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Is it possible to limit the total number of database connections in .NET?

I have an API server that interacts with an Azure-hosted PostgreSQL database instance. There are a large number (>50) of postgres databases on that one postgres server. Any given API request may have to interact with any given database.
Unfortunately, our Azure plan for Postgres only allows 50 connections. I regularly have requests fail because Postgres won't accept more. My ADO.NET connection pool is still holding onto database connections for recently used databases, while connections to other databases error out.
I've tried setting the Max Pool Size on my connection strings, but it appears that the connection pool limit is applied per database, not per server. I still need as much pooling as I can get, opening new connections can take >1500ms, which is beyond my SLA if it happens on every request.
Is there a way to ask .NET to never open more than 50 database connections, either per server or total?

Set Max Pool Size and instead of connecting to a separate database connect to the same database on the server and then execute the \connect statement to change to the desired database. The following code fragment demonstrates creating an initial connection to the master database and then switching to the desired database specified in the databaseName string variable.
// Assumes that command is a NpgsqlCommand object and that
// connectionString connects to master.
command.Text = "\connect DatabaseName";
using (NpgsqlConnection connection = new NpgsqlConnection(
connectionString))
{
connection.Open();
command.ExecuteNonQuery();
}

Is connecting to a database using SqlConnection() AND a Connection string safe?

I am using the following code in conjunction with dapper ORM to connect to a database :
using (IDbConnection db = new SqlConnection(ConnectionString()))
{
return db.Query<object>(Sql).ToList();
}
The connection string contains database name and login information. I am wondering if while establishing connection to the database server, if any of that information could be visible to someone else.

If you mean in transit: you can force SQL Server to use encrypted connections - https://technet.microsoft.com/en-us/library/ms189067(v=sql.105).aspx
If you mean in-process - the key parts are removed by default so they won't be trivially available to other code with the SqlConnection instance; this is related to the "Persist Security Info" parameter on SqlConnection's connection-string, which defaults to false. Basically, the .ConnectionString property does not expose the credentials once provided. Note that the string will still have existed in memory at some point, so someone with raw access to the process and memory analysis tools may still be able to obtain it; see https://msdn.microsoft.com/en-us/library/system.data.sqlclient.sqlconnection.connectionstring(v=vs.110).aspx
However, you could also just use Windows authentication via SSPI - this then just uses the app-domain's executing user identity info to connect. Same link as above, but see the "Integrated Security" connection-string parameter.

On the Local Computer: Yes, it would be possible to get access to the information
Over the Network DB Connections: Depends on DB, SQL Server supports SSL, but if you don't use that then you'd be exposing information in your traffic

This would entirely depend on where the connection is being established from and where the connection is being established to.
If either end is in the hands of someone for example, in a distributed client, then they will be able to get hold of the connection details. Typically however, a connection is established "behind the scenes", something like from a web server to a database. Because a connection established like this is all "server side", the connection string is never visible to the "client" of the application and is therefore generally perceived to be safe - of course it is still at the mercy of the infrastructure! :)
It's worth nothing that if this is something like a thick client running on a domain then using something like Windows credentials is an option and would be as secure as the account.

Watching state of OracleConnection

I'm using ODP on my Oracle DB application.I registered to OracleConnection.StateChange event for watching the state of the connection. When i disconnect the db connection the event fires but for example if the internet connection is lost there is no action.
How can i handle these kind of(internet conenction lost etc.) situations via the StateChange event?
Or do i have to create a thread for checking the connnection state regularly?
If yes how can i check the connection because i checked the state and it seems to be open even i unplug the internet cable.
Regards.

The common way is to issue an unexpensive sql statement just before the connection is used for something. On oracle it is something like 'select 1 from dual', on ms-sql 'select 1'. This sql forces a roundtrip to the server and lost connection to the server is reported.
JBoss is using something called check-valid-connection-sql. Others have other names.

Getting Connection lost contact even though a new connection is created

I'm writing an ASP.NET web application. The database is Oracle. I've recently noticed that if I leave the application open for a while without doing anything and then try to access the database I get "ORA-03135: connection lost contact" error. One reason I can think of first is the connection timeout but I create a fresh new connection, open it, do what I need to do and "finally" close it every time I access the database. What's more interesting is after I get this error, I don't have any problems any more with connecting to the database. Do you have any ideas as to what the problem might be?

Are you using a connection pool?
In this case you can get this behaviour because the Oracle Connection Pool returns a "disconnected" connection. Try adding Validate Connection=True; to your connection string.

