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Why call Dispose()? Memory leak won't occur?

Edit: My question isn't getting the main answer that I was looking for. I wasn't clear. I would really like to know two things:
Can NOT calling Dispose() cause memory leaks?
What's the worst thing that can happen if you have a large program and never call Dispose() on any of your IDisposable objects?
I was under the impression that memory leaks could occur if Dispose() isn't called on IDisposable objects.
Per the discussion on this thread, my perception was incorrect; a memory leak will NOT occur if Dispose() isn't called.
Why ever bother calling Dispose() then? Is it just to free the resource immediately, instead of sometime later? What's the worst thing that can happen if you have a large program and never call Dispose() on any of your IDisposable objects?

Dispose is used to release non-managed resources. This could mean memory, if a class allocated non-managed memory, but it is more often native objects and resources like open files and database connections.
You often need to call Dispose on a class that itself doesn't have any non-managed resources, but it does contain another class that is disposable and may have non-managed resources.
It's also sometimes useful for developers to implement dispose to ensure deterministic finalization--guaranteeing the order in which resources are freed.
Also note that classes that implement dispose often also have a finalizer to release resourcdes if Dispose is not called. Objects with a finalizer have a different life-cycle than classes without one. When they are ready for GC, the GC will see that they have a finalizer and instead of immediately collecting the object when the GC is ready to, it puts it into the finalization queue. This means that the object lives for one extra GC iteration. When you call dispose, the implementation usually (but is not required to) calls GC.SuppressFinalize() which means the finalizer no longer needs to be called.
If a class implements IDisposable, you should always call Dispose().

While some other answers seem to be suggesting that you can get away with not calling it, this is really bad advice. You should always call Dispose on any IDisposable resource.
Some .NET objects have what's called a "finalizer" - something you can define on your own classes as well, but that you rarely see done in a typical C# programmer's code. The finalizer is what runs when the garbage collector destroys the object, and sometimes it will call Dispose - but only if the implementer of the class made it that way.
The best practice is to always Dispose - no matter what. There are plenty of libraries I've used where not calling Dispose on a resource results in a memory leak, a connection leak, a operating system resource leak, or other kinds of horribleness. And the garbage collector will not reconcile the problem, because they don't implement any custom finalizer.
See related: Will the Garbage Collector call IDisposable.Dispose for me?

The convention is that if an object implements IDisposable you should call Dispose() or use the "using" pattern. The difference between Dispose() and waiting for the destructor (finalizer) to execute is that Dispose() is called right away and can be used for freeing some important resources like db connections, files, devices, unmanaged oejects, etc.
So to summarize - if it is IDisposable - Dispose() it!

Not calling Dispose will not ever (*see note 2 on wrong implementation) cause traditional "memory leak" (memory is never freed till end of the process).
The "only" thing that will happen in relation to memory is it will be freed in non-deterministic moment in the future.
One interesting case of non Dispose objects is when very small managed objects hold large amounts of unmanaged memory (i.e. allocated with some flavor of Win32 memory management functions i.e. HeapAlloc ). In this case managed memory manager may not be able to detect memory pressure properly to trigger Gen2 GC and (especially in case of x86 - 32bit process) it may prematurely fail to allocate managed memory for your process. The other issue in this case is fragmentation of address space by "waiting for GC to be de-allocated" (again mostly in x86 case) - when smaller chunks of native memory are allocated with some relatively large space between them preventing allocation of large blocks needed for managed memory management.
Notes:
This answer explicitly talks about true memory leaks/memory allocation issues cased by not disposing IDisposable object managing memory. While it is true that there are no "true memory leaks" caused by such practice most people will consider growing memory usage as memory leak (similar to storing large amount of objects in static list/dictionary for lifetime of an application).
One can create object that manages native memory and incorrectly implements IDisposable pattern. In this case it is possible to really leak native memory (irrespective of calling Dispose).
In most cases objects that implement IDisposable don't managed memory at all. For most practical C# programs native resources managed by such objects are handles for system resources like files, bitmaps, fonts, synchronization objects or COM native objects. Not disposing them in timely manner will cause other issues.
Dispose all objects properly. There is no excuse not to.

