threadsafe; to make them threadsafe you can use locks, or InterlockedIncrement.
Similarly with queues. Not-threadsafe-queues can "lose" enqueues the same way that not-threadsafe counters can lose increments. Worse, not threadsafe queues can even crash or produce crazy results if you use them in a multi-threaded scenario improperly.
The difficulty with "thread safe" is that it is not clearly defined. Does it simply mean "will not crash"? Does it mean that sensible results will be produced? For example, suppose you have a "threadsafe" collection. Is this code correct?
if (!collection.IsEmpty) Console.WriteLine(collection[0]);
No. Even if the collection is "threadsafe", that doesn't mean that this code is correct; another thread could have made the collection empty after the check but before the writeline and therefore this code could crash, even if the object is allegedly "threadsafe". Actually determining that every relevant combination of operations is threadsafe is an extremely difficult problem.
Now to come to your actual situation: anyone who is telling you "you should use the Queue class, it is better because it is threadsafe" probably does not have a clear idea of what they're talking about. First off, Queue is not threadsafe. Second, whether Queue is threadsafe or not is completely irrelevant if you are only using the object on a single thread! If you have a collection that is going to be accessed on multiple threads, then, as I indicated in my example above, you have an extremely difficult problem to solve, regardless of whether the collection itself is "threadsafe". You have to determine that every combination of operations you perform on the collection is also threadsafe. This is a very difficult problem, and if it is one you face, then you should use the services of an expert on this difficult topic.

A type that is thread safe can be safely accessed from multiple threads without concern for concurrency. This usually means that the type is read-only.
Interestingly enough, Queue<T> is not thread safe - it can support concurrent reads as long as the queue isn't modified but that isn't the same thing as thread safety.
In order to think about thread safety consider what would happen if two threads were accessing a Queue<T> and a third thread came along and began either adding to or removing from this Queue<T>. Since this type does not restrict this behavior it is not thread safe.

In dealing with multithreading, you usually have to deal with concurrency issues. The term "concurrency issues" refers to issues that are specifically introduced by the possibility of interleaving instructions from two different execution contexts on a resource shared by both. Here, in terms of thread safety, the execution contexts are two threads within a process; however, in related subjects they might be processes.
Thread safety measures are put in place to achieve two goals primarily. First is to regain determinism with regard to what happens if the threads context-switch (which is otherwise controlled by the OS and thus basically nondeterministic in user-level programs), to prevent certain tasks from being left half-finished or two contexts writing to the same location in memory one after the other. Most measures simply use a little bit of hardware-supported test-and-set instructions and the like, as well as software-level synchronization constructs to force all other execution contexts to stay away from a data type while another one is doing work that should not be interrupted.
Usually, objects that are read-only are thread-safe. Many objects that are not read-only are able to have data accesses (read-only) occur with multiple threads without issue, if the object is not modified in the middle. But this is not thread safety. Thread safety is when all manner of things are done to a data type to prevent any modifications to it by one thread from causing data corruption or deadlock even when dealing with many concurrent reads and writes.

lock keyword in C#

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.

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.

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().

Bear in mind that there might be reasons why your application is not as single-threaded as you think. Async I/O in .NET may well call-back on a pool thread, for example, as do some of the various timer classes (not the Windows Forms Timer, though).

Generally speaking if your application is single threaded, you're not going to get much use out of the lock statement. Not knowing your application exactly, I don't know if they're useful or not - but I suspect not. Further, if you're application is using lock everywhere I don't know that I would feel all that confident about it working in a multi-threaded environment anyways - did the original developer actually know how to develop multi-threaded code, or did they just add lock statements everywhere in the vague hope that that would do the trick?

lock should be used around the code that modifies shared state, state that is modified by other threads concurrently, and those other treads must take the same lock.
A lock is actually a memory access serializer, the threads (that take the lock) will wait on the lock to enter until the current thread exits the lock, so memory access is serialized.
To answer you question lock is not needed in a single threaded application, and it does have performance side effects. because locks in C# are based on kernel sync objects and every lock you take creates a transition to kernel mode from user mode.
If you're interested in multithreading performance a good place to start is MSDN threading guidelines

You can have performance issues with locking variables, but normally, you'd construct your code to minimize the lengths of time that are spent inside a 'locked' block of code.
As far as removing the locks. It'll depend on what exactly the code is doing. Even though it's single threaded, if your object is implemented as a Singleton, it's possible that you'll have multiple clients using an instance of it (in memory, on a server) at the same time..

Yes, there will be some performance penalty when using lock but it is generally neglible enough to not matter.
Using locks (or any other mutual-exclusion statement or construct) is generally only needed in multi-threaded scenarios where multiple threads (either of your own making or from your caller) have the opportunity to interact with the object and change the underlying state or data maintained. For example, if you have a collection that can be accessed by multiple threads you don't want one thread changing the contents of that collection by removing an item while another thread is trying to read it.

Lock(token) is only used to mark one or more blocks of code that should not run simultaneously in multiple threads. If your application is single-threaded, it's protecting against a condition that can't exist.
And locking does invoke a performance hit, adding instructions to check for simultaneous access before code is executed. It should only be used where necessary.

See the question about 'Mutex' in C#. And then look at these two questions regarding use of the 'lock(Object)' statement specifically.

There is no point in having locks in the app if there is only one thread and yes, it is a performance hit although it does take a fair number of calls for that hit to stack up into something significant.