在 LINQ 表达式中不使用 IComparer

Question

I'm seeing some weird behavior and not able to figure out how it can occur. I have 2 sorted sets which I am using a buffers to reorder TCP packets by sequence number, and I am reading packets out of these sets within a lock statement.

Here is a simplified example:

SortedSet<Packet> in = new SortedSet<TcpPacket>(new SeqComparer()); // inbound buffer
SortedSet<Packet> out = new SortedSet<TcpPacket>(new SeqComparer()); // outbound buffer

public void BufferPacket(Packet packet) {
    // Some code which does not modify buffers
    bool buffer = isOutbound ? out : in;
    lock (buffer) {
        buffer.Add(packet);

        Packet bufferedPacket;
        // Removing retransmitted packets
        while (buffer.Count > 0 && (bufferedPacket = buffer.Min()).SeqNum < expected) {
            buffer.Remove(bufferedPacket);
        }

        // Process buffer
        while (buffer.Count > 0 && (bufferedPacket = buffer.Min()).SeqNum == expected) {
            buffer.Remove(bufferedPacket);
            expected += bufferedPacket.Length;
            ...
        }
    }
}

And something is causing this exception:

System.ArgumentException: At least one object must implement IComparable.
   at System.Collections.Comparer.Compare(Object a, Object b)
   at System.Linq.Enumerable.Min[TSource](IEnumerable`1 source)

which seems to imply that after checking Count > 0 , Min() found no objects. I am very confused as to how this could happen while locked.

Answer 1

This is answer extracted from comments below the question

This is not a lock related issue.

As per the comments, OP implemented IComparer<Packet> and passed it into SortedSet<Packet> . The comparer whilst it is used by the collection, Min method being a LINQ extension does not use it.

Instead, Min expects the Packet class to implement IComparable . And because the class does not seem to have implemented the interface, the exception is thrown.

Min does not compare objects when collection only contains 0 or 1 items. When the collection contains 2 or more items, Min compares Packet and the exception is thrown.

As per the OP's comment, this is where the confusion was:

Most of the time there is just 1 packet in the buffer (except for rare cases of out-of-order packets which is what I am trying to solve with this buffer. )

The error message however might also be misleading in this scenario:

At least one object must implement IComparable.

It implies there might be no items in the set.

To fix this issue, because the sets are already ordered, using First or Last as per the implementation of IComparer<Packet> should work.

Answer 2

Instead of using the Enumerable.Min extension method of the LINQ library, use the SortedSet.Min property instead:

Gets the minimum value in the SortedSet<T> , as defined by the comparer.