How do I query LDAP from C# to resolve Oracle TNS hostname while using managed ODP.NET?

Further to my second comment in the accepted Answer, this is the code for performing an LDAP lookup which improves the original version I found here. And it also handles server lists in the ldap.ora file that includes multiple delimited port numbers.

private static string ResolveServiceNameLdap(string serviceName)
{
    string tnsAdminPath = Path.Combine(@"C:\Apps\oracle\network\admin", "ldap.ora");
    string connectionString = string.Empty;

    // ldap.ora can contain many LDAP servers
    IEnumerable<string> directoryServers = null;

    if (File.Exists(tnsAdminPath))
    {
        string defaultAdminContext = string.Empty;

        using (var sr = File.OpenText(tnsAdminPath))
        {
            string line;

            while ((line = sr.ReadLine()) != null)
            {
                // Ignore commetns
                if (line.StartsWith("#"))
                {
                    continue;
                }

                // Ignore empty lines
                if (line == string.Empty)
                {
                    continue;
                }

                // If line starts with DEFAULT_ADMIN_CONTEXT then get its value
                if (line.StartsWith("DEFAULT_ADMIN_CONTEXT"))
                {
                    defaultAdminContext = line.Substring(line.IndexOf('=') + 1).Trim(new[] {'\"', ' '});
                }

                // If line starts with DIRECTORY_SERVERS then get its value
                if (line.StartsWith("DIRECTORY_SERVERS"))
                {
                    string[] serversPorts = line.Substring(line.IndexOf('=') + 1).Trim(new[] {'(', ')', ' '}).Split(',');
                    directoryServers = serversPorts.SelectMany(x =>
                    {
                        // If the server includes multiple port numbers, this needs to be handled
                        string[] serverPorts = x.Split(':');
                        if (serverPorts.Count() > 1)
                        {
                            return serverPorts.Skip(1).Select(y => string.Format("{0}:{1}", serverPorts.First(), y));
                        }

                        return new[] {x};
                    });
                }
            }
        }

        // Iterate through each LDAP server, and try to connect
        foreach (string directoryServer in directoryServers)
        {
            // Try to connect to LDAP server with using default admin contact
            try
            {
                var directoryEntry = new DirectoryEntry("LDAP://" + directoryServer + "/" + defaultAdminContext, null, null, AuthenticationTypes.Anonymous);
                var directorySearcher = new DirectorySearcher(directoryEntry, "(&(objectclass=orclNetService)(cn=" + serviceName + "))", new[] { "orclnetdescstring" }, SearchScope.Subtree);

                SearchResult searchResult = directorySearcher.FindOne();

                var value = searchResult.Properties["orclnetdescstring"][0] as byte[];

                if (value != null)
                {
                    connectionString = Encoding.Default.GetString(value);
                }

                // If the connection was successful, then not necessary to try other LDAP servers
                break;
            }
            catch
            {
                // If the connection to LDAP server not successful, try to connect to the next LDAP server
                continue;
            }
        }

        // If casting was not successful, or not found any TNS value, then result is an error message
        if (string.IsNullOrEmpty(connectionString))
        {
            connectionString = "TNS value not found in LDAP";
        }
    }
    else
    {
        // If ldap.ora doesn't exist, then return error message
        connectionString = "ldap.ora not found";
    }

    return connectionString;
}

To get you started, try:

using System.DirectoryServices;
...

DirectoryEntry de = new DirectoryEntry();
de.Username = "[email protected]";
de.Password = "password";
de.Path = "LDAP://DC=mysite,DC=com";
de.AuthenticationType = AuthenticationTypes.Secure;
DirectorySearcher des = new DirectorySearcher(de);

Now you should be able to hit ldap using the DirectorySearcher (filter, findone, etc) described here

Here a more complete sample based on the other answers:

using System;
using System.Data;
using System.DirectoryServices;
using System.Text;
using Oracle.ManagedDataAccess.Client;

namespace BAT
{
    internal class Program
    {
        private static void Main(string[] args)
        {
            string directoryServer = "myServer:389";
            string defaultAdminContext = "cn=OracleContext,dc=world";
            string serviceName = "MYDB";
            string userId = "MYUSER";
            string password = "MYPW";

            using (IDbConnection connection = GetConnection(directoryServer, defaultAdminContext, serviceName, userId,
                    password))
            {
                connection.Open();

                connection.Close();
            }

        }

        private static IDbConnection GetConnection(string directoryServer, string defaultAdminContext,
            string serviceName, string userId, string password)
        {
            string descriptor = ConnectionDescriptor(directoryServer, defaultAdminContext, serviceName);
            // Connect to Oracle
            string connectionString = $"user id={userId};password={password};data source={descriptor}";

            OracleConnection con = new OracleConnection(connectionString);
            return con;
        } 

        private static string ConnectionDescriptor(string directoryServer, string defaultAdminContext,
            string serviceName)
        {
            string ldapAdress = $"LDAP://{directoryServer}/{defaultAdminContext}";
            string query = $"(&(objectclass=orclNetService)(cn={serviceName}))";
            string orclnetdescstring = "orclnetdescstring";

            DirectoryEntry directoryEntry = new DirectoryEntry(ldapAdress, null, null, AuthenticationTypes.Anonymous);
            DirectorySearcher directorySearcher = new DirectorySearcher(directoryEntry, query, new[] { orclnetdescstring },
                SearchScope.Subtree);

            SearchResult searchResult = directorySearcher.FindOne();
            byte[] value = searchResult.Properties[orclnetdescstring][0] as byte[];

            if (value != null)
            {
                string descriptor = Encoding.Default.GetString(value);
                return descriptor;
            }

            throw new Exception("Error querying LDAP");
        }
    }
}