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C# Connection timeout to oracle database when application pool recycles

We have a web page which uses ODP.Net to connect to oracle database from ASP MVC page. During the heavy user load if application pool recycles for any reason, than we get few connection timeout errors from internal oracle class.
Inner Exception: StackTrace: at OracleInternal.ConnectionPool.PoolManager`3.Get(ConnectionString csWithDiffOrNewPwd, Boolean bGetForApp, String affinityInstanceName, Boolean bForceMatch)
How can we handle this scenario ? what could be the reason for this error ?

If you are using a new connection string that differs per user or with some other variable parameter you can exceed your max connections. Pooling only works if you use the exact same string. Under the covers your "close" is ignored and the connection manager keeps the connection open. In SQL Server you can see a connection reset in the trace console every time an existing connection is reused.
Oracle probably has something similar.
Tracing your connections and possibly connection resets will most likely shed some light on what is creating so many connections.

Restore NHibernate after lost Oracle database connection

I have a long running application that uses NHibernate.ISessionFactory to connect to an Oracle database.
Occasionally the database goes offline (e.g. for weekend maintenance), but even once the database is back online, subsequent queries fail with the following exception (inner exceptions also shown):
NHibernate.Exceptions.GenericADOException: could not execute query
[ select .....]
>> Oracle.ManagedDataAccess.Client.OracleException: ORA-03135: Connection lost contact
>> OracleInternal.Network.NetworkException: ORA-03135: Connection lost contact
>> System.Net.Sockets.SocketException: An established connection
was aborted by the software in your host machine
Restarting the application restores the functionality, but I would like the application to be able to automatically cope without a restart, by "resetting" the connection.
I have tried the following with my ISessionFactory when I hit this exception:
sf.EvictQueries();
sf.Close();
sf = null;
sf = <create new session factory>
but see the same exception after recreating the ISessionFactory. I assume this is because NHibernate is caching the underlying broken connection in some kind of connection pool?
How can I persuade NHibernate to create a genuinely new connection (or even just reset all state completely), and hence allow my application to fix the connection issue itself without an application restart?
EDIT:
Following A_J's answer, note that I am already calling using (var session = _sessionFactory.OpenSession()) for each database request.

I suspect you are opening ISession (call to ISessionFactory.OpenSession()) at startup and closing it at application end. This is wrong approach for any long running application.
You should manage connection at lower level of time. In web application, this is generally handled per request. In your case, you should find what that should be. If yours is windows service that does some activity after specified time then Timer_Tick event is good place.
I cannot suggest what that location could be in your application; you need to find out on your own.
Edit 1
Looking at your edit and comment, I do not think this has anything to do with NHibernate. May be that the connection pool is returning a disconnected/stale connection to NHibernate.
Refer this and this accepted answer.

sql timeout expired

Logging:System.Data.SqlClient.SqlException: Timeout expired. The
timeout period elapsed prior to completion of the operation or the
server is not responding.
i am a beginner when i saw in the application log files the above is the most frequent error i saw and also it is getting repeated everyday. on the database when i saw time taken for executing the particular procedure which the above function is calling is less than 5 secs.
But in the application we gave connection timeout=200s and by default command timeout=30 secs our manager says we don't have to increase the command timeout by anymore further as it is true. But still the exception is keep coming.
can anyone suggest me any solution so i can get rid of the above problem thanks

The setting in the web config, if it's the timeout in the connection string setting, is the connection timeout. It only applies to the time it takes to make a connection. From your problem description, it doesn't sound like a connection timeout is what's happening.
Command timeouts are specified in other ways. If you are using DataContext, for example, the timeout is set using the CommandTimeout property.
http://msdn.microsoft.com/en-us/library/system.data.linq.datacontext.commandtimeout.aspx
If you can give a code snippet of how you are hitting the database so we can see what classes you are using, more specific recommendations can be made.

problem regarding maximum pool size in asp.net

I have been working on a small file manager module in a project where a list of folders are shown in a treeview. I have done the whole thing in javascript. Everytime I click a node, a list of data is fetched into a datareader and populated in the front end.
But when I deploy the application in IIS, after about 18 subsequent clicks, the IIS is halted and I have to reset it again. When I checked the event viewer I got the following error
Exception type: InvalidOperationException
Exception message: Timeout expired. The timeout period elapsed prior to obtaining a connection from the pool. This may have occurred because all pooled connections were in use and max pool size was reached.
So in my connection string in the web.config, I set pooling to True and max pool size to 200 and the problem was solved.
But I wonder is it a good practice to use connection pool size in this way. Or how do we prevent so many connections from being opened.
Thanks!

I think what's happening is that you don't free up unused resources. More specifically, you absolutely must call Dispose() on all database-related objects, like SqlConnection, SqlDataReader, etc. Or, better yet, wrap them in using statements.

A sample connection string for SQL Server:
"Data Source=(local);Initial Catalog=pubs;User ID=sa;Password=;Max Pool Size=75;Min Pool Size=5;"
Do like this may help You:)
Default value of Max Pool size is 100.
You can set it to a higher number also so far as performance of the server is not a issue..

How to get better control over connection pool?

I've been getting this error recently, after several reloads of the same page:
System.InvalidOperationException: Timeout expired. The timeout period elapsed prior to obtaining a connection from the pool. This may have occurred because all pooled connections were in use and max pool size was reached
So I am thinking there must be some queries or calls in the app I used incorrectly that causes them not to release the connection. Is there any tools out there that allows me to somehow peek into the pool to see who is hanging on to what?

There's a timeout property on your connection object that you can change. This will change the time it waits to get a connection, there's also a command timeout which controls how long it waits until the command times out once it is running (but the first one sounds like what you need) see here (anything that inherits from DBConnection should have this if you arn't using sql server).
Have a look here too, might help :)