I want to locate the DNS server, Global catalog server and domain contorller

I am looking for some code in either vb or C# to locate the DNS server, the Global Catalog server and the primary domain controller.  Please, don't offer links unless they actually articulate source code.  I am comfortable with AD programming and can find all computers as demonstrated below, but I just want to find servers with specific purposes.
    Public Function GetCatalogServer() As String
        Dim oDE As DirectoryEntry
        Dim oSearch As DirectorySearcher
        Dim oResults As SearchResultCollection

            oDE = GetDirectoryObject("Default")
            oSearch = New DirectorySearcher(oDE)
            oSearch.PageSize = 1000
            oSearch.Filter = "(objectClass=Computer)"
            oSearch.SearchScope = SearchScope.Subtree

            oResults = oSearch.FindAll()

        Catch ex As Exception
            Me.Severity = 3
            Return (Nothing)
        End Try

        Return (Me.m_sServer)
    End Function

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To identify GC

strComputer = "atl-dc-01"
Set objRootDSE = GetObject("LDAP://" & strComputer & "/rootDSE")
strDsServiceDN = objRootDSE.Get("dsServiceName")
Set objDsRoot  = GetObject("LDAP://" & strComputer & "/" & strDsServiceDN)
intOptions = objDsRoot.Get("options")
If intOptions And NTDSDSA_OPT_IS_GC Then
    WScript.Echo strComputer & " is a global catalog server."
    Wscript.Echo strComputer & " is not a global catalog server."
End If

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To identify Domain controllers:

Set objDomain = getObject("LDAP://rootDse")
objDC = objDomain.Get("dnsHostName")
Wscript.Echo objDC

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wjolsonAuthor Commented:

I presume that in your first example, you are suggesting that I compare a computer in the domain against the constant to determine if it is a DC.  If that is the case, I would have to get all computers in the domain; which I can do.  I was hoping the process was not so brute force.  Hey, any thought on knowing if you have a workgroup server or an actual Domain Controller.

I will test these 2 options out later today.
wjolsonAuthor Commented:
I still wanted to know about a DC, but no big deal: 2 out of 3 ain't bad.  Besides, the leaves me with something more to learn without being totally spoon fed.  Thanks.
Unfortunately GC only can be found in a DC. So, knowing the DC status before you check for GC would be easier.

Also, you may want to take a look at this one--to determine the role of a computer(DC, member sever or workstation):
strComputer = "."
Set objWMIService = GetObject("winmgmts:" _
    & "{impersonationLevel=impersonate}!\\" & strComputer & "\root\cimv2")
Set colComputers = objWMIService.ExecQuery _
    ("Select DomainRole from Win32_ComputerSystem")
For Each objComputer in colComputers
    Select Case objComputer.DomainRole 
        Case 0 
            strComputerRole = "Standalone Workstation"
        Case 1        
            strComputerRole = "Member Workstation"
        Case 2
            strComputerRole = "Standalone Server"
        Case 3
            strComputerRole = "Member Server"
        Case 4
            strComputerRole = "Backup Domain Controller"
        Case 5
            strComputerRole = "Primary Domain Controller"
    End Select
    Wscript.Echo strComputerRole

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