Dispose() is intended to free resources that the garbage collector won't free, such as database connections. These resources should also be freed in the finalizer, however the finalizer is much slower than the Dispose() method.

For me:
Dispose can be used in using() scope. This can help me to determine the lifespan of a IDisposeable component. I usually use this in StreamWriter/Reader or SqlConnection classes.
Another use of Dispose is it can end a lifespan of a component explicitly. Such as calling Form.Dispose() in C# winform will close the form. However, for SqlConnection, people said that just calling Dispose alone without explicitly calling Close won't guarantee the connection to be closed. Advised to call both Close and Dispose. I haven't tried this though.
And another thing, after Dispose() is called, the GC can immediaetly free the memory, because they know that the object lifespan is end, rather than waiting for the lifespan to end.
Similiar question may be C# disposing IDisposable

Can NOT calling Dispose() cause memory leaks?
Yes, of course. Below is just one example.
Assume you have a main window in your application and you create a child control that has an event subscription to the main window. You unsubscribe from them on Dispose. If you don't dispose, main window can hold the reference to your child control until you close the application.
What's the worst thing that can happen if you have a large program and
never call Dispose() on any of your IDisposable objects?
The worse case is not releasing some unwanted memory until you close the application.
And the other question is, what if you never implement IDisposable or finalization when they are required?
The worse case of having a memory leak is holding on to that memory until you restart the PC. This can happen only if you have un-managed resources and you don't implement dispose/finalize. If you implement Idisposable interface and implement the finalizer, finalization process will execute the Dispose for you.
Another reason why you should call Dispose is to Suppress the finalization.
As I've indicated before if there is any object with Finalize method and you didn't call the Dispose. That object can reside in memory for two GC cycles. In the first cycle, it enqueue that instance to the finalization queue and finalization happens after GC process. So, only next GC cycle can release that memory.

If an object has been disposed, does suppressing the gc finalizer save it some time?

Garbage Collection can become a time consuming process. In this regard, the GC tends to work only when it has to. If an object has been disposed, to help save time and aid the GC, should the GC's finalizer be suppressed?
using(var sO = new someObject())
{
//work with object
}
public class someObject : IDisposable
{
internal Stream someResource;
internal Context someContext;
public void Dispose()
{
someContext.Dispose();
someResource.Dispose();
//A: does suppressing the GC finalizer here save time for the GC?
//B: can this cause memory leaks?
//GC.SuppressFinalize(this);
}
}

To clear up some confusion:
You only need a finalizer if you need to clean up unmanaged resources in some special way.
If you have no unmanaged resources, then you don't need a finalizer.
If you have a finalizer, then you should definitely implement IDisposable.
If you have a finalizer, then you should call GC.SuppressFinalize in Dispose, because the finalizer doesn't need to be called if you've already cleaned up after yourself.
You only need to call GC.SuppressFinalize if you have a finalizer. If you don't, you probably still want to call it anyway as a defensive measure.
does suppressing the GC finalizer here save time for the GC?
If there are unmanaged resources and your object has a finalizer, then yes; your object might live a long time on the finalization queue if you don't suppress this. The resources you dispose of may also take a long time to finalize (closing an expensive I/O channel, for example).
If you don't have a finalizer, you're saying that you don't have anything unmanaged to clean up. You should still call SuppressFinalize anyway as a defensive measure.
From the FxCop rules:
"Failure to suppress finalization degrades performance and provides no benefits."
For your other question,
can this cause memory leaks?
In this example, yes, if any of the resources are unmanaged. If your object is terminated in some unusual way before it has a chance to invoke Dispose, unmanaged resources won't be properly freed. Consider what happens if someResource is unmanaged and throws an exception while you're using a someObject instance, for example.
If your object holds onto unmanaged resources and you need to guarantee those resources are cleaned up, you need a finalizer too (which would be named ~someObject in your case).

If you actively Dispose an object then yes you should suppress the finalization. There is no need for the object to be finalized if you actively / deterministically release all of the resources. Letting it hang around for the finalizer is just wasting time later on.

