Operator + cannot be applied to operands of type IntPtr and int - .Net 3.5

Question

The following code (used for looking up DNS TXT records) works fine in .NET 4, however I have had to downgrade the project to .NET 3.5 for various reasons, and now I am being presented with several errors

public String DnsGetTxtRecord(String name) {
  const Int16 DNS_TYPE_TEXT = 0x0010;
  const Int32 DNS_QUERY_STANDARD = 0x00000000;
  const Int32 DNS_ERROR_RCODE_NAME_ERROR = 9003;
  const Int32 DNS_INFO_NO_RECORDS = 9501;
  var queryResultsSet = IntPtr.Zero;
  try {
    var dnsStatus = DnsQuery(
      name,
      DNS_TYPE_TEXT,
      DNS_QUERY_STANDARD,
      IntPtr.Zero,
      ref queryResultsSet,
      IntPtr.Zero
    );
    if (dnsStatus == DNS_ERROR_RCODE_NAME_ERROR || dnsStatus == DNS_INFO_NO_RECORDS)
      return null;
    if (dnsStatus != 0)
      throw new Win32Exception(dnsStatus);
    DnsRecordTxt dnsRecord;
    for (var pointer = queryResultsSet; pointer != IntPtr.Zero; pointer = dnsRecord.pNext) {
      dnsRecord = (DnsRecordTxt) Marshal.PtrToStructure(pointer, typeof(DnsRecordTxt));
      if (dnsRecord.wType == DNS_TYPE_TEXT) {
        var lines = new List<String>();
        var stringArrayPointer = pointer
          + Marshal.OffsetOf(typeof(DnsRecordTxt), "pStringArray").ToInt32();
        for (var i = 0; i < dnsRecord.dwStringCount; ++i) {
          var stringPointer = (IntPtr) Marshal.PtrToStructure(stringArrayPointer, typeof(IntPtr));
          lines.Add(Marshal.PtrToStringUni(stringPointer));
          stringArrayPointer += IntPtr.Size;
        }
        return String.Join(Environment.NewLine, lines);
      }
    }
    return null;
  }
  finally {
    const Int32 DnsFreeRecordList = 1;
    if (queryResultsSet != IntPtr.Zero)
      DnsRecordListFree(queryResultsSet, DnsFreeRecordList);
  }
}

[DllImport("Dnsapi.dll", EntryPoint = "DnsQuery_W", ExactSpelling = true, CharSet = CharSet.Unicode, SetLastError = true)]
static extern Int32 DnsQuery(String lpstrName, Int16 wType, Int32 options, IntPtr pExtra, ref IntPtr ppQueryResultsSet, IntPtr pReserved);

[DllImport("Dnsapi.dll")]
static extern void DnsRecordListFree(IntPtr pRecordList, Int32 freeType);

[StructLayout(LayoutKind.Sequential, CharSet = CharSet.Unicode)]
struct DnsRecordTxt {
  public IntPtr pNext;
  public String pName;
  public Int16 wType;
  public Int16 wDataLength;
  public Int32 flags;
  public Int32 dwTtl;
  public Int32 dwReserved;
  public Int32 dwStringCount;
  public String pStringArray;
}

var stringArrayPointer = pointer + Marshal.OffsetOf(typeof(DnsRecordTxt), "pStringArray").ToInt32(); triggers "Operator + cannot be applied to operands IntPtr and int

return String.Join(Environment.NewLine, lines); returns "The best overload method has some invalid arguments"

Assistance appreciated!

Answer 1

This

var stringArrayPointer = pointer + Marshal.OffsetOf(typeof(DnsRecordTxt), "pStringArray").ToInt32();

can be changed to

var stringArrayPointer = (IntPtr)((long)pointer + Marshal.OffsetOf(typeof(DnsRecordTxt), "pStringArray").ToInt32());

(in .NET 4.0 they added some basic operations to IntPtr, but in .NET 2.0/3.5 you had to cast it to int/long. Casting to long is safer because it's 64bit ready)

This

return String.Join(Environment.NewLine, lines);

to

return String.Join(Environment.NewLine, lines.ToArray());

(In .NET 4.0 they added a String.Join overload that accepted a IEnumerable<string> as a parameter. In .NET 2.0/3.5 you had to use the overload that accepts a string[])