C# Signature:

/// <summary>
///     Retrieves the connected state of the local system.
///     <para>
///     For more information go to
///     https://msdn.microsoft.com/en-us/library/windows/desktop/aa384702%28v=vs.85%29.aspx
///     </para>
/// </summary>
/// <param name="description">
///     C++ ( lpdwFlags [out]. Type: LPDWORD )<br />Pointer to a variable that receives the connection description. This
///     parameter may return a valid flag even when the function returns FALSE. This parameter can be one or more of the
///     following values.
///     <list type="table">
///     <listheader>
///         <term>Internet Connection State Possible Flags</term>
///     </listheader>
///     <item>
///         <term>INTERNET_CONNECTION_CONFIGURED (0x40)</term>
///         <description>
///         Local system has a valid connection to the Internet, but it might or might not be currently
///         connected.
///         </description>
///     </item>
///     <item>
///         <term>INTERNET_CONNECTION_LAN (0x02)</term>
///         <description>Local system uses a local area network to connect to the Internet.</description>
///     </item>
///     <item>
///         <term>INTERNET_CONNECTION_MODEM (0x01)</term>
///         <description>Local system uses a modem to connect to the Internet.</description>
///     </item>
///     <item>
///         <term>INTERNET_CONNECTION_MODEM_BUSY (0x08)</term>
///         <description>No longer used.</description>
///     </item>
///     <item>
///         <term>INTERNET_CONNECTION_OFFLINE (0x20)</term>
///         <description>Local system is in offline mode.</description>
///     </item>
///     <item>
///         <term>INTERNET_CONNECTION_PROXY (0x04)</term>
///         <description>Local system uses a proxy server to connect to the Internet.</description>
///     </item>
///     <item>
///         <term>INTERNET_RAS_INSTALLED (0x10)</term>
///         <description>Local system has RAS installed.</description>
///     </item>
///     </list>
/// </param>
/// <param name="reservedValue">C++ ( dwReserved [in].Type: )<br />This parameter is reserved and must be 0.</param>
/// <returns>
///     <c>true</c> if there is an active modem or a LAN Internet connection, <c>false</c> if there is no Internet
///     connection, or if all possible Internet connections are not currently active.
/// </returns>
/// <remarks>
///     A return value of TRUE from InternetGetConnectedState indicates that at least one connection to the Internet is
///     available. It does not guarantee that a connection to a specific host can be established. Applications should
///     always check for errors returned from API calls that connect to a server. InternetCheckConnection can be called to
///     determine if a connection to a specific destination can be established.
///     <br />A return value of TRUE indicates that either the modem connection is active, or a LAN connection is active
///     and a proxy is properly configured for the LAN.A return value of FALSE indicates that neither the modem nor the LAN
///     is connected.If FALSE is returned, the INTERNET_CONNECTION_CONFIGURED flag may be set to indicate that autodial is
///     configured to "always dial" but is not currently active.If autodial is not configured, the function returns FALSE.
///     <br />Like all other aspects of the WinINet API, this function cannot be safely called from within DllMain or the
///     constructors and destructors of global objects.
///     <br />Note WinINet does not support server implementations. In addition, it should not be used from a service. For
///     server implementations or services use Microsoft Windows HTTP Services (WinHTTP).
/// </remarks>
[DllImport("wininet.dll", SetLastError=true)]
extern static bool InternetGetConnectedState(out int lpdwFlags, int dwReserved);

[Flags]
enum ConnectionStates
{
    Modem = 0x1,
    LAN = 0x2,
    Proxy = 0x4,
    RasInstalled = 0x10,
    Offline = 0x20,
    Configured = 0x40,
}

VB Signature:

  Private Declare Function InternetGetConnectedState Lib "wininet.dll" _
  (ByRef lpdwFlags As Int32, ByVal dwReserved As Int32) As Boolean

Private Enum ConnectionStates
    Modem = &H1
    LAN = &H2
    Proxy = &H4
    RasInstalled = &H10
    Offline = &H20
    Configured = &H40
End Enum

User-Defined Types:

InternetGetConnectedStateFlags

Notes:

Tips & Tricks:

Please add some!

Sample Code:

C: http://support.microsoft.com/default.aspx?scid=kb;EN-US;242558

C#:

using System;
using System.Runtime.InteropServices;

namespace ConsoleApplication2
{
  internal class Program
  {
   [DllImport("wininet.dll", SetLastError = true)]
   private static extern bool InternetGetConnectedState(out int lpdwFlags, int dwReserved);

   private static void Main(string[] args)
   {
    int flags;
    bool isConnected = InternetGetConnectedState(out flags, 0);
    Console.WriteLine(string.Format("Is connected :{0} Flags:{1}", isConnected, flags));
   }
  }
}

Function Get_InternetConnectedState(ByRef p_lngFlags As Long, Optional ByRef p_return_str As String = "") As Boolean

   Dim lngFlags As Long

      lngFlags = 0
      Get_InternetConnectedState = False

      If InternetGetConnectedState(lngFlags, 0) Then
     'connected.
     If lngFlags And ConnectionStates.LAN Then
        'LAN connection.
        p_return_str = "LAN connection."
     ElseIf lngFlags And ConnectionStates.Modem Then
        'Modem connection.
        p_return_str = "Modem connection."
     ElseIf lngFlags And ConnectionStates.Proxy Then
        'Proxy connection.
        p_return_str = "Proxy connection."
     End If

     Get_InternetConnectedState = True

      Else
     'not connected.
     p_return_str = "Not connected."
     Get_InternetConnectedState = False
      End If

      p_lngFlags = lngFlags

End Function

Alternative Managed API:

Try NetworkInterface.GetIsNetworkAvailable() in the the System.Net.NetworkInformation namespace. It returns true when a network is available and false otherwise. See also: http://msdn2.microsoft.com/en-us/library/system.net.networkinformation.networkinterface.getisnetworkavailable(VS.80).aspx. NOTE: this is all new in .NET 2.0!

Direct Link: http://msdn.microsoft.com/library/default.asp?url=/library/en-us/wininet/wininet/internetgetconnectedstate.asp

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