When opening an oracle connection, connection object is null

I had same problem and solved adding to my C# Project References Oracle.ManagedDataAccess instead of Oracle.DataAccess.
To do this go to (Tools / Nugget Package Manager / Nugget Package Manager for Solution) browse for Oracle References and select Oracle.ManagedDataAccess.
Then you need to comment or delete your actual DataAccess calls to ManagedDataAccess as:

//using Oracle.DataAccess.Client; 
//using Oracle.DataAccess.Types;
using Oracle.ManagedDataAccess.Client;

No need to change your Connection Code, only the using coding. After it works you can delete Oracle.DataAccess from your Project References

I had the same problem when I started to use ODP.NET.

You can adjust your code like this:

try
{
    OracleConnection con;
    con = new OracleConnection();

    con.ConnectionString = "DATA SOURCE=<DSOURCE_NAME>;PERSIST SECURITY INFO=True;USER ID=******;PASSWORD=*******";

    con.Open();
}
catch (OracleException ex)
{
    Console.WriteLine("Oracle Exception Message");
    Console.WriteLine("Exception Message: " + ex.Message);
    Console.WriteLine("Exception Source: " + ex.Source);
}
catch (Exception ex)
{
    Console.WriteLine("Exception Message");
    Console.WriteLine("Exception Message: " + ex.Message);
    Console.WriteLine("Exception Source: " + ex.Source);
}

You can get more information about your error here: ORA-12154

The problem is your Data Source in your connection string. I assume that it looks like this: Data Source=Server.Source as you could find in your TNSNAMES.ORA file on your computer.

The problem is that ODP.NET does not read the TNSNAMES.ORA file as Visual Studio does.

You have multiple choices to solve this issue:

1. Edit your Web.Config or App.config file to tell ODP.NET how to handle your source.
2. Copy your TNSNAMES.ORA file in the same directory as the .exe. (If it's not a web app)
3. Change the Data Source in order to write the long version instead of the alias. E.g. : Data Source=(DESCRIPTION=(ADDRESS=(PROTOCOL=tcp)(HOST=sales-server)......)))

My favorite method is the #3. It's a lot easier to debug when you have a problem.

You can find more information in the dataSources Section of the Configuring Oracle Data Provider for .NET documentation.