Unit testing the app.config file with NUnit

Question

I usually isolate external dependencies like reading a config file in their own facade-class with very little functionality. In tests I can create a mock version of this class that implements and use that instead of the real config file. You can create your own mockup's or use a framework like moq or rhino mocks for this.

That way you can easily try out your code with different configuration values without writing complex tests that first write xml-configuration files. The code that reads the configuration is usually so simple that it needs very little testing.

You can modify your config section at runtime in your test setup. E.g:

// setup
System.Configuration.Configuration config = 
     ConfigurationManager.OpenExeConfiguration(ConfigurationUserLevel.None);
config.Sections.Add("sectionname", new ConfigSectionType());
ConfigSectionType section = (ConfigSectionType)config.GetSection("sectionname");
section.SomeProperty = "value_you_want_to_test_with";
config.Save(ConfigurationSaveMode.Modified);
ConfigurationManager.RefreshSection("sectionname");

// carry out test ...

You can of course setup your own helper methods to do this more elegantly.

You can call the set method of ConfigurationManager.AppSettings to set the values required for that particular unit test.

[SetUp]
public void SetUp()
{
  ConfigurationManager.AppSettings.Set("SettingKey" , "SettingValue");
  // rest of unit test code follows
}

When the unit test runs it will then use these values to run the code

You can both read and write to the app.config file with the ConfigurationManager class