Counting the number of messages in a Message Queue in .NET - safe version

MSMQCountSafe.cs

using System;
using System.Messaging;
using System.Runtime.InteropServices;
 
public static class MessageQueueExtensions {
 
    [DllImport("mqrt.dll")]
    private static extern int MQMgmtGetInfo([MarshalAs(UnmanagedType.BStr)]string computerName, [MarshalAs(UnmanagedType.BStr)]string objectName, ref MQMGMTPROPS mgmtProps);
 
    private const byte VT_NULL = 1;
    private const byte VT_UI4 = 19;
    private const int PROPID_MGMT_QUEUE_MESSAGE_COUNT = 7;
 
    //size must be 16
    [StructLayout(LayoutKind.Sequential)]
    private struct MQPROPVariant {
        public byte vt;       //0
        public byte spacer;   //1
        public short spacer2; //2
        public int spacer3;   //4
        public uint ulVal;    //8
        public int spacer4;   //12
    }
 
    //size must be 16 in x86 and 28 in x64
    [StructLayout(LayoutKind.Sequential)]
    private struct MQMGMTPROPS {
        public uint cProp;
        public IntPtr aPropID;
        public IntPtr aPropVar;
        public IntPtr status;
    }
 
    public static uint GetCount(this MessageQueue queue) {
        return GetCount(queue.Path);
    }
 
    private static uint GetCount(string path) {
        if (!MessageQueue.Exists(path)) {
            return 0;
        }
 
        MQMGMTPROPS props = new MQMGMTPROPS { cProp = 1 };
        try {
            props.aPropID = Marshal.AllocHGlobal(sizeof(int));
            Marshal.WriteInt32(props.aPropID, PROPID_MGMT_QUEUE_MESSAGE_COUNT);
 
            props.aPropVar = Marshal.AllocHGlobal(Marshal.SizeOf(typeof(MQPROPVariant)));
            Marshal.StructureToPtr(new MQPROPVariant { vt = VT_NULL }, props.aPropVar, false);
 
            props.status = Marshal.AllocHGlobal(sizeof(int));
            Marshal.WriteInt32(props.status, 0);
 
            int result = MQMgmtGetInfo(null, "queue=Direct=OS:" + path, ref props);
            if (result != 0 || Marshal.ReadInt32(props.status) != 0) {
                return 0;
            }
 
            MQPROPVariant propVar = (MQPROPVariant)Marshal.PtrToStructure(props.aPropVar, typeof(MQPROPVariant));
            if (propVar.vt != VT_UI4) {
                return 0;
            } else {
                return propVar.ulVal;
            }
        } finally {
            Marshal.FreeHGlobal(props.aPropID);
            Marshal.FreeHGlobal(props.aPropVar);
            Marshal.FreeHGlobal(props.status);
        }
    }
}
 
public class MessageQueueExtensionsTest {
 
    public static void Main() {
 
        string queueName = @".\Private$\MyQueue";
 
        if (MessageQueue.Exists(queueName)) {
            MessageQueue.Delete(queueName);
        }
 
        MessageQueue queue = MessageQueue.Create(queueName);
        Console.WriteLine("Count should be 0: " + queue.GetCount());
 
        queue.Send("ping", "ping");
        Console.WriteLine("Count should be 1: " + queue.GetCount());
 
        queue.Send("ping2", "ping2");
        Console.WriteLine("Count should be 2: " + queue.GetCount());
 
        queue.Receive();
        Console.WriteLine("Count should be 1: " + queue.GetCount());
 
        queue.Send("ping3", "ping3");
        Console.WriteLine("Count should be 2: " + queue.GetCount());
 
        MessageQueue.Delete(queueName);
        Console.WriteLine("Count should be 0: " + queue.GetCount());
    }
}