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    <p>Dictionary expects immutable object as a key, with a stable GetHashCode / Equals implementation. This means that after object is placed into dictionary, value returned by GetHashCode should not change, and any changes made to this object should not affect Equals method.</p> <p>Although PhysicalAddress class was designed immutable, it still contains a few extension points, where its immutability is flawed. </p> <p>First, it can be changed through input byte array, which is not copied but passed by reference, like this:</p> <pre><code>var data = new byte[] { 1,2,3 }; var mac = new PhysicalAddress(data); data[0] = 0; </code></pre> <p>Second, PhysicalAddress is not a sealed class, and can be changed by derived implementation through overriding Constructor / GetHashCode / Equals methods. But this use case looks more like a hack, so we will ignore it, as well as modifications through reflection.</p> <p>Your situation can only be achieved by first placing PhysicalAddress object into dictionary, and then modifying its source byte array, and then wrapping it into new PhysicalAddress instance.</p> <p>Luckily, PhysicalAddress' GetHashCode implementation computes hash only once, and if same instance is modified, it is still placed into same dictionary bucket, and located again by Equals.</p> <p>But, if source byte array is passed into another instance of PhysicalAddress, where hash was not yet computed - hash is recomputed for new byte[] value, new bucket is located, and duplicate is inserted into dictionary. In rare cases, same bucket can be located from new hash, and again, no duplicate is inserted.</p> <p>Here's the code which reproduces the problem:</p> <pre><code>using System; using System.Collections.Generic; using System.Net.NetworkInformation; class App { static void Main() { var data = new byte[] { 1,2,3,4 }; var mac1 = new PhysicalAddress(data); var mac2 = new PhysicalAddress(data); var dictionary = new Dictionary&lt;PhysicalAddress,string&gt;(); dictionary[mac1] = "A"; Console.WriteLine("Has mac1:" + dictionary.ContainsKey(mac1)); //Console.WriteLine("Has mac2:" + dictionary.ContainsKey(mac2)); data[0] = 0; Console.WriteLine("After modification"); Console.WriteLine("Has mac1:" + dictionary.ContainsKey(mac1)); Console.WriteLine("Has mac2:" + dictionary.ContainsKey(mac2)); dictionary[mac2] = "B"; foreach (var kvp in dictionary) Console.WriteLine(kvp.Key + "=" + kvp.Value); } } </code></pre> <p>Note the commented line - if we will uncomment it, "ContainsKey" method will precompute hash for mac2, and it will be the same even after modification.</p> <p>So my recommendation is to locate piece of code which generates PhysicalAddress instances, and create new byte array copy for each constructor call.</p>
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    1. COThank you for the well constructed answer :) Unfortunately we are already creating a new byte array with each constructor call. See the code recently added to the question. The MacAddress property on TagReport is never assigned to after that and is only used. Also the instance created there eventually makes it's way to the GetTagReports call.
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    2. COWhat happens with physicalAddressBytes after TagReport is constructed? Is it reused somewhere? What is the average size of dictionary? How often is it modified?
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    3. COFew more ideas - try to test memory of server where you are able to reproduce this issue. Put all your dictionary access methods into lock(), to make sure there are no multithreading problems.
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