Based on your description and the provided code, the issue seems to lie in the configuration or the connection process itself rather than with the Oracle database services since SQL Developer can successfully connect. Here are some steps to diagnose and resolve the issue:

1. Verify Connection String

Ensure your DBConnection connection string in the configuration file (e.g., App.config or Web.config) is correct. The connection string for Oracle typically looks like:

<connectionStrings>
  <add name="DBConnection" 
       connectionString="User Id=<username>;Password=<password>;Data Source=<datasource>" 
       providerName="Oracle.ManagedDataAccess.Client" />
</connectionStrings>

• Replace <username> and <password> with the correct Oracle credentials.

• Replace <datasource> with the appropriate Oracle Data Source, such as:

  • For EZConnect: hostname:port/service_name
    • Example: localhost:1521/XEPDB1
  • For TNS alias: Use the alias defined in tnsnames.ora.

2. Check for Oracle.ManagedDataAccess.Client Setup

Ensure you have the Oracle.ManagedDataAccess NuGet package installed in your project. It can be added using:

dotnet add package Oracle.ManagedDataAccess

Make sure the version of this library is compatible with the Oracle database version.

3. Debug Exception Details

Update the catch block in your constructor and log detailed exception information:

catch (OracleException oracleEx)
{
    Console.WriteLine($"OracleException: {oracleEx.Message}");
    Console.WriteLine($"Error Code: {oracleEx.ErrorCode}");
    Console.WriteLine($"Stack Trace: {oracleEx.StackTrace}");
}
catch (Exception ex)
{
    Console.WriteLine($"General Exception: {ex.Message}");
    Console.WriteLine($"Stack Trace: {ex.StackTrace}");
}

This will provide more specific details about why the connection fails.

4. Verify Network Configuration

• Check TNS Listener: Use the tnsping command to verify the Oracle listener is reachable.

  tnsping <datasource>
  

  Replace <datasource> with your TNS alias or hostname.

• Firewall: Ensure no firewall rules block the connection on port 1521 (or the port used by Oracle).

5. Test Connection Using a Minimal C# Example

Try connecting with a minimal example to isolate the problem:

using Oracle.ManagedDataAccess.Client;

class TestOracleConnection
{
    static void Main(string[] args)
    {
        string connectionString = "User Id=<username>;Password=<password>;Data Source=<datasource>";
        
        try
        {
            using (var connection = new OracleConnection(connectionString))
            {
                connection.Open();
                Console.WriteLine("Connection successful!");
            }
        }
        catch (Exception ex)
        {
            Console.WriteLine($"Error: {ex.Message}");
        }
    }
}

6. Check Connection Timeout

If the database connection is slow, increase the Connection Timeout in the connection string:

<add name="DBConnection" 
     connectionString="User Id=<username>;Password=<password>;Data Source=<datasource>;Connection Timeout=60" 
     providerName="Oracle.ManagedDataAccess.Client" />

7. Ensure Disposal of Resources

The Dispose method in your DBManager class is not implemented. Ensure proper disposal of resources:

public void Dispose()
{
    if (_connection != null)
    {
        if (_connection.State != ConnectionState.Closed)
        {
            _connection.Close();
        }
        _connection.Dispose();
        _connection = null;
    }
}

8. Use Explicit .NET Assembly Version

Sometimes, mismatches in versions can cause issues. Verify the correct version of Oracle.ManagedDataAccess assembly is loaded using:

Console.WriteLine(typeof(OracleConnection).Assembly.FullName);

9. Check Logs

Look at the Oracle Database logs for any relevant errors, or enable tracing for your Oracle client to gather more information.

If none of the above resolves the issue, please share the exact error message and stack trace for further assistance.