In the presentation about Good API Design I talked about in yesterdays post one of the key points made was that once an API is defined you should never make changes to it that will break your client’s code. An example cited throwing exceptions based on values previously considered fine.

As luck would have it I encountered an actual example of precisely this problem today while installing the .NET 2.0 Runtime on a development server. This server runs a number of .NET 1.1 applications and a number of classic ASP applications consuming COM components written in .NET 1.1.

Things didn’t start well, with the framework installer stopping the IIS instance for the better part of 10 minutes while installing, however it did restart it again once it was done (unlike MSDTC and SQL Server when installing anything from the Windows Components section of Add Remove Programs on Windows 2003).

Matters got worse when someone mentioned that one of the components on the server was now misbehaving - specifically one that uses the ASP.NET Cache to provide caching capabilities.

Whenever a web application tried to create this object (via Server.CreateObject) it was getting an invalid pointer error. Other COM components developed in a similar way were working fine, so I assumed there was something wrong with the registration of the component. Un-registering and re-registering the component gave no joy - neither did calling it from a simple VBScript file.

To make matters worse, a simple .NET test application was working just fine using the exact same library.

After a bit of head scratching and pondering the SysInternals (Now a part of Microsoft) Process Explorer revealed that instead of using the .NET 1.1 version of System.Web both CScript and the IIS DLLHost were loading the .NET 2.0 version. The code for the component hadn’t changed, so maybe the .NET framework had.

Loading the source code for the component into Visual Studio 2005 and attempting to compile and run a the simple test application revealed the problem, a Null Reference Exception from within the framework.

As the COM Component was using the ASP.NET System.Web.Cache it was creating a HTTP Context instance internally. This code looked like this:

private System.Web.HttpContext context = new System.Web.HttpContext(null);

Poking round the disassembled code of System.Web in Reflector didn’t reveal what it was that was causing the exceptions, although I did only go a few functions deep, however it did reveal an alternative way of getting to the cache.

Changing our code to use a call to System.Web.HttpRuntime.Cache to obtain the cache instance fixed our problem, and a quick rebuild of the component against .NET 2 and redeploy to the server and we were back up and running.

Lessons learned from all this:

  • The .NET Framework installer will stop IIS and keep it stopped for a large part of the install - useful to know considering I’ll be installing it on some production servers soon
  • Both IIS and CScript seem to run all .NET COM Components through the most up to date version of the .NET framework, regardless of the version the component is registered or compiled with
  • .NET applications (like our test applet) will run in the .NET framework version they were compiled against if available