Merlin’s Delphi Forge

Whenever you try to connect to connect to a Internet server while no Internet connection is present you can be almost certain an Access Violation will fire.

The only way around this is to check whether a connection is present or not. You can use the code below.

uses
  Windows, SysUtils, Registry, WinSock, WinInet;
...
type
  TConnectionType = (ctNone, ctProxy, ctDialup);
...
function ConnectedToInternet: TConnectionType;
function RasConnectionCount: Integer;
...
implementation
...
const
  ERROR_BUFFER_TOO_SMALL = 603;
  RAS_MAXENTRYNAME = 256;
  RAS_MAXDEVICENAME = 128;
  RAS_MAXDEVICETYPE = 16;
type
  ERasError = class(Exception);

  HRASConn = DWORD;
  PRASConn = ^TRASConn;
  TRASConn = record
    dwSize: DWORD;
    rasConn: HRASConn;
    szEntryName: array[0..RAS_MAXENTRYNAME] of Char;
    szDeviceType: array[0..RAS_MAXDEVICETYPE] of Char;
    szDeviceName: array[0..RAS_MAXDEVICENAME] of Char;
  end;

  TRasEnumConnections =
    function(RASConn: PrasConn; // Buffer to receive Connections data
      var BufSize: DWORD; // Size of buffer (in bytes)
      var Connections: DWORD // Number of Connections written to buffer
    ): Longint;
  stdcall;
...
function ConnectedToInternet: TConnectionType;
var
  UseProxy: LongWord;
  Reg: TRegistry;
begin
  Result := ctNone;
  Reg := TRegistry.Create;
  with Reg do
    try
      try
        RootKey := HKEY_CURRENT_USER;
        if OpenKey('Software\\Microsoft\\Windows\\CurrentVersion\\Internet settings', False) then begin
          if GetDataType('ProxyEnable') = rdBinary then
            ReadBinaryData('ProxyEnable', UseProxy, SizeOf(Longword))
          else
            UseProxy := Integer(ReadBool('ProxyEnable'));
          if (UseProxy <> 0) and (ReadString('ProxyServer') <> '') then
            Result := ctProxy;
        end;
      except
        // No connection through proxy
      end;
    finally
      Free;
    end;

  if Result = ctNone then
    if RasConnectionCount > 0 then
      Result := ctDialup;
end;

function RasConnectionCount: Integer;
var
  Conns: array[1..4] of TRasConn;
  BufSize, NumConns: DWORD;
  RasDLL: HInst;
  RasResult: Longint;
  RasEnums: TRasEnumConnections;
begin
  Result := 0;
  // Load the RAS DLL
  RasDLL := LoadLibrary('rasapi32.dll');
  if RasDLL = 0 then
    Exit;

  try
    RasEnums := GetProcAddress(RasDLL, 'RasEnumConnectionsA');
    if @RasEnums = nil then
      raise ERasError.Create('RasEnumConnectionsA not found in rasapi32.dll');

    Conns[1].dwSize := SizeOf(Conns[1]);
    BufSize         := SizeOf(Conns);
    RasResult       := RasEnums(@Conns, BufSize, NumConns);
    if (RasResult = 0) or (Result = ERROR_BUFFER_TOO_SMALL) then
      Result := NumConns;
  finally
    FreeLibrary(RasDLL);
  end;
end;

Tags: access violation, borland, class exception, connection, Delphi, dialup, hkey current user, ie, internet, internet explorer, internet server, internet settings, proxy, proxyserver, ras, System, tip, trick, windows, winsock