C# codes could not connect to ldap after it restart

Client: win XP embedded; server: win server 2008

code:

    private DirectorySearcher getDirectorySearcher()
    {
        Runtime.Log.Info("Root Path Format: " + _rootPath);
        _directoryEntry = new DirectoryEntry(_rootPath, _username, _password);
        try
        {
            _directoryEntry.RefreshCache();

            return new DirectorySearcher(_directoryEntry)
            {
                PageSize = 1000
            };
        }
        catch (Exception ex)
        {
            _directoryEntry.Close();
            _directoryEntry.Dispose();
            throw;
        }
    }

These codes works well in normal situation.

It will say error after connecting to the not started LDAP server, but when LDAP server restart-up, it also could not connect to the server. restart the exe in client will resolve this issue.

The error info is:

    Exception authenticating user 'Administrator'

    System.Runtime.InteropServices.COMException (0x8007203A): The server is not operational.

       at System.DirectoryServices.DirectoryEntry.Bind(Boolean throwIfFail)
       at System.DirectoryServices.DirectoryEntry.Bind()
       at System.DirectoryServices.DirectoryEntry.RefreshCache()
       at ThreeVR.EnterpriseServer.Ldap.LdapContext.getDirectorySearcher()

I tried every method to refresh or dispose DirectoryEntry and DirectorySearcher objects, but it not work.

Could anybody give me some advises? Any ideas?

May be flushing the DNS cache can help?

using System.Runtime.InteropServices;

...

[DllImport("dnsapi.dll",EntryPoint="DnsFlushResolverCache")]
private static extern UInt32 DnsFlushResolverCache ();

...

public static void FlushResolverCache()
{
    UInt32 result = DnsFlushResolverCache();
}