One point not yet mentioned: an object need only be considered "live" if there is some execution path via which code might need to access its fields or its object header (a data structure that holds information about the object's type, whether it's been used as a monitor lock, etc.) If an object field holds an unmanaged handle and the last thing the Dispose method does is to close the handle, it's possible that the object may become eligible for finalization while the Dispose method is running. Oops. Not only will the call to GC.SuppressFinalize serve to prevent the finalizer from getting enqueued after it executes; its placement as the last thing Dispose does will prevent the object from becoming eligible for finalization before it executes.

How much work in the Dispose method?

How much work should be done in a Dispose method? In constructors I've always taken the stance that you should only do what is absolutely necessary to instantiate the object. This being the case I've also always taken the approach that you should ONLY be cleaning up open resources when disposing. Closing files, freeing memory, disposing of child disposable object, etc. You shouldn't be doing lengthy processes like touching files, accessing databases and such in the Dispose method.
Am I wrong? Are those action's OK as long as you are handling any possible exceptions so they don't bubble out of the method? I just don't think doing a lot in Dispose is a good idea. I would like to know what the community thinks.

Am I wrong?
No, you are right. In general the Dispose method is used to clean the unmanaged resources that your class might have allocated.
But that's difficult to generalize. There are cases where the Dispose method is simply used to ensure that some operation executes. For example in ASP.NET MVC there's the Html.BeginForm helper which is used like that:
using (Html.BeginForm())
{
}
and all that the Dispose method does is render a closing </form> tag. So as you can see people could be creative with the pattern and it is very difficult to draw conclusions without a specific scenario.
But in the most common situations it's used to release unmanaged resources.

"It depends". What kind of database/file access are we talking about? Say for example that your disposable object is some sort of logger and you use it in the following pattern
using(Logger logger = new Logger())
{
foo.PerformTask();
}
I think it would be perfectly acceptable for logger to write out "Log started" in the constructor "Log Completed" in Dispose.

How much work should be done in a Dispose method?
This depends, are you implementing the IDispose interface just for the convenience of a 'using' statement, or are you implementing the full IDisposable pattern? In the later case of a full disposable pattern it is still acceptable to perform more complex actions provided that you're 'disposing' parameter is true (i.e. you are not in GC).
When you are defining a finalizer that calls the Dispose method there really is not too much to be concerned about. Similar uses/abuses of the IDisposable interface mentioned already be others (i.e. using (Html.BeginForm())) are capable of performing any action. Often this can greatly reduce code complexity and prevent coders from accidentally forgetting to perform some closing action. One down (or up) side to this is that code executes a little differently inside a finally block.
In constructors I've always taken the stance that you should only do what is absolutely necessary to instantiate the object.
Objects, IMHO, should be valid post construction. So if you have a lot of work to do to construct something so be it. Don't think of the workload involved, think of the consumer of you're object and it's usability. Post-construction Initialize() methods suck ;)
This being the case I've also always taken the approach that you should ONLY be cleaning up open resources when disposing. Closing files, freeing memory, disposing of child disposable object, etc. You shouldn't be doing lengthy processes like touching files, accessing databases and such in the Dispose method.
Actually let's break this down a bit...
Disposing from the GC call to the Finalizer
When you implement the IDisposable pattern (not the interface, the pattern, finalizer and all) you are essentially saying that your object has an unmanaged resource that nobody else knows about. That means you have PInvoked a call to Win32's CreateFile, or maybe you called Marshal.AllocHGlobal or something like that. Essentially you likely have an IntPtr instance member you need to do something with to prevent a memory leak. These are the ONLY types of things that should be done when the disposing parameter is false (i.e. called from the finalizer on the GC thread).
Generally you DO NOT call the Dispose method on children. You should not expect any child object to be valid. Simply touching a member of the child object can accidentally 'revive' or resurrect it.
So when you are writing code that executes in a Dispose method called from the Finalizer you have to be careful. You are executing on the GC thread while the rest of your application waits for you. You should perform as few operations as possible to release the unmanaged memory/resource and quit. Never throw an exception and if you are calling an API that may throw you should catch any exception raised. Propagating exceptions back to the GC will prematurely abort the finalizer thread and the remaining objects to be finalized will not have a chance to clean up.
Disposing from the IDisposable.Dispose() method
As I’ve already said, using the Dispose method is safe enough and can safely accommodate any amount of code/process. This is where you would free unmanaged resources, call the dispose method of child objects, flush and close files, etc. Most of the Dispose methods I’ve written do not have an associated Finalizer and therefore do not follow the IDisposable pattern, yet they implement IDisposable just for the convenience of the using statement.
Am I wrong? Are those action's OK as long as you are handling any possible exceptions so they don't bubble out of the method? I just don't think doing a lot in Dispose is a good idea. I would like to know what the community thinks.
You are absolutely right when the dispose method in question is used from a finalizer. You’re assertions about what you should and should not do in a Dispose method should actually be reworded to apply to anything called by a Finalizer. The fact that this is generally done in a method called Dispose is a matter of convention, the IDisposable pattern, but these issues could easily exist in other methods used by the Finalizer.

