Checking if an Obj exists with specific fieldname?

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Hope anyone could point to the right destination, I am trying to figure out how I can check for the existence of a specific object with a specific fieldname, hope you can help! this is how I imagined it:

pseudocode:

C#

if ((fileReaderObj with field name == test)== null)
   {
     Filereader fileReaderObj = new Filereader(test2);
   }
   else 
   {
    // do something else 
    Console.WriteLine("Hey");
   }

Said in other words:

When i create the object I assign a specific name to a public field in the object:

C#

Filereader fileReaderObj = new Filereader(test2);

when checking for object, how could I check if an object with that field name already exists?

Posted 20-Oct-11 0:35am

dahund

Updated 20-Oct-11 2:05am

v2

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I.explore.code 20-Oct-11 6:38am

What do yo mean "existence of a specific object with a specific fieldname"?? just a null check?? or existence in a collection?? or something else??

dahund 20-Oct-11 7:59am

No iam trying to create and object for each user that uses the system, and I need to separate these objects from one of the other so I don't have to recreate them all the time (When the object is created it pulls in allot of data from an SQL database, which takes some time) this time.

My idea was that I could use a specific field name for identification of an object, thereby checking if an object with a specific field name exists!!

Maybe there is another way?? naming the object, maybe but dont now how.

3 solutions

Solution 2

Do you mean whether there's a file handle open for a specific file? Because when you're trying to open an already opened file you get an IO Exception that indicates that the file could not be opened because it's already being used.

Posted 20-Oct-11 0:51am

ShacharK

Updated 20-Oct-11 0:52am

v2

Solution 5

If you are about to read text file.. here is the way you do. if its bitmap you can use bmpImg.Save() for creation..whereas for existance verification FILE.EXISTS() method you need to use.

C#

if (system.IO.FILE.Exists(filePath))
    //File.OpenRead(); read file body
else
  //File.CreateText(); Create file and write text into it.

Posted 20-Oct-11 2:28am

Zubair Alie

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BobJanova · Accepted Answer · 2011-10-20T01:20:00

Solution 4

There is a fundamental misunderstanding of source vs compiled code here. The field name you give an object in source only has meaning at coding time – the fact you called that variable 'fileReaderObj' is only relevant at compile time, at runtime that information is not present.

What you're doing sounds like it could be replaced by using an existing entity management system (like NHibernate), but if you are doing it yourself for good reason: you need to store a runtime reference, either in the object or in a hash map that you look up into whenever you try to get one. For example:

Dictionary<string, FileReader> cache = new Dictionary<string, FileReader>();

FileReader GetFileReader(string key){
 FileReader r;
 if(!cache.TryGetValue(key, out r)) cache[key] = r = new FileReader(key);
 return r;
}

void SomeMethod(){
 // instead of this:
 // FileReader fileReaderObj = new FileReader(test);
 // use this:
 FileReader fileReaderObj = GetFileReader("test");
}

Note that the key is set at runtime to a string, not a field name. (Depending on what 'test' is you might need to pass it as well, if the FileReader needs information within that object.)

Posted 20-Oct-11 1:20am

BobJanova

Updated 20-Oct-11 1:21am

v2

Comments

dahund 20-Oct-11 7:41am

Looks like this is what i need, hopefully you can answer some questions, or correct me I am wrong.

As I understand this, each time I create and object I assign a key(string) which is stored in the Dictionary(cache) next time I want to create a object i first check if the key(string) exists in Dictionary if not I create, else I do something different ( working with object); is this correct ?

and one last thing; Does this check if object runtime reference exists??:

if(!cache.TryGetValue(key, out r)) cache[key] = r = new FileReader(key);

Thx so very much :-)

BobJanova 20-Oct-11 16:36pm

Roughly speaking, yes. The GetFileReader method handles the look up and checking if an entry with that key already exists, so in code that wants a FileReader, it is as simple as
FileReader myFileReader = GetFileReader(key);
(as in the last line of the solution).

Yes, the line you quoted tries to get the existing value, and if there wasn't one, TryParse returns false, so a new FileReader is created and inserted into the cache. It's also assigned to r so it gets returned from the method.

dahund 20-Oct-11 7:45am

sry, one last question, when working with this object reference. how do i use referenc (key.somemethod()) instead of obj.somemethod ?

BobJanova 20-Oct-11 16:36pm

Once you've got a reader out of the cache with
FileReader myFileReader = GetFileReader(key);
... you have a normal reference and you can just call myFileReader.SomeMethod() directly.

dahund 20-Oct-11 17:07pm

perfect :-) i think this will work!!! but i would have to clean dictionary from time to time, since garbage collection could have removed some of the objects making my reference invalid!!! But again thank you, have been looking for something like this in a long time.

BobJanova 20-Oct-11 17:09pm

Anything in the cache is still referenced and therefore won't be garbage collected (unless you drop references to the cache, of course). If you are sure you are done with a FileReader you should remove it from the cache (cache.Remove(key)).