I've posted this question in
'microsoft.public.dotnet.framework.compactframework' as well, but
despite the great help I still haven't solved the problem. So before
going the C++ route I would like to see if anybody in this group may
have the answer.

What I'm trying to do is change the route table of my Windows Mobile 5
Pocket PC (.Net CF) device (TyTN) using C#. This is probably similar as
doing it on standard CF. Using Iphlpapi.dll I can retrieve the route
table, but I am unable to change/add a route. I have specified the
following structs/pinvokes:

=========START===========
[StructLayout(LayoutKind.Sequential)]
public struct MIB_IPFORWARDROW {
   public UInt32 dwForwardDest;        //destination IP address.
   public UInt32 dwForwardMask;        //Subnet mask
   public UInt32 dwForwardPolicy;      //conditions for multi-path
route. Unused, specify 0.
   public UInt32 dwForwardNextHop;     //IP address of the next hop.
Own address?
   public UInt32 dwForwardIfIndex;     //index of interface
   public UInt32 dwForwardType;        //route type
   public UInt32 dwForwardProto;       //routing protocol.
   public UInt32 dwForwardAge;         //age of route.
   public UInt32 dwForwardNextHopAS;   //autonomous system number. 0
if not relevant
   public int    dwForwardMetric1;     //-1 if not used (goes for all
metrics)
   public int    dwForwardMetric2;
   public int    dwForwardMetric3;
   public int    dwForwardMetric4;
   public int    dwForwardMetric5;
}

[StructLayout(LayoutKind.Sequential)]
public struct MIB_IPFORWARDTABLE {
   public int dwNumEntries;            //number of route entries in
the table.
   public MIB_IPFORWARDROW[] table;
}

[DllImport("Iphlpapi.dll")]
[return: MarshalAs(UnmanagedType.U4)]
public static extern int CreateIpForwardEntry(ref MIB_IPFORWARDROW
pRoute);

[DllImport("Iphlpapi.dll")]
[return: MarshalAs(UnmanagedType.U4)]
public static extern int DeleteIpForwardEntry(ref MIB_IPFORWARDROW
pRoute);

[DllImport("Iphlpapi.dll")]
[return: MarshalAs(UnmanagedType.U4)]
public static extern int SetIpForwardEntry(ref MIB_IPFORWARDROW
pRoute);

[DllImport("Iphlpapi.dll")]
[return: MarshalAs(UnmanagedType.U4)]
public static extern int GetIpForwardTable(byte[] pIpForwardTable, out
int pdwSize, bool bOrder);

=========END===========

I invoke these methods as follows:

=========START===========
public int createIpForwardEntry(UInt32 destIPAddress, UInt32 destMask,
                                           UInt32 nextHopIPAddress,
UInt32 ifIndex) {
   MIB_IPFORWARDROW mifr = new MIB_IPFORWARDROW();
   mifr.dwForwardDest = destIPAddress;
   mifr.dwForwardMask = destMask;
   mifr.dwForwardPolicy = Convert.ToUInt32(0);
   mifr.dwForwardNextHop = nextHopIPAddress;
   mifr.dwForwardIfIndex = ifIndex; //?
   mifr.dwForwardType = Convert.ToUInt32(4);
   mifr.dwForwardProto = Convert.ToUInt32(3);
   mifr.dwForwardAge = Convert.ToUInt32(0);
   mifr.dwForwardNextHopAS = Convert.ToUInt32(0);
   mifr.dwForwardMetric1 = -1;
   mifr.dwForwardMetric2 = -1;
   mifr.dwForwardMetric3 = -1;
   mifr.dwForwardMetric4 = -1;
   mifr.dwForwardMetric5 = -1;
   return CreateIpForwardEntry(ref mifr);
}
=========END===========

All IP addresses are added as unsigned ints (double checked whether
they are correct. I even tried adding a row that I just successfully
retrieved (thereby skipping my own structure). If that gives me an
error code 87 (invalid param).

Is there anybody here that has successfully done this or has a few
relevant pointers? Preferably C# examples, but C++ examples are a step
in the right direction. Thanks!