If an object does something to the state of some outside entities in a way which makes them more useful to that but less useful to everyone else, the Dispose method of the object should do whatever is necessary to restore outside those entities to more generally-useful state. If one wishes to avoid having an object do too much work in Dispose, one should design the object so as never leave any outside entities in a state which would be onerous to clean up.
BTW, Microsoft likes to use the term "unmanaged resources", and gives examples, but never really offers a good definition. I would suggest that an object holds an "unmanaged resource" if an outside entity is altering its behavior on behalf of that object, in a fashion which is detrimental to other objects or entities, and if that outside entity will continue to alter its behavior until the object stops it from doing so.

You should lean toward the conclusion you have already come to. However there are situations where you need to ensure that services are stopped and that could include things like messages being logged for the service shutdown, or saving the current runtime state to data store. This type of disposal usually only applies to things that have a lifestyle that is application scope, meaning they exist the whole time the application is running. So there are situations outside of the expected norm. As is the case with every rule you should follow when writing code.

IDisposable implementation - What should go in 'if (disposing)'

I have been fixing some memory leak issues in a winforms application and noticed some disposable objects that are not Disposed explicitly (developer hasn't called Dispose method). Implementation of Finalize method also doesn't help because it doesn't go in if (disposing) clause. All the static event unregistering and collection clearing have been put in if (disposing) clause. The best practice is calling the Dispose if the object is disposable, but unfortunately this happens sometimes
If there are unmanaged objects, static event handlers and some managed collections that needs to clear when disposing. What's the way to decide what should go in and what should go out of if (disposing) clause.
Dispose method.
// Dispose(bool disposing) executes in two distinct scenarios.
// If disposing equals true, the method has been called directly
// or indirectly by a user's code. Managed and unmanaged resources
// can be disposed.
// If disposing equals false, the method has been called by the
// runtime from inside the finalizer and you should not reference
// other objects. Only unmanaged resources can be disposed.
protected virtual void Dispose(bool disposing)
{
if (!disposed)
{
if (disposing)
{
// Free other state (managed objects).
}
// Free your own state (unmanaged objects).
// Set large fields to null.
disposed = true;
}
}
It says managed objects should in if (disposing) which executes normally only when explicitly call Dispose method by the developer. If the Finalize method has been implemented and developer forgets to call the Dispose method the execution that comes here through the Finalizer does not go in if (disposing) section.
Below are my questions.
If I have static event handlers that causes memory leaks where should I un-register them? In or out of if (disposing) clause?
If I have some collections that causes memory leaks where should I clear them? In or out of if (disposing) clause?
If I am using third party disposable objects (eg: devExpress winform controls) that I am not sure whether they are managed or unmanaged objects. Let's say I want to dispose them when disposing a form. How can I know what are managed and what are non-managed objects? Being disposable doesn't say that? In such cases how to decide what should go in and what should go out of if (disposing) clause?
If I am not sure something managed or unmanaged what can be the bad consequences of disposing/clearing/unregistering-events out of the if (disposing) clause? Let's say it checks for null before disposing?
Edit
What I mean as event un-registering is something like below. Publisher is a long lived instance and below line is in the subscriber's constructor. In this case subscriber need to unregister the event and dispose before the publisher.
publisher.DoSomeEvent += subscriber.DoSomething;

