C# Signature:

[DllImport("Netapi32.dll", CharSet=CharSet.Auto, SetLastError=true)]
static extern int DsGetDcName
(
    [MarshalAs(UnmanagedType.LPTStr)]
    string ComputerName,
    [MarshalAs(UnmanagedType.LPTStr)]
    string DomainName,
    [In] GuidClass DomainGuid,
    [MarshalAs(UnmanagedType.LPTStr)]
    string SiteName,
    int Flags,
    out IntPtr pDOMAIN_CONTROLLER_INFO
);

VB Signature:

Declare Function DsGetDcName Lib "netapi32.dll" (TODO) As TODO

User-Defined Types:

DOMAIN_CONTROLLER_INFO, GuidClass

Notes:

Pointer to a PDOMAIN_CONTROLLER_INFO value that receives a pointer to a DOMAIN_CONTROLLER_INFO structure that contains data about the domain controller selected. This structure is allocated by DsGetDcName. The caller must free the structure using the NetApiBufferFree function when it is no longer required.

Tips & Tricks:

Please add some!

Sample Code:

[StructLayout(LayoutKind.Sequential, CharSet = CharSet.Unicode)]
  struct DOMAIN_CONTROLLER_INFO
  {  
    [MarshalAs(UnmanagedType.LPTStr)]  
    public string DomainControllerName;
    [MarshalAs(UnmanagedType.LPTStr)]    
    public string DomainControllerAddress;
    public uint   DomainControllerAddressType;
    public Guid   DomainGuid;
    [MarshalAs(UnmanagedType.LPTStr)]    
    public string DomainName;
    [MarshalAs(UnmanagedType.LPTStr)]    
    public string DnsForestName;
    public uint   Flags;
    [MarshalAs(UnmanagedType.LPTStr)]    
    public string DcSiteName;
    [MarshalAs(UnmanagedType.LPTStr)]    
    public string ClientSiteName;
  }

[DllImport("Netapi32.dll", CharSet=CharSet.Auto, SetLastError=true)]        
  static extern int DsGetDcName  
  (
    [MarshalAs(UnmanagedType.LPTStr)]
    string ComputerName,
    [MarshalAs(UnmanagedType.LPTStr)]
    string DomainName,
    [In] int DomainGuid,
    [MarshalAs(UnmanagedType.LPTStr)]
    string SiteName,
    [MarshalAs(UnmanagedType.U4)]
    DSGETDCNAME_FLAGS flags,
    out IntPtr pDOMAIN_CONTROLLER_INFO
  );

  [DllImport("Netapi32.dll", SetLastError=true)]    
   static extern int NetApiBufferFree(IntPtr Buffer);

  [Flags]
  public enum DSGETDCNAME_FLAGS : uint
  {
    DS_FORCE_REDISCOVERY = 0x00000001,
    DS_DIRECTORY_SERVICE_REQUIRED = 0x00000010,
    DS_DIRECTORY_SERVICE_PREFERRED = 0x00000020,
    DS_GC_SERVER_REQUIRED = 0x00000040,
    DS_PDC_REQUIRED = 0x00000080,
    DS_BACKGROUND_ONLY = 0x00000100,
    DS_IP_REQUIRED = 0x00000200,
    DS_KDC_REQUIRED = 0x00000400,
    DS_TIMESERV_REQUIRED = 0x00000800,
    DS_WRITABLE_REQUIRED = 0x00001000,
    DS_GOOD_TIMESERV_PREFERRED = 0x00002000,
    DS_AVOID_SELF = 0x00004000,
    DS_ONLY_LDAP_NEEDED = 0x00008000,
    DS_IS_FLAT_NAME = 0x00010000,
    DS_IS_DNS_NAME = 0x00020000,
    DS_RETURN_DNS_NAME = 0x40000000,
    DS_RETURN_FLAT_NAME = 0x80000000
   }

   private DOMAIN_CONTROLLER_INFO GetDomainInfo()
   {
    DOMAIN_CONTROLLER_INFO domainInfo;
    const int ERROR_SUCCESS = 0;
    IntPtr pDCI = IntPtr.Zero;
    try
    {
     int val = DsGetDcName("","",0,"",
     DSGETDCNAME_FLAGS.DS_DIRECTORY_SERVICE_REQUIRED|
     DSGETDCNAME_FLAGS.DS_RETURN_DNS_NAME|
     DSGETDCNAME_FLAGS.DS_IP_REQUIRED, out pDCI);
       //check return value for error
       if(ERROR_SUCCESS == val)
       {
    domainInfo = (DOMAIN_CONTROLLER_INFO)Marshal.PtrToStructure(pDCI, typeof(DOMAIN_CONTROLLER_INFO));

    string msg = "Forest : " + domainInfo.DnsForestName  + "\r\n";
    msg += "DC-Site: " + domainInfo.DomainControllerName + "\r\n";
    msg += " Client: " + domainInfo.ClientSiteName + "\r\n";
    System.Windows.Forms.MessageBox.Show ( msg);
       }
    else
       {
     throw new Win32Exception(val);
       }
    }
  finally
  {
    NetApiBufferFree(pDCI);
  }
  return domainInfo;
}

Alternative Managed API:

System.DirectoryServices.ActiveDirectory.Domain.GetCurrentDomain()