While developing Internet applications, this is important to check whether the internet connection is available or not. We can achieve this with few lines of code.

Step 1: Include the following Namespace to your class.

using System.Runtime.InteropServices;

This Namespace is necessary to call a function exported from unmanaged DLL.

Step 2:  Use the following lines of code to import a function from the unmanaged DLL.

[DllImport("wininet.dll",CharSet=CharSet.Auto)] 

static extern bool InternetGetConnectedState(ref ConnectionState lpdwFlags, int dwReserved); 

Parameters

lpdwFlags - Pointer to a variable that receives the connection description. This parameter can be one or more of the following values:

dwReserved - Must be zero.

Return Values: Returns TRUE if there is an Internet connection, or FALSE otherwise.

Create a following enumeration for first parameter [lpdwFlags].

[Flags] enum ConnectionState: int
{
  INTERNET_CONNECTION_MODEM = 0x1,
  INTERNET_CONNECTION_LAN = 0x2,
  INTERNET_CONNECTION_PROXY = 0x4,
  INTERNET_RAS_INSTALLED = 0x10,
  INTERNET_CONNECTION_OFFLINE = 0x20,
  INTERNET_CONNECTION_CONFIGURED = 0x40
}

Step 3: Create a instance of ConnectionState enumeration type and set value to 0.

ConnectionState Description = 0;

Step 4: Call the InternetGetConnectedState function in your class.

txtConnectionFlag.Text = InternetGetConnectedState(ref Description, 0).ToString();
txtConnectionState.Text = Description.ToString();