Broadly, managed resources are disposed inside if (disposing) and unmanaged resources outside of it. The dispose pattern works as such:
if (disposed) {
If this object is already disposed, don't dispose of it a second time.
if (disposing) {
If disposal was requested programatically (true), dispose of managed resources (IDisposable objects) owned by this object.
If disposal was caused by the garbage collector (false), do not dispose of managed resources because the garbage collector may have already disposed of the owned managed resources, and will definitelty dispose of them before the application terminates.
}
Dispose of unmanaged resources and release all references to them. Step 1 ensures this only happens once.
disposed = true
Flag this object as disposed to prevent repeated disposal. Repeated disposal may cause a NullReferenceException at step 2 or 3.
Question 1
Don't dispose of them in the Dispose method at all. What would happen if you disposed of multiple instances of the class? You'd dispose the static members each time, despite them already being disposed. The solution I found was to handle the AppDomain.DomainUnloaded event and perform static disposal there.
Question 2
It all depends if the items of the collection are managed or unmanaged. It's probably worth creating managed wrappers that implement IDisposable for any unmanaged classes you are using, ensuring all objects are managed.
Question 3
IDisposable is a managed interface. If a class implements IDisposable, it's a managed class. Dispose of managed objects inside if (disposing). If it doesn't implement IDisposable, it is either managed and does not require disposing, or is unmanaged and should be disposed outside of if (disposing).
Question 4
If the application terminates unexpectedly, or doesn't use manual disposal, the garbage collector disposes of all objects in random order. The child object may be disposed before it's parent is disposed, causing the child to be disposed a second time by the parent. Most managed objects can safely be disposed multiple times, but only if they've been built correctly. You risk (though, unlikely) causing the gargabe collection to fail if an object is disposed multiple times.

The key to remember here is the purpose of IDisposable. It's job is to help you deterministically release resources that your code is holding, before the object is garbage collected. This is actually why the C# language team chose the keyword using, as the brackets determine the scope that the object and it's resources are required for by the application.
For instance, if you open a connection to a database, you want to release that connection and close it ASAP after you have finished with it, rather than waiting for the next garbage collection. This is where and why you implement a Disposer.
The second scenario is to assist with unmanaged code. Effectively this is anything to do with C++/C API calls to the operating system, in which case you are responsible for ensuring that the code isn't leaked. As much of .Net is written to simply P/Invoke down to the existing Win32 API this scenario is quite common. Any object which encapsulates a resource from the operating system (e.g. a Mutex) will implement a Disposer to allow you to safely and deterministically release it's resources.
However these APIs will also implement a destructor, to guarantee that if you don't use the resource correctly that it will not be leaked by the operating system. When your finalizer is called, you do not know whether or not other objects you were referencing have already been garbage collected, which is why it is not safe to make function calls upon them (as they could throw NullReferenceException), only the unmanaged references (which by definition cannot be garbage collected) will be available to the finalizer.
Hope that helps a bit.

If I have static event handlers that causes memory leaks where should I un-register them? In or out of if (disposing) clause?
Dispose method is called on instances where as static event handler are used at class level. So you should not un-register them at all in dispose. Usually static event handler should be un-register when class is unloading or at some point during the execution of application you derive that this event handler is no more required.
For all manages and un-managed resources better implement IDisposable pattern.
See here
http://msdn.microsoft.com/en-us/library/fs2xkftw%28VS.80%29.aspx
and
Finalize/Dispose pattern in C#

It sounds like you mainly have managed objects, i.e. objects that implement IDisposable. Unmanaged code would be things like IntPtr's or handles, which would normally mean calling unmanaged code or P/Invoke to get to them.
As Maheep pointed out, Dispose is not meant for this. When an object is done receiving events it should unregister itself. If that's not possible, consider using WeakReferences instead.
This probably shouldn't go in dispose unless these collections contain objects that need to be disposed. If they are disposable objects then it should go in the "if disposing" block and you should call dispose on each item in the collection.
If it implements IDisposable it's managed
You shouldn't access other managed code objects when called by the finalizer which is what being outside the "if (disposing)" block means.

