CODE EXAMPLE FOR CSHARP

Get the Default gateway address c#

[DllImport("iphlpapi.dll", CharSet = CharSet.Auto)]
private static extern int GetBestInterface(UInt32 destAddr, out UInt32 bestIfIndex);

public static IPAddress GetGatewayForDestination(IPAddress destinationAddress)
{
    UInt32 destaddr = BitConverter.ToUInt32(destinationAddress.GetAddressBytes(), 0);

    uint interfaceIndex;
    int result = GetBestInterface(destaddr, out interfaceIndex);
    if (result != 0)
        throw new Win32Exception(result);

    foreach (var ni in NetworkInterface.GetAllNetworkInterfaces())
    {
        var niprops = ni.GetIPProperties();
        if (niprops == null)
            continue;

        var gateway = niprops.GatewayAddresses?.FirstOrDefault()?.Address;
        if (gateway == null)
            continue;

        if (ni.Supports(NetworkInterfaceComponent.IPv4))
        {
            var v4props = niprops.GetIPv4Properties();
            if (v4props == null)
                continue;

            if (v4props.Index == interfaceIndex)
                return gateway;
        }

        if (ni.Supports(NetworkInterfaceComponent.IPv6))
        {
            var v6props = niprops.GetIPv6Properties();
            if (v6props == null)
                continue;

            if (v6props.Index == interfaceIndex)
                return gateway;
        }
    }

    return null;
}