Friday, July 21, 2006

Unit tests - Changing app.config values at runtime

It would be useful for a number of unit tests to change the configuration values used in the app.config file of a unit test DLL. Actually changing the value in the file is straightforward enough i.e.

  1. Get access to the app.config file full path from the process using AppDomain.CurrentDomain.SetupInformation.ConfigurationFile
  2. Load the file into an XmlDocument
  3. Extract the relevant node using code such as XmlNode node = xmlDocument.SelectSingleNode(@"/configuration/appSettings/add[@key='" + configurationName + "']");
  4. Set the value of the “value” attribute using node.Attributes.GetNamedItem("value").Value = configurationValue;

However by default changes to the configuration file won’t be reflected at runtime i.e. code such as

String fromEmailAddress = System.Configuration.ConfigurationSettings.AppSettings["EmailFromAddress"];

will not reflect the new value if EmailFromAddress is changed in the app.config file. The AppSettings class caches the values retrieved within the AppDomain and never refreshes them. ASP.NET as a CLR host actually does refresh the settings when the web.config file is changed however by default the standard CLR host won't.

Since there is no way to force the settings to be re loaded the simplest approach would be to ensure that all clients use the existing fa├žade defined in Tranmit.Common.ConfigurationSupport. The GetValue method looks like this

public static Object GetValue( String key,
Type typeOfObject,
Object defaultValue)
// Assign default value to object if no key found
Object value = defaultValue;

System.Configuration.AppSettingsReader appSettingsReader = new System.Configuration.AppSettingsReader();

// Check value is present first to avoid exception
// being thrown from AppSettingsReader.GetValue
if(ConfigurationSettings.AppSettings.Get(key) != null)
value = appSettingsReader.GetValue(key, typeOfObject);

return value;
} // GetValue

This is fine for production code however when executing unit tests we need to make sure the value is loaded directly from the app.config file rather than through the AppSettings or AppSettingsReader class. We could do this by

  • Set a value within the AppDomain to indicate a unit test is being executed e.g. AppDomain.CurrentDomain.SetData(“UnitTestExecuting”,true);
  • Modifying the existing GetValue method to check the app domain value UnitTestExecuting. If this is enabled then extract the configuration value using the steps 1 to 3 above directly from the file. Otherwise use the existing code which uses the AppSettings class

There should only be a slight performance impact for unit test code which is fine, but production code should execute the same. This will obviously require that client code uses the GetValue method rather than the classes in the ConfigurationSettings namespace directly.