Microsoft Enterprise Library Caching Gotcha

Recently, a developer where I work added caching to some methods that retrieve contacts from a table in our database. Afterwords, a tester complained that when he went to one screen that allows selecting a contact, he saw the text "[Select a contact]" repeated. Not only that, every time he returned to that screen, the problem became worse.

Before caching was added, the method that retrieves the list of contacts got them from the database, then it added the text "[Select a contact]" to the beginning of the list. When caching was added, the list was retrieved from the cache (if available), and then the text "[Select a contact]" was added to the beginning.

It turns out that when a list is added to the cache, and then later the list is modified, that modified list will be returned the next time the cache is accessed. Here is some sample code to make it more clear:

private void btnTestCaching_Click(object sender, EventArgs e)
{
  List<string> testList = TestCaching(1);
  PrintList(testList);
  testList = TestCaching(2);
  PrintList(testList);
}

private void PrintList(List<string> testList)
{
  foreach (string s in testList)
  {
      Debug.WriteLine(s);
  }
}

private List<string> TestCaching(int id)
{
  ICacheManager cache = CacheFactory.GetCacheManager();
  List<string> testList = cache["TestList"] as List<string>;
  if (testList == null)
  {
      testList = new List<string>();
      testList.Add("Test 1");
      testList.Add("Test 2");
      testList.Add("Test 3");

      AbsoluteTime expiry = new AbsoluteTime(new TimeSpan(2, 0, 0, 0));
      cache.Add("TestList", testList, CacheItemPriority.High, null, new ICacheItemExpiration[] { expiry });
  }
  testList.Add("[Select a contact] " + id.ToString());
  return testList;
}

I added a counter to make it slightly more clear what's happening. Here's the output from Visual Studio:

Test 1
Test 2
Test 3
[Select a contact] 1
Test 1
Test 2
Test 3
[Select a contact] 1
[Select a contact] 2

I was surprised by the outcome. I thought that the list would be persisted only when cache.Add() is called, apparently that's not the case.

Viewing GAC Assemblies in an Explorer Window

I saw a neat trick somewhere on the internet...at the moment I can't find the original source. I like to give attributions where possible, but oh well.

As many developers probably know, it's possible to navigate to the GAC by using the command line to go to C:\Windows\Assembly (this is on Windows Server 2003).

However, it's also possible to get Explorer to show items in the GAC. All of the assemblies I have created with Visual Studio end up in the GAC_MSIL folder. I can take the following steps to view them: click Start / Run, then key in "%SYSTEMROOT%\assembly\gac_msil", and click OK.

An Explorer window will pop up showing all of the assemblies in that part of the GAC:



Why would you want to do this? This trick allows you to debug a DLL directly from the GAC. You can now navigate to the exact location of the DLL, and then drop the .PDB in the same folder. If you then have the project for the DLL open in Visual Studio, you can attach to the process calling your DLL and set breakpoints, etc.

This doesn't always work smoothly, but there are some situations where it was the only way to get the job done. In particular, if the client for the DLL is a program like BizTalk or SharePoint, it's the only way I could debug the DLL.

I have encountered at least one time when I had to reboot to be able to set breakpoints, but it beat the hell out of my alternatives.

Remember to remove the PDB from the GAC before you try to UN-install your DLL.