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.NET Forum / .NET Framework / New Users / October 2004*URGENT Comparing objects stored in hashtable/collection
Hi! I need to compare objects that I store in HashTable by using Add method and passing object as first param, and a unique key string as second. Later on I iterate through this collection and need to compare the object stored in the HashTable to another object that's obtained as one of the params in a call to an event. Example code (i'm skipping most declarations to avoid long post): Private Shared Sub OnChanged(ByVal source As Object, ByVal e As FileSystemEventArgs) Dim fsw as FileSystemWatcher fsw = source myEnumerator = hshtable.GetEnumerator() While myEnumerator.MoveNext if myEnumerator.Value Is fsw then 'do something if the object is the same end if End While I also tried using myEnumerator.Current when performing comparison, as well as trying to cast the .Value and .Current to the same type of object that I compare it to (FileSystemWatcher). I'm open to any suggestions. The basic goal of what I'm doing is to store a reference to an object and an associated string somewhere so that I can later when the event of FileSystemObject fires identify that object and associated string to perform some relevant action. I have a pretty short Deadline on this so any help would be appreciated! Thanks! > Hi! I need to compare objects that I store in HashTable by using Add > method and passing object as first param, and a unique key string as > second. Later on I iterate through this collection and need to > compare the object stored in the HashTable to another object that's > obtained as one of the params in a call to an event. > Example code (i'm skipping most declarations to avoid long post): [snip] > I also tried using myEnumerator.Current when performing comparison, > as well as trying to cast the .Value and .Current to the same type of [quoted text clipped - 6 lines] > > Thanks! Can you cast objects in VB.NET? In C# I would use: // Get corresponding TreeNode object TreeNode tn = (TreeNode)hashTable[currentHandle]; Where the HashTable contains pairs of TreeNodes/TreeNode handles. There must be something similaer in VB, perhaps hashTable[currentHandle] as TreeNode?  SignatureJeff Gaines Damerham Hampshire UK Thanks for suggestion Jeff, but as I mentioned in my initial post I have tried that approach. I might have gone wrong about doing though, so any suggestions as to how to do it the right way would be great. VB.net has CType function that allows you to do the casting/converting between types, so I tried using it by saying CType(ojbect1, FileSystemWatcher) and then comparing it to the source object that already suppose to be of that type. I've also tried casting the source object to the same type, but VB throws exceptions not liking one of those particualar conversions. > > Hi! I need to compare objects that I store in HashTable by using Add > > method and passing object as first param, and a unique key string as [quoted text clipped - 27 lines] > There must be something similaer in VB, perhaps > hashTable[currentHandle] as TreeNode? What exactly is in the hashtable? If you want to map a string to the FileSystemWatcher, then you should use the fsw's as keys and strings as values. And if you are doing so, simply casting the return value of the hshtable[fsw] call to string should be enough. So I guess your code should be: Private Shared Sub OnChanged(ByVal source As Object, ByVal e As FileSystemEventArgs) Dim fsw as FileSystemWatcher Dim val as String fsw = source val = hshtable[fsw] 'if the val isn't Nothing, you've got what you need. HTH, Stefan > Thanks for suggestion Jeff, but as I mentioned in my initial post I have > tried that approach. I might have gone wrong about doing though, so any [quoted text clipped - 40 lines] >> There must be something similaer in VB, perhaps >> hashTable[currentHandle] as TreeNode? Stefan, I don't think you fully understand what I'm doing thus far and why I am having the problem. Hashtable (in case you don't know) is a collection of sorts where you can store associated pairs of objects(anything, strings, numbers, objects,etc...) and keys (strings) that identify them. You can then use various methods of HashTable class to walk through the list of items you have stored in it and return the objects you stored and the associated keys, or use the key to locate a specific object. My problem is that I cannot use the key to search for the object I need, I need to compare the object stored in the HashTable to the object that's being passed to the OnChanged event of the FileWatcher that fires at a particular time. Once I am able to establish which specific object stored in the HashTable is the one that's currently reaising it's event, I can get the associated Key that I stored in HashTable to perform further actions in the database. I hope you or anyone else will have some suggestions either to improve this method or to do it some other way to achieve end result. "©tefan ©imek" wrote: > What exactly is in the hashtable? > [quoted text clipped - 59 lines] > >> There must be something similaer in VB, perhaps > >> hashTable[currentHandle] as TreeNode? > Stefan, I don't think you fully understand what I'm doing thus far and why I > am having the problem. Hashtable (in case you don't know) is a collection of > sorts where you can store associated pairs of objects(anything, strings, > numbers, objects,etc...) and keys (strings) that identify them. Um, hashtable keys don't have to be strings. Indeed, if (as per your first post) you're calling Add and passing an object as the first parameter and a string as the second parameter, then that object is the key, and the string is the value - so no wonder the value is never the FileSystemWatcher you're interested in. SignatureJon Skeet - <skeet@pobox.com> http://www.pobox.com/~skeet If replying to the group, please do not mail me too Heh! That sure explains a lot :) Thanks for pointing out my obvious mistake, I'll try the same code I had, but will take into account the sequence of params now. I hope this will work (i know it would defenetly not work the way it's written now). I'll let you know where I get witht this, but thanks again... sometimes you need a fresh eye/person to find something as simple as this. > > Stefan, I don't think you fully understand what I'm doing thus far and why I > > am having the problem. Hashtable (in case you don't know) is a collection of [quoted text clipped - 6 lines] > key, and the string is the value - so no wonder the value is never the > FileSystemWatcher you're