Solving a timeout error for SQL query

I am getting this error:
Timeout expired. The timeout period elapsed prior to completion of the operation or the server is not responding.
I know there are already guides out there to help solve this but they are not working for me. What am I missing or where should I add the code to these SQL statements in my C# program:
String sql = project1.Properties.Resources.myQueryData;
SqlDataAdapter sqlClearQuestDefects = new SqlDataAdapter(sql,
"Data Source=ab;Initial Catalog=ac;User ID=ad; Password =aa");
DataSet lPlanViewData = new DataSet();
sqlClearQuestDefects.Fill(lPlanViewData, "PlanViewData");
I am getting the timeout error at this line:
SqlDataAdapter sqlClearQuestDefects = new SqlDataAdapter(sql,
"Data Source=ab;Initial Catalog=ac;User ID=ad; Password =aa");

SqlDataAdapter adp = new SqlDataAdapter();
adp.SelectCommand.CommandTimeout = 0; // Set the Time out on the Command Object

You're trying to connect to a SQL Server, and it is taking longer than ADO.NET is willing to wait.
Try connecting to the same server, using the same username and password, using SQL Server Management Studio. If you get the same error, there is either something wrong with your connection string, the server you specify is not running, or you can't get to the server across the network from where you are (maybe you're on a public IP address trying to get in to an internal server name). I can't think of a scenario in which you'd enter the exact same server and credentials into SSMS and connect, then do the same in ADO.NET and fail.
If you're on a slow network, you can try increasing the timeout value. However, if a connection is going to happen at all, it should happen pretty quickly.
Take a look at both your SQL Native Client settings, and the SQL Server settings on the server. There is a section for allowed protocols; SQL can connect using a variety of protocols. Usually, you want TCP/IP for a server on the network, and Named Pipes for a server running on your own computer.
EDIT FROM YOUR COMMENT: Oh, that's normal; happens all the time. From time to time on a TCP network, packets "collide", or are "lost" in transmission. It's a known weakness of packet-switching technologies, which is managed by the TCP protocol itself in most cases. One case in which it isn't easily detected is when the initial request for a connection is lost in the shuffle. In that case, the server doesn't know there was a request, and the client didn't know their request wasn't received. So, all the client can do is give up.
To make your program more robust, all you have to do is expect a failure or two, and simply re-try your request. Here's a basic algorithm to do that:
SqlDataAdapter sqlClearQuestDefects;
short retries = 0;
while(true)
{
try
{
sqlClearQuestDefects = new SqlDataAdapter(sql, "Data Source=ab;Initial Catalog=ac;User ID=ad; Password =aa");
break;
}
catch(Exception)
{
retries++;
//will try a total of three times before giving up
if(retries >2) throw;
}
}

Since the exact command to increase connection time out wasn't mentioned in the other answers (of yet)- if you do determine a need to increase your connection time out, you would do so in your connection string as follows:
Data Source=ab;Initial Catalog=ac;User ID=ad; Password =aa; Connection Timeout=120
Where 120 = 120 seconds. Default is 20 or 30 as I recall.

This is probably a connection issue with your database, for example if you had the following connection string:
"Data Source=MyDatabaseServer...
Then you need to make sure that:
The machine MyDatabaseServer is connected to the network and is accessible from the machine you are running your application from (under the name "MyDatabaseServer")
The database server is running on MyDatabaseServer
The database server on MyDatabaseServer is configured to accept connections from remote machines
The firewall settings both on the local machine and MyDatabaseServer are correctly set up to allow SQL Server connections through
Your username / password etc... are correct
You can also try connecting to the given database instance using SQL Server Management Studio from the client machine as a diagnosis step.
There are plenty of articles that address SQL Server connectivity issues - do a Google search for the specific error message that comes up or failing that as a specific question on Server Fault

Faced this problem recently and found the resolution that worked for me.
By the way, setting Timeout = 0 helped to avoid the exception, but the execution time was unreasonable, while manual execution of the store procedure took a few seconds.
Bottom line:
I added SET IMPLICIT_TRANSACTIONS OFF to the stored procedure that is used to fill the data set.
From MSDN:
The SQL Server Native Client OLE DB Provider for SQL Server and the
SQL Server Native Client ODBC driver automatically set
IMPLICIT_TRANSACTIONS to OFF when connecting. SET
IMPLICIT_TRANSACTIONS defaults to OFF for connections with the
SQLClient managed provider, and for SOAP requests received through
HTTP endpoints.
[...]
When SET ANSI_DEFAULTS is ON, SET IMPLICIT_TRANSACTIONS is ON.
So I believe that in my case defaults weren't as required. (I couldn't check that. Don't have enough privileges on SQL server). But adding this line to my SP solved the problem.
IMPORTANT: In my case I didn't need the transaction, so I had no problem to cancel the implicit transaction setting. If in your case transaction is a must you, probably, shouldn't use this solution.