Hashtable question

[Guest]

Hi,

I am using a hashtable as a cache. I am only allowed for the cache to take
up a certain amount of memory.

I think I can state how large I want the hashtable to be on creation.

What I need is someway to monitor the size of the hashtable so that as it
nears the maximum size I have set, I can delete some item or empty the table
altogether.

Can anyone tell me how I can implement this?

Also what happens if I try to add items to the table when it has reached its
maximum size?

Thanks In Advance
Macca

 

[Guest]

Hi Vadym,

I will be defining my own cache manager class which will contain a
hashtable.
The question I have is how do i find the size of the hashtable at runtime so
i can control its size, i.e stop it becoming full.

Thanks,

Macca

 

[Chris Mullins]

I am using a hashtable as a cache. I am only allowed for the cache to take
up a certain amount of memory.

There may be a better options for you: Weak References.

This will allow your cache to grow without bounds so long as there's no
memory pressure. As memory presure builds, the Garbage Collector will
collect the Weak References and your app keeps on going.

You can find more information at:
http://msdn2.microsoft.com/en-us/library/ms404247.aspx
http://msdn2.microsoft.com/en-us/library/system.weakreference.aspx

 

[martin.zarate]

There may be a better options for you: Weak References.

This will allow your cache to grow without bounds so long as there's no
memory pressure. As memory presure builds, the Garbage Collector will
collect the Weak References and your app keeps on going.

You can find more information at:
http://msdn2.microsoft.com/en-us/library/ms404247.aspx
http://msdn2.microsoft.com/en-us/library/system.weakreference.aspx

I've often contemplated this approach myself... a question though, does
the MS hashtable implementation have some sort of container object? I
suppose even the weak reference object must have some overhead. Either
way - if you use this approach, I suppose you must occaisionally go
through and prune all the empty WeakReferences out of your hashtable...
unless you can bind an event to the Weak Reference and have it remove
itself... but at this point we're getting into the sort of complicated
three-star programming that is best left to C++.

 

[Chris Mullins]

The standard generic dictionary uses KeyValuePair<> as it's container.
There's no way I know of to wrap this with a weak reference and have this
"just work". You would need (as you said) some sort of monitor thread to
prune the null references out of the Dictionary.

Poking around the web, it looks like a few people have implemented the
IDictionary interface on a class that is Weak Reference aware. I haven't
used any of them, and have a healthy skepticism for people's ability to
implement complex data structures without bugs.

On the other hand, the Asp.Net Cache is weak reference aware, and it can be
used in any type of application. I've used this as a cache before and had a
good experience.