运算符+不能应用于IntPtr和int-.Net 3.5类型的操作数

Question

以下代码（用于查找DNS TXT记录）在.NET 4中运行良好，但由于各种原因，我不得不将该项目降级到.NET 3.5，现在我遇到了几个错误

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();触发"运算符+不能应用于操作数IntPtr和int

return String.Join(Environment.NewLine, lines);返回"最佳重载方法有一些无效参数"

感谢协助！

Answer 1

此

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

可以更改为

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

（在.NET 4.0中，他们向IntPtr添加了一些基本操作，但在.NET 2.0/3.5中，您必须将其转换为int/long。转换为long更安全，因为它已准备好64位）

这个

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

至

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

（在.NET 4.0中，他们添加了一个接受IEnumerable<string>作为参数的String.Join重载。在.NET 2.0/3.5中，您必须使用接受string[]的重载）

Answer 2

您不能添加到IntPtr，但可以执行以下操作：

long address = pointer.ToInt64()
             + Marshal.OffsetOf(typeof(DnsRecordTxt), "pStringArray").ToInt64();
var stringArrayPointer = (IntPtr)address;

（正如@xanatos在回答中所说，使用ToInt64而不是ToInt32使其在x64和x86中都能工作。）

---

至于String.Join的问题，您需要向它传递一个数组作为第二个参数：

string.Join(Environment.NewLine, lines.ToArray());