interested in. > Heh! That sure explains a lot :) > Thanks for pointing out my obvious mistake, I'll try the same code I [quoted text clipped - 3 lines] > again... sometimes you need a fresh eye/person to find something as > simple as this. [snip] Alex Don't forget you don't need to iterate through the Hashtable - my C# example and tefan's VB example shows how to read it directly. Will save some time :-) SignatureJeff Gaines Damerham Hampshire UK Hi Jeff! Great news, the bloody thing is working finally. Although there as some confusion after I set all the sequenses right, the actual comparison of the objects had to be debugged a couple of times since it's very missleading the way hashtable works. I assumed that by comparing the .Value or .Current property of the HT object to the current fired FSW object I'd see the results, but apparently the actual object is stored in .Key property, which is the confusing part since I though it would return the Key I stored for that particular object. But at the end it all came down to comparing the .Key property to the instance that raised the event, and by getting the actual key i stored using the .Value property. I'll take a look at the suggestion about not using the iteration and just finding the correct object as you said, but I'm not sure that you can access the actual .Value and .Key property of whatever you find with the method you mentioned. Also, any suggesions on how the best way either dispose of this filewatchers once they are processed (don't want them sitting in memory) or re-use them by pooling them somehow (with threading?). Thanks again! Cheers > > Heh! That sure explains a lot :) > > Thanks for pointing out my obvious mistake, I'll try the same code I [quoted text clipped - 11 lines] > example and tefan's VB example shows how to read it directly. Will save > some time :-) > Hi Jeff! Great news, the bloody thing is working finally. Although there as > some confusion after I set all the sequenses right, the actual comparison of [quoted text clipped - 4 lines] > is the confusing part since I though it would return the Key I stored for > that particular object. Hashtable itself doesn't have either Current or Value properties. Are you talking about IDictionaryEntry or something similar? If so, I suspect it's a case of getting things the wrong way round again. I've never seen keys and values getting swapped round as you're describing. > But at the end it all came down to comparing the .Key property to the > instance that raised the event, and by getting the actual key i > stored using the .Value property. I'll take a look at the suggestion > about not using the iteration and just finding the correct object as > you said, but I'm not sure that you can access the actual .Value and > .Key property of whatever you find with the method you mentioned. The method Jeff was suggesting is fetching the value using the key - it's the normal way of accessing a hsahtable. You need to be very clear on which is the key and which is the value though - this seems to be the cause of much of the problem. SignatureJon Skeet - <skeet@pobox.com> http://www.pobox.com/~skeet If replying to the group, please do not mail me too Hi Jon! To answer your question, I use the IDictionaryEnumerator to get those values to walk through the hashtable. I think I got a handle on the whole key/value issue, and will try to use this method to identify the necessary object without scrolling through all entries. I do need some suggestions in regard to properly releasing the FSW objects from memory once they are fired and actions performed in their OnChanged event. I'm thinking of first removing their instance from the hashtable, and then calling the dispose method. I don't think that will remove them from memory though, I belive the GC will do that later one. Thanks again! > > Hi Jeff! Great news, the bloody thing is working finally. Although there as > > some confusion after I set all the sequenses right, the actual comparison of [quoted text clipped - 23 lines] > on which is the key and which is the value though - this seems to be > the cause of much of the problem. > Hi Jon! To answer your question, I use the IDictionaryEnumerator to get > those values to walk through the hashtable. I think I got a handle on the > whole key/value issue, and will try to use this method to identify the > necessary object without scrolling through all entries. Good. > I do need some > suggestions in regard to properly releasing the FSW objects from memory once > they are fired and actions performed in their OnChanged event. I'm thinking > of first removing their instance from the hashtable, and then calling the > dispose method. I don't think that will remove them from memory though, I > belive the GC will do that later one. That sounds right to me, yes. SignatureJon Skeet - <skeet@pobox.com> http://www.pobox.com/~skeet If replying to the group, please do not mail me too See inline: > Stefan, I don't think you fully understand what I'm doing thus far and why > I [quoted text clipped - 8 lines] > keys, > or use the key to locate a specific object. Except that using a hashtable just to walk through it is something like using a DataTable to store integeres from 20 to 1 and retrieve them by index ;) And no, the keys are not strings. Hashtable is a generic object to object dictionary, so you just might use the FSW's as keys, and your related strings as values. And if my example hasn't been clear enough, I'm sorry, 'cause I don't use VB.NET, though I usually can read and understand it. > My problem is that I cannot use the key to search for the object I need, I > need to compare the object stored in the HashTable to the object that's [quoted text clipped - 7 lines] > database. I hope you or anyone else will have some suggestions either to > improve this method or to do it some other way to achieve end result. And I hope someone will persuade you not to iterate over the hashtable. Doing so is usefull only if you really need to do something with EVERY key/value pair, like serializing, etc. The primary function of hashtable is to lookup a value by it's key, through it's indexer (or Item property, don't know how it's called in VB). HTH, Stefan > "Ctefan Cimek" wrote: > [quoted text clipped - 67 lines] >> >> There must be something similaer in VB, perhaps >> >> hashTable[currentHandle] as TreeNode? Free MagazinesGet these publications absolutely FREE for up to 12 months. There are no hidden fees and no obligation. Simply choose a title, complete the application form and submit it. Read more ...
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