A C# Server Drop-down Control

Introduction

The sample code below shows the API's and code necessary to enumerate the servers on a domain. The ServerType enum determines the filter for the NetServerEnum call.

The tricky part (I found) was using the unsafe keyword to actually get the pointer to the array of structures, and then marshalling the pointers to strings (server names) back into managed code. I spent a couple of hours trying to get this to work...but the frustration has paid off.

The demo project contains a usercontrol that implements the NetServerEnum API. The user of the control then only has to set the ServerType property, and the drop-down will be populated.

public enum ServerTypeEnum
{
    steNone = 0,
    steWorkstation = 0x00000001,
    steAll = 0x00000002,
    steSQLServer = 0x00000004,
    steDomainController = 0x00000008
}

[sysimport(dll="netapi32.dll")]
private static extern void NetApiBufferFree([marshal(UnmanagedType.U4)]uint bufptr);

[sysimport(dll="netapi32.dll")] 
unsafe private static extern uint NetServerEnum([marshal(UnmanagedType.LPWStr)] string ServerName, 
        uint level,
        [marshal(UnmanagedType.LPVoid)]uint* bufptr,
        uint prefmaxlen,
        ref uint entriesread,
        ref uint totalentries,
        uint servertype,
        [marshal(UnmanagedType.LPWStr)] string domain, 
        uint resume_handle);

[System.Runtime.InteropServices.StructLayoutAttribute (LayoutKind.Sequential, CharSet=CharSet.Auto)]
public struct SERVER_INFO_101 
{ 
    public int dwPlatformID; 
    public int lpszServerName;
    public int dwVersionMajor; 
    public int dwVersionMinor; 
    public int dwType; 
    public int lpszComment;
}

protected void GetServers()
{
    string servername = null;
    string domain = "YourDomainName"; 
    uint level = 101, prefmaxlen = 0xFFFFFFFF, entriesread = 0, 
         totalentries = 0, resume_handle = 0; 
    
    cboServers.Items.Clear();

    unsafe
    {    
        //get a pointer to the server info structure
        SERVER_INFO_101* si = null;
        SERVER_INFO_101* pTmp;    //temp pointer for use when looping 
                                  //through returned (pointer) array

        //this api requires a pointer to a byte array...
        //which is actually a pointer to an array of SERVER_INFO_101 structures
        //If the domain parameter is NULL, the primary domain is implied. 
        uint nRes = NetServerEnum(servername, level, 
                (uint *) &si, prefmaxlen, ref entriesread, ref totalentries, 
                (uint)_ServerType, domain, resume_handle);
                
        if (nRes == 0)
        {
            if ((pTmp = si) != null)    //assign the temp pointer 
            {
                for (int i = 0; i < entriesread; i++)    //loop through the entries
                {
                    try
                    {
                        //the lpszServerName member of the structure contains a pointer to 
                        //the server name string...marshal the pointer from unmanaged
                        //memory to a managed string
                        cboServers.Items.Add(Marshal.PtrToStringAuto(pTmp->lpszServerName));
                    }
                    catch (Exception e)
                    {
                        MessageBox.Show(e.Message) ;
                    }
                    pTmp++;        // increment the pointer...essentially move to the next 
                                   // structure in the array
                }
            }
        }
        NetApiBufferFree((uint)si);
    }             
}

History

30 May 2002 - updated source files