Network computer picker control

• CompPicker Class Library - 282 Kb
• NetSend Sample App - 283 Kb

Introduction

A Windows class library for selecting networked computers.

Background

While having authored several Windows Forms apps, I've frequently needed to browse the network specifically for selecting a computer, however there is no managed method to accomplish this. I was inspired by Michael Potter's Finding SQL Servers on the Network article, so I decided to take it a little further and allow more granular control of the types of computers one can select.

Enumerating Computers

The first thing we have to do is define the function which will perform this task for us. NetServerEnum is located in the NetApi32.dll library.

// enumerates network computers

[DllImport("Netapi32", CharSet=CharSet.Unicode)]
private static extern int NetServerEnum( 
    string servername,        // must be null

    int level,        // 100 or 101

    out IntPtr bufptr,        // pointer to buffer receiving data

    int prefmaxlen,        // max length of returned data

    out int entriesread,    // num entries read

    out int totalentries,    // total servers + workstations

    uint servertype,        // server type filter

    [MarshalAs(UnmanagedType.LPWStr)]
    string domain,        // domain to enumerate

    IntPtr resume_handle );

The third parameter of NetServerEnum will populate a structure containing information about the computers it finds. With the exception of sv101_platform_id & sv101_type, these values are exposed as public properties in the NetworkComputers struct.

// Holds computer information

[StructLayoutAttribute(LayoutKind.Sequential, CharSet=CharSet.Unicode)]
internal struct SERVER_INFO_101
{
    public int sv101_platform_id;
    public string sv101_name;
    public int sv101_version_major;
    public int sv101_version_minor;
    public int sv101_type;
    public string sv101_comment;
}

To get our list, we simply call one of the CompEnum constructors and pass it one of the ServerType values

ce = new CompEnum(CompEnum.ServerType.SV_TYPE_DOMAIN_CTRL | 
  CompEnum.ServerType.SV_TYPE_MASTER_BROWSER)

or it's equivalent bit-mapped value (GetServerTypeValues performs this step for us)

private void GetServerTypeValues(object sender, System.EventArgs e)
{
    int filterVal = 0x00;
    bool itemsChecked = false;
    foreach (CheckBox cb in groupBoxServerTypes.Controls)
    {
        if (cb.Checked)
        {
            filterVal += Int32.Parse((string)cb.Tag,
                System.Globalization.NumberStyles.HexNumber);
            itemsChecked = true;
        }
    }

    checkBoxAll.Enabled = !itemsChecked;
    DisplayComputerTypes((uint)filterVal);
}

Now we can enumerate through our collection of computers.

internal void DisplayComputerTypes(uint serverType)
{             
    Cursor.Current = Cursors.WaitCursor;
    lbComputers.Items.Clear();
    ce = new CompEnum(serverType, cbDomainList.SelectedItem.ToString());
    int numServer = ce.Length;
    
    if (ce.LastError.Length == 0)
    {
        IEnumerator enumerator = ce.GetEnumerator();

        int i = 0;
        while (enumerator.MoveNext())
        {
            lbComputers.Items.Add(ce[i].Name);
            i++;
        }
    }
...

Enumerating SQL Servers Using SQL-DMO

Short for SQL Server Distributed Management Objects, SQL-DMO is a far more reliable way to retrieve the names of SQL Servers on your network. My code first performs a check to see if SQL-DMO is possible, otherwise it uses the regular NetServerEnum API.

private void GetSqlServersUsingSQLDMO(object sender, System.EventArgs e)
{
    SQLDMO.Application app = new SQLDMO.ApplicationClass();
    SQLDMO.NameList nameList = app.ListAvailableSQLServers();
    string srvName = "";
    _sqlServerList = new string[nameList.Count];

    for (int i=0; i<nameList.Count; i++)
    {                
        srvName = nameList.Item(i + 1);
        _sqlServerList[i] = srvName;
    }
}

ListAvailableSQLServers() returns a NameList object that enumerates SQL Server names. From here we use the Item method of NameList to retrieve the actual server name. That's all there is to it.

Sample Usage

CompPicker cp = new CompPicker();

// show selected computer

if (cp.ShowDialog(this) == DialogResult.OK)
    MessageBox.Show(this, cp.SelectedComputerName);

Extending

To extend the library to fit your needs, simply add or remove checkboxes in the groupBoxServerTypes group box and place their tag values equal to one of the ServerType bitmap values. These values are originally defined in LMServer.h and can also be found in the CompEnum class.

Performance Issues

Performance is hampered in a non-domain environment, presumably because there is no browse master defined? The same holds true when browsing in a different domain than the one you are currently in.

Compatibility

Since the NetServerEnum WinAPI is only available for Windows NT or greater, it will not work on Win9x machines.

History

• Version 1.0 - 06.01.2004 - First release version.
• Version 1.1 - 07.15.2004 - Utilize SQL-DMO for enumerating SQL Servers, if available.