Unless the sole purpose of a class is to encapsulate some resource(*) which needs to be cleaned up if abandoned, it shouldn't have a finalizer, and Dispose(bool) should never be called with a value of False, but calling Dispose(False) should have no effect. If an inherited class would need to hold a resource requiring cleanup if abandoned, it should encapsulate that resource into an object devoted solely to that purpose. That way, if the main object gets abandoned and nobody else holds any reference to the object encapsulating the resource, that object can perform its cleanup without having to keep the main object alive for an extra GC cycle.
Incidentally, I dislike Microsoft's handling of the Disposed flag. I would suggest that the non-virtual Dispose method should use an integer flag with Interlocked.Exchange, to ensure that calling Dispose from multiple threads will only result in the dispose logic being performed once. The flag itself may be private, but there should be a protected and/or public Disposed property to avoid requiring every derived class to implement its own disposal flag.
(*) A resource isn't some particular type of entity, but rather a loose term that encompasses anything a class may have asked some outside entity to do on its behalf, which that outside entity needs to be told to stop doing. Most typically, the outside entity will have granted the class exclusive use of something (be it an area of memory, a lock, a GDI handle, a file, a socket, a USB device, or whatever), but in some cases the outside entity may have been asked to affirmatively do something (e.g. run an event handler every time something happens) or hold something (e.g. a thread-static object reference). An "unmanaged" resource is one which will not be cleaned up if abandoned.
BTW, note that while Microsoft may have intended that objects which encapsulate unmanaged resources should clean them up if abandoned, there are some types of resources for which that really isn't practical. Consider an object that stores an object reference in a thread-static field, for example, and blanks out that object reference when it is Dispose'd (the disposal would, naturally, have to occur on the thread where the object was created). If the object gets abandoned but the thread still exists (e.g. in the threadpool), the target of the thread-static reference could easily be kept alive indefinitely. Even if there aren't any references to the abandoned object so its Finalize() method runs, it would be difficult for the abandoned object to locate and destroy the thread-static reference sitting in some thread.

What should go in 'if (disposing)'
All Managed objects should go inside if(disposing) clause. Managed objects should not go out side of it (which will be executed through the finalization).
The reason is that Garbage collectors finalization process can execute the Dispose(false) if that class has a Destructor. Normally there is a Destructor only if there are unmanaged resources.Garbage collector's finalization doesn't have a particular order to execute the Finalize method. So, other managed objects may not be in memory by the time finalization occurs.

Should GC.SuppressFinalize be called on objects that do not have a finalizer?

For some reason FXCop seems to think I should be calling GC.SuppressFinalize in Dispose, regardless of whether I have a finalizer or not.
Am I missing something? Is there a reason to call GC.SuppressFinalize on objects that have no finalizer defined?

There's no need to call GC.SuppressFinalize(this) in Dispose, unless:
You are the base class that implements virtual Dispose methods intended for overriding (again, it might not be your responsibility even here, but you might want to do it in that case)
You have a finalizer yourself. Technically, every class in .NET has a finalizer, but if the only finalizer present is the one in Object, then the object is not considered to need finalizing and isn't put on the finalization list upon GC
I would say, assuming you don't have any of the above cases, that you can safely ignore that message.

There is always a finalizer in IL - System.Object.Finalize() exists in every class, so if you make a custom class, it has a finalizer you want to suppress. That being said, not all objects are put on the finalization queue, so you only techncially should need to suppress finalization if you implement your own finalizer.
If you're implementing IDisposable to wrap unmanaged resources, you should include a finalizer, and you should prevent this from running, since in theory you're doing the cleanup already when Dispose is called.

It looks like FxCop simply inspects the Dispose() and doesn't check for the presence of a destructor.
It should be safe to ignore.

All objects have a finalizer method, even if you have not implemented one by using a c# destructor (which is not actually guaranteed to be called by the GC). It's just good practice to supress the call if you have implemented IDisposable because that means you have decided to perform the finalization explictly.
devx article

I don't see any need to call SuppressFinalize() if there's no finalizer defined. If you want to be defensive then it can be good to have a finalizer as well as Dispose(), so you don't need to rely on clients to always call Dispose(). Then you won't leak resources when they forget.