how to user oracle client is asp.net c# in 2015

We need much more detailed information!
What error? How do you connect (method, connection string...)?

Are you u sing instant client? unmanaged or managed driver?

Currently we are using Oracle 11g and .NET 2010. In the said environment we are using connection string to connect database and we used reference System.Data.OracleClient;

Now we are change to Oracle 19c and .NET 2015. We migrated database to 19c and upgraded project to 2015.
We are using Oracle 19c client and managed driver

We are using  
public GetCon()
{
            connection = new OracleConnection(System.Configuration.ConfigurationManager.ConnectionStrings["ProjectMgmt"].ConnectionString);
            connection.Open();        
}
We are getting message "Unable to open database connection".

Need solution on the same. 

Can you post the ProjectMgmt setup from your App.config?

Yes, we need that info as slightwv suggested, but my feeling tells me, that we might end up seeing a connection string that worked in 11 but due to CDB/PDB stuff, this setup won't work anymore ;-)
(connection string with SID, but with 19, you'll end up rather in the CDB than the PDB... we had some serious issues with stuff like that when we migrated back then)