ASP.NET Core with Docker support: Connection refused

[dapalmi]

I have a ASP .NET Core (v2.0) app with Docker support. And I want to start a Oracle Database when starting my app. This is how my docker-compose file looks like:

version: '3'
  services:
   devarttestapp:
     image: devarttestapp
     build:
       context: ./DevartTestApp
       dockerfile: Dockerfile

    oracledb:
     image: sath89/oracle-12c
     ports:
        - "1521:1521"

I use Devart data provider dotConnect for Oracle.

var conn = new Devart.Data.Oracle.OracleConnection(); 
conn.ConnectionString = Environment.GetEnvironmentVariable("ORACLE_CONNECTION_STRING"); 
try 
{
     conn.Open(); 
} 
catch (Exception e) 
{
     Console.WriteLine(e);
     throw;
}

But when I try to connect to the database of the created Oracle DB container I get following exception:

System.Net.Internals.SocketExceptionFactory+ExtendedSocketException (Connection refused 127.0.0.1:1521)

I also tried to include the depends_on as well as the networks option in my docker-compose file with the same results.

What could be the cause of this exception?
And how do I solve this problem?

[EDIT] Add C# syntax highlight by @karelz

[karelz]

Does it work outside of Docker on regular VMs?

[tarekgh]

would be good to include the exception